Chang 1998 Monterey Laminae PALAIOS

RESEARCH REPORTS 439

Copyright Q 1998, SEPM (Society for Sedimentary Geology) 0883-1351/98/0013-0439/$3.00

Diatomaceous Sediments from the Miocene MontereyFormation, California: A Lamina-Scale Investigation of

Biological, Ecological, and Sedimentary Processes

ALICE S. CHANG and KURT A. GRIMMDepartment of Earth and Ocean Sciences, University of British Columbia,

Vancouver, British Columbia V6T 1Z4, Canada

LISA D. WHITEDepartment of Geosciences, San Francisco State University, San Francisco, CA 94132

PALAIOS, 1998, V. 13, p. 439–458

Finely laminated diatomaceous sediments from the Mio-cene Monterey Formation, Lompoc, Santa Barbara County,California, record the mutualistic coupling of life processesand environmental evolution at subannual and subseason-al resolution. In this study we present a new classification oflamina and couplet styles based on couplet bimodality,lamina thickness, compositional domination, lamina spac-ing, and cyclicity. We also describe five distinct laminatypes with emphasis on paleoenvironmental settings, paleo-ecological associations, and biologically mediated sedimen-tary/taphonomic processes. Detrital laminae, consisting ofsilt, clay, and robust diatoms, were deposited from conti-nental runoff during rainy seasons. Thin biosiliceous lam-inae consist of either moderately preserved high-diversitydiatom assemblages, or well-preserved monogeneric phyto-plankton assemblages. Most thick, continuous diatoma-ceous laminae are composed of well-preserved monogenericand monospecific diatom assemblages that likely experi-enced biologically induced aggregation and rapid sedimen-tation without grazing. Thick, discontinuous diatomaceouslaminae consist of either Thalassiothrix longissima mats orChaetoceros setae. Mat laminae reflect stratified water con-ditions and high biomass conditions developed via vertical-ly migrating diatom mats. Setae laminae are problematicto interpret. Macerated biosilica laminae, consisting ofclosely packed and highly fragmented biosilica from a va-riety of taxa, reflect intense zooplankton maceration anddissolution of diverse phytoplankton assemblages. Our re-sults illustrate how different lamina types and associationscan be used to proxy specific ecological and oceanographicconditions. This study provides a foundation for developinga high-resolution time series of paleoenvironmental vari-ability recorded by biological event strata in hemipelagicsediments.

INTRODUCTION

Laminated diatomites of the Miocene Monterey Forma-tion are a unique but underutilized paleoenvironmentalarchive, providing an excellent opportunity to explore pa-leoecology and fine-scale sedimentary processes in a tec-tonically active region influenced by coastal upwelling.Monterey Formation diatomites consist of well-defined,

mm-scale couplets of diatomaceous and detrital laminae.The finely laminated fabric of Monterey Formation diato-mites preserves a high-resolution record of biological, eco-logical, and sedimentary processes operating in the Cali-fornia Borderland region during the Miocene (Chang andGrimm, in press).

Earlier studies of the Monterey Formation and its bio-siliceous analogs focused on comparisons between lami-nated and non-laminated intervals, and assessed theirbroad-scale composition, origin, and paleoenvironmentalsignificance (e.g., Garrison et al., 1981). Studies from thelast decade involved fine-scale analysis of distinct laminatypes, and interpretations of paleoecology from ODP cores(e.g., Kemp, 1990). Recent detailed lamina-scale studies,utilizing high-resolution backscattered electron microsco-py, of laminated diatomaceous hemipelagites from regionssuch as the Santa Barbara Basin, the Gulf of California,and the Peruvian margin have shed light on ecological as-semblages, biological activity, and interannual sedimen-tary processes (Brodie and Kemp, 1994; Grimm et al.,1996, 1997; Pike and Kemp, 1997). These studies have ad-vanced the understanding of biologically mediated sedi-mentary processes and oceanic biogeochemical cycles.

Laminated diatomaceous sediments from the MontereyFormation are widely regarded as true seasonal rhyth-mites (i.e., varves; Pisciotto and Garrison, 1981; Anderson,1986), however, this interpretation is not well-substanti-ated. The examination of such laminae and the classifica-tion of laminae and sedimentary couplets in economic di-atomites from the Celite Corporation diatomite quarry,Lompoc, Santa Barbara County, California (Fig. 1), is thefocus of this study. We attribute diatom assemblages in-vestigated in this study to biologically mediated sedimen-tary processes operating at seasonal and subseasonal timescales. From the data, we interpret sedimentary/taphon-omic processes involved in the production of individuallaminae and couplet types, and illustrate how such signa-tures can be used to proxy paleoenvironmental settingsand environmental evolution.

METHODS AND MATERIALS

Approximately 218 m of laminated diatomite are ex-posed at Celite Quarry. Updated chronostratigraphy plac-es the age of the sediments at the base of the exposed sec-tion at 8.2 Ma (Thalassiosira antiqua Zone) and the top of

440 CHANG ET AL.

FIGURE 1—Map of study location, Celite Quarry, Lompoc, California(modified from Grimm and Orange, 1997).

the economically mineable deposits at 7.3 Ma (Nitzschiareinholdii Zone; J. A. Barron, written communication,1997; Barron and Isaacs, in press). Our studies concen-trated on diatomites from a location within the economicdeposits in an area that was topographically the lowest inthe quarry, darkest in color, and the least weathered.These beds constitute part of the southern limb of an east-west trending syncline (Chang, 1997).

Two stratigraphically continuous sections totalling ;59m were cleared by bulldozers and scraped clean withsharpened hoes. We subdivided the sections into unitsbased on the respective diatom or mud richness of the sed-iments (Fig. 2). The sections were measured and de-scribed, focusing on both laminated and non-laminated in-tervals. Samples were cut with a gas-powered concretesaw, producing blocks ranging in size from ;15 cm (depth)by 10–15 cm (breadth) by 20–50 cm (length). Additionalsamples, from a correlative interval on the topographicallyhigher and more extensively weathered northern limb ofthe syncline, 850 m northwest of our main site, were col-lected to assess lamina-scale correlation across the quarry.

In the laboratory, the blocks were slabbed perpendicu-lar to bedding with a bandsaw to a thickness of 1 cm. Sev-eral slabs were cut for hand-sample description, x-radiog-raphy, light microscopy, and scanning electron microsco-py. Detailed methodology is reported in Chang (1997).

Microstratigraphic descriptions of slabbed and sandedhand samples focused on physical characteristics of lami-nated intervals (e.g., lamina thickness). Non-laminatedand deformed intervals were also examined for their rela-tionships with laminated intervals.

X-radiography was performed with a CGR 300-T mam-mography instrument at UBC Hospital. X-radiography bymammography produces sharper and higher-resolutionimages than conventional x-ray techniques because thecollimated beam reduces parallax between discrete lami-nae (Algeo et al., 1994; Collins, 1996). Each slab was ex-posed at 26 keV, with mud-rich samples exposed at 50mAs, and diatom-rich samples at 32 mAs. Saturation of x-ray film is inversely proportional to sediment bulk densi-ty; porous diatomaceous sediments have high x-ray trans-

missivity and appear light-colored in the x-ray contactprints, whereas dense detrital sediments appear dark.Contact photographic prints were developed, and fivesamples (2–8, 2–9, M-3, M-6 and M-BP; Fig. 2) were se-lected for detailed analysis because of the superior clarityof their x-radiographs and the presence of diverse sedi-mentary features and lamina types.

Hyrax-mounted smear and strewn slides were used toidentify microfossils, assess preservation states, and inter-pret ecological associations. Semi-quantitative countswere performed on a petrographic microscope at a magni-fication of 10003 by examining at least half of each slide;evaluation criteria and frustule preservation are ex-plained in Table 1. Full quantitative counts involved thecounting of at least 200 diatoms per slide; the counts ofeach taxon were normalized to 100%.

High-magnification imaging was achieved with second-ary electron microscopy (SEM) and backscattered electronmicroscopy (BSEM). Conventional SEM permits the im-aging of specimen topography and a bedding-plane view ofmicrofossil taxa and their states of preservation. Centi-meter-sized cubes were cut with a bandsaw, where thelamina to be imaged was carefully sectioned parallel tobedding with a single-edge razor blade, and Au-Pd coated.BSEM analyses of epoxy-impregnated samples producescross-sectional images of sediments and allows for optimalanalysis of submillimeter fabrics (Kemp, 1990). Sampleswere sawed perpendicular to bedding into centimeter-sized cubes and rods, vacuum-impregnated with low-vis-cosity epoxy (Struers brand Epofix), highly polished, andcarbon coated. Both SEM and BSEM imaging were per-formed on a Philips XL-30 scanning electron microscope atan accelerating voltage of 15 kV (Chang, 1997).

GENERAL SEDIMENTOLOGY ANDOCEANOGRAPHIC CONDITIONS

Monterey Formation

Organic-rich hemipelagic sediments of the MontereyFormation were deposited within an oxygen-minimumzone in either a series of fault-bounded basins similar tothose along the modern California Borderland (Gorslineand Emery, 1959) or on low-gradient slopes along an opencontinental margin (Isaacs, in press).

The Monterey Formation has been traditionally de-scribed as a tripartite sequence, consisting of a lower cal-careous facies dominated by coccolithophores, a middletransitional phosphatic facies, and an upper siliceous fa-cies dominated by diatoms. Previous environmental mod-els proposed that the Monterey Formation was depositedwithin fault-bounded basins in which broad-scale paleoen-vironmental changes were neatly preserved at all loca-tions (Pisciotto and Garrison, 1981). Recent reevaluationof these models illustrates that deposition of the MontereyFormation is more complicated than previously document-ed; facies were deposited on gentle slopes along an opencontinental margin, governed by a dynamic progradingmargin, and affected by local water depths, bottom topog-raphy, and sediment supply (Isaacs et al., 1996; Isaacs, inpress). Furthermore, differential production and preser-vation of siliceous and calcareous components from one de-positional site to another, along with varying degrees of

LAMINATED DIATOMITES: BIOLOGICAL, ECOLOGICAL, AND SEDIMENTARY PROCESSES 441

FIGURE 2—Stratigraphy and sample locations from two continuous measured sections from our main study area. The positions of correlativesamples collected elsewhere in the quarry have been plotted on the stratigraphic columns.

442 CHANG ET AL.

TABLE 1—Microfossil abundance and preservation criteria for semi-quantitative smear and strewn slide analyses.

AbundanceAbundantCommonFewRareBarren

$2 specimens per field of view1 specimen per field of view1 specimen per vertical traverse,1 specimen per vertical traverse0 specimens observed in sample

PreservationExcellent pristine; primary aggregation and depo-

sition of frustules; no alteration (i.e.,fragmentation and/or dissolution)

Good lightly silicified and heavily robust speci-mens present; minor alteration

Moderate lightly silicified specimens present butaltered

Poor lightly silicified specimens absent or rareand altered; assemblage dominated byrobust forms

phosphatization, was likely superimposed on major paleo-ceanographic trends (White et al., 1992). Hence, the tri-partite sequence is not present at all locations.

The upper siliceous facies of the Monterey Formationwas deposited in response to intensified coastal upwelling.A series of middle Miocene cooling steps induced by Ant-arctic ice-sheet expansion at ;16–13 Ma resulted in asteepened pole-to-equator thermal gradient, enhancedwind strengths, and intensified coastal upwelling (Ingle,1981; Barron and Baldauf, 1989). Initial deposition of theupper siliceous facies was determined to have began at14.8–14.3 Ma in north-central California, and at 12.7 Main southern California (White, 1989; White et al., 1992).This north-south difference in age is attributed to the pro-gressive southward intensification of the California Cur-rent during the interval of stepwise Miocene cooling(White et al., 1992).

The switch from coccolithophore-dominated to diatom-dominated sedimentation reflects increased nutrient de-livery to the surface waters via enhanced upwelling at theonset of middle Miocene cooling (Pisciotto and Garrison,1981). Cooler waters and high nutrient levels permittedthe displacement of coccolithophores by diatoms. Al-though diatoms became the dominant phytoplanktongroup, other microfossils, such as those of radiolaria,planktic foraminifera, benthic foraminifera, and coccolith-ophores, were deposited and are preserved within diato-maceous sediments, indicating the persistence of otherphytoplankton groups throughout the Miocene.

Formation of Laminated Sediments

The formation of laminated hemipelagic sediments in-volves both the production and preservation of laminae.Visually distinct laminae derive from heterogeneities insediment composition and texture, where a lack of physi-cal and/or biological reworking permits lamina preserva-tion (Grimm et al., 1996).

A well-defined oxygen-minimum zone, commonly situ-ated beneath highly productive coastal upwelling regions,excludes bioturbating macrobenthos. Fluctuations of the

oxygen-minimum zone over time may result from chang-ing eustatic sea levels or changes in the biological oxygendemand via changes in primary productivity and exportefficiency (Kennett et al., 1994). Oxygen-minimum zonefluctuation in Monterey Formation diatomites is recordedby the presence, absence, and/or relative distinctivenessoflaminae, and the abundance of associated bioturbation(Ozalas et al., 1994; Savrda, 1995).

Laminated sediments may also be preserved in oxygen-ated environments. Laminated Neogene sediments fromthe North Atlantic and equatorial Pacific possess thickmat laminae consisting of the elongate diatom Thalas-siothrix longissima (Kemp and Baldauf, 1993; Boden andBackman, 1996). Mats, consisting of entangled diatoms,are buoyant, macroscopic communities that photosynthe-size in the pelagic zone and take up nutrients in deeper,subphotic waters (Villareal and Carpenter, 1989; Villarealet al., 1993). The entangled meshwork of T. longissimamats essentially produced ‘‘preconsolidated’’ laminae inthe surface waters. Zooplankton apparently avoid grazingon mats because they are uningestible (Paffenhofer et al.,1980), and bioturbating bottom-dwelling organisms aredeterred from destroying the mats because of their hightensile strength and impenetrability (Kemp et al., 1995;Boden and Backman, 1996).

Overview of the Monterey Formation at Lompoc

The upper siliceous facies of the Monterey Formationexposed at Celite Quarry consists of both laminated andnon-laminated intervals. Finely laminated diatomite, con-sisting of millimeter-scale diatomaceous and detrital lam-inae, comprises the majority of the sediments. The mostopal-rich sediments in the quarry occur in economicallymined intervals designated as DE-1 and DE-2 zones byCelite Corporation (E. Morlan, personal communication,1996; Fig. 2). Hybrid fault/vein structures termed intra-stratal microfractured zones are also abundant in this in-terval, and represent synsedimentary deformation events(Grimm and Orange, 1997). Non-laminated intervals, rep-resenting episodic events that punctuated the laminatedsequence, include three main types: massive muddy beds,bioturbated beds, and speckled beds.

Massive muddy beds are tan-colored, decimeter-thickbands that are generally siltier than the surrounding lam-inated sediments. Most massive muddy beds have sharpupper and lower contacts and a homogeneous texture.Some beds contain intraclasts or convolute laminae. Mul-tiple deposits form amalgamated beds #50 cm in thick-ness. Massive muddy beds were likely deposited by muddydebris flows (Chang and Grimm, in press).

Bioturbated beds are markedly siltier than the sur-rounding laminated sediments, confirming the observa-tions of Ozalas et al. (1994). We observed 17 bioturbatedbeds in our 59-m measured section. Bioturbated beds havesharp concordant or erosive basal contacts, sharp uppercontacts, and consist of an upper massive and burrowedunit and a lower unit of burrow-imprinted laminae (Sa-vrda, 1995). Burrow-imprinted laminae are always asso-ciated with superjacent massive units that are abruptlyoverlain by undisturbed laminated sediments. No evi-dence for transitional fabrics (i.e., minimally burrowed ordisturbed laminae) between bioturbated beds and laminae


FIGURE 3—Comparison of a slab from the unweathered sample M-BP (left) and its weathered equivalent (right) from the correlative in-terval, showing correlation of laminae and other sedimentary featuresacross a distance of 850 m. (a) speckled beds (Chang and Grimm, inpress), (b) mud-rich couplets, (c) diatom-rich couplets. The only dif-ferences between these samples are localized slump folds.

TABLE 2—Classification of lamina and couplet styles.

Category Descriptor Specifications

Thickness razor-stripedpin-stripedchalk-striped

,0.2 mm0.2–1.0 mm1.0–1.5 mm

Spacing closely spacedmoderately spacedwidely spaced

,10 laminae/cm5–10 laminae/cm,5 laminae/cm

Bimodality high bimodalitymoderate bimodalitylow bimodality

large contrast in grayscale and bulk densitymoderate contrast in grayscale and bulk densitysmall contrast in grayscale and bulk density

Domination mud-dominateddiatom-dominatedsubequal

.1.5:1 mud or detritus

.1.5:1 biosilica;1:1

Cyclicity coupletpacketbundle

single pair of dark and light laminaesuite of coupletssuite of packets alternating with muddy intervals

overlying them are present, suggesting abrupt termina-tion of burrowing episodes (Follmi and Grimm, 1990;Grimm and Follmi, 1994). The only ichnogenus observedin our sections was Thalassinoides (cf. Savrda and Ozalas,1993).

Speckled beds range in thickness from 3 mm to 10 cmand possess bedding-parallel fabrics. Speckled beds con-sist of sand- to granule-sized diatomaceous and detritalaggregates that are distributed uniformly throughout amatrix of mud and macerated biosilica (Chang et al., 1997;Chang and Grimm, in press). Speckled beds have sharpupper and lower contacts and are not crosscut by biotur-bation. We attribute the formation of speckled beds to thedisruption of laminated diatomaceous sediments by step-wise rheological transformation, where formerly rigidslump or slide blocks evolved into fluid turbulent flowsand subsequently transformed into viscous debris flows.The processes of initial sediment disruption to final depo-sition by debris flows occurred within the oxygen-mini-mum zone (Chang and Grimm, in press).

Both laminated and non-laminated features weretracked across the quarry by comparing samples extractedfrom sections at both the southern and northern limbs ofthe syncline. Most non-laminated beds and laminae asthin as 0.5 mm were laterally continuous across a distanceof 850 m with only minor localized differences (e.g., slumpfolding) between the two sample sites (Fig. 3).

CLASSIFICATION OF LAMINA STYLES

We introduce a new method of classifying lamina andcouplet styles based on five main parameters: couplet bi-modality, lamina thickness, compositional domination,lamina spacing, and cyclicity (Table 2). This classificationprovides a convenient way for grouping sedimentary lam-inae, and is useful for the description and subsequent in-terpretation of different lamina types.

Bimodality

Grimm et al. (1996) define bimodality as the relative dif-ference in x-radiograph grayscale, and thus bulk density,between adjacent laminae of different compositions withina sedimentary couplet. This concept was expanded for ap-plication in the field and on slabbed samples by comparingthe color differences between juxtaposed laminae of differ-ent compositions. The color differences on hand samplesparallel contrasts in x-radiograph grayscale (i.e., light-col-ored diatom-rich laminae correlate to light shades on x-ra-diograph contact prints). Adapting bimodality to fieldstudies provides a quick, semi-quantitative assessment ofsediment composition and bulk density.

In the field, couplets displaying a stark color differencebetween detrital and diatomaceous laminae are termedhigh-bimodality couplets (cf. Grimm et al., 1996). In x-ra-diographs, these couplets show a large contrast in gray-scale and thus bulk density (Figs. 4 and 5). Moderate-bi-modality couplets possess a moderate color difference be-tween detrital and weakly diatomaceous laminae.Low-bimodality couplets display slight color differencesbetween detrital and poorly diatomaceous laminae; thiscorresponds to a low contrast in x-ray transmissivity(Grimm et al., 1996).

444 CHANG ET AL.

FIGURE 4—Photographs of scraped outcrop surfaces depicting dif-ferent lamina and couplet styles (cf. Table 2). (A) Razor- to pin-striped,low-bimodality (white asterisk) and high-bimodality (black asterisk)couplets. Diameter of coin is 2.4 cm. (B) Pin- to chalk-striped laminaecomprising diatom-dominated (black asterisk) and mud-dominated(white asterisk) couplets. Three packets of couplets (a, b, c) are sep-arated by thin muddy layers. Coin 5 2.4 cm. (C) Four distinct, chalk-striped diatomaceous laminae. Arrow denotes lamina containingmonospecific Rouxia californica. Coin 5 1.8 cm.

Thickness

Three thicknesses were ascribed for most laminae in oursection (Figs. 4 and 5). We refer to laminae measuring,0.2 mm as razor-striped laminae; these can be either dis-tinct and continuous, or indistinct and discontinuous (Fig.5A). Laminae between 0.2–1.0 mm thick are consideredpin-striped and are usually distinct. We term diffuse todistinct laminae with thicknesses of 1.0–1.5 mm as chalk-striped laminae (Fig. 5B).

Domination

Domination refers to the relative mud or diatom rich-ness of a couplet or laminated interval. Intervals charac-terized by low 151 bimodality couplets, thick chalk-striped detrital laminae, and thin muddy gravity-flow lay-ers are mud-dominated, where diatomaceous laminae arethin and/or indistinct (Figs. 4 and 5). Diatom-dominatedintervals are characterized by high 151 bimodality cou-plets and thick diatomaceous laminae (Fig. 4B). Subequalcouplets are characterized by equal proportions of detritaland diatomaceous laminae.

Spacing

We term intervals with .10 laminae/cm as closelyspaced, where the laminae are often razor- to pin-striped.Intervals possessing 5–10 laminae/cm are moderatelyspaced, with pin- to chalk-striped detrital laminae, and in-creasing mud content. Intervals containing ,5 laminae/cm are widely spaced, are usually mud-dominated, andhave diffuse diatomaceous laminae (Fig. 5B).

Cyclicity

The laminae in our section occurred in couplets, pack-ets, and bundles. Couplets are individual pairings of dia-tomaceous and detrital laminae; as discussed below, manyof these mm-scale couplets may be true varves. Packets arecentimeter-scale associations of uninterrupted coupletsthat are synonymous with the first-order clastic-biogeniccycle defined by Pisciotto and Garrison (1981). Bundles(second-order clastic biogenic cycle) consist of laminatedpackets alternating with mud-rich units on a centimeterto decimeter scale, and may represent deposition on deca-dal to millennial time scales (Fig. 4B).

Stratigraphic oscillations between mud-dominated, di-atom-dominated, and non-laminated intervals may havebeen influenced by two sets of processes: allocyclic mech-anisms (external forces such as climate and sea-level fluc-tuations) and autocyclic mechanisms (internal forces suchas gravity-flow lobe switching). A combination of bothmechanisms operate to give rise to cyclic deposition.

DESCRIPTION OF LAMINA TYPES

Laminated diatomites from Celite Quarry consist ofmm-scale diatomaceous/detrital couplets. Laminated in-tervals may be either mud-dominated or diatom-dominat-ed, depending on the thickness and/or abundance of detri-tal or diatomaceous laminae.

For the description of each lamina type, mesoscale de-

scriptions will be discussed first, followed by microscaledescriptions. Microfossils such as coccoliths, agglutinatedforaminifera, and radiolaria were observed in minor abun-dance. Phytoplankton taxa were identified to species levelwhere possible. Lamina color was determined in the fieldon freshly unearthed samples, with color designationsadapted from conventional soil-color usage (Munsell ColorCompany, 1971).

Detrital Laminae

Detrital laminae are brown (5Y 2.5/2) to olive (2.5Y 6/2),razor, pin and chalk stripes of constant thicknesses; thick-


FIGURE 5—X-radiograph contact prints of slabbed samples. (A) Sample 2–8. A relatively diatom-rich sample with razor- to pin-stripedlaminae. The upper portion of this sample has distinct, well-defined razor-striped laminae. Numbered black arrows show positions wheresubsamples were collected. (B) Sample M-6. A muddy sample containing numerous low-bimodality (LB) to moderate-bimodality (MB)couplets with both distinct and diffuse razor- to chalk-striped laminae. Numbered black arrows show positions where subsamples werecollected.

er and more abundant detrital laminae occur in mud-dom-inated intervals. Microscopic constituents observed fromsmear and strewn slides include silt grains, clay, spongespicules, and a variety of diatom taxa (Fig. 6). Some of thediatom taxa are Coscinodiscus sp., Actinoptychus sp., Rha-phoneis miocenica, Navicula sp., the benthic taxon Arach-nodiscus sp., and resting spores (Chaetoceros, Stephano-pyxis, and Leptocylindrus; see Appendices 1 and 3). Thesediatoms are all robust, heavily silicified forms. Lightly si-licified diatoms, such as Thalassiosira antiqua, are alsopresent but have been fragmented and/or altered by dis-solution.

Thin Biosiliceous Laminae

Thin biosiliceous laminae range in color from white toolive gray (10YR 8/1 to 2.5Y 7/2) in high-bimodality cou-plets, to increasingly darker shades towards low-bimodal-ity couplets; thicknesses range from razor to pin stripes(Figs. 4 and 5). High-diversity diatom assemblages, con-taining a combination of any taxa listed in Table 3, com-prise most laminae (also see Appendices 2 and 3). Howev-er, some thin biosiliceous laminae are composed of mono-generic Coscinodiscus assemblages or monospecific as-semblages of the silicoflagellate Dictyocha speculum (Fig.

446 CHANG ET AL.

FIGURE 6—Backscattered electron micrographs of epoxy-impregnat-ed sediment from sample 2–8. (A) High-bimodality couplet showing alarge contrast between a detrital lamina (below) with abundant siltgrains (sg), and a porous diatomaceous lamina (above). Asterisk de-marcates the boundary between the two laminae; white box indicateswhere high-magnification image in Figure 6B was collected. (B) Detailof area outlined in Figure 6A. The detrital lamina contains angular siltgrains (sg), tubular sponge spicules (sp), and macerated biosilica.Some diatom frustules are visible but most have been fragmented.Unfragmented and porous diatom frustules comprise the diatoma-ceous lamina.

FIGURE 7—High-magnification images of monospecific thin biosili-ceous laminae. (A) Backscattered electron micrographs of two razor-striped Coscinodiscus-rich laminae (C1 and C2) within a moderate-bimodality interval from sample M-6 (Fig. 5B, arrow 1). Highly reflec-tive angular silt grains (sg) are abundant in this sample. White boxindicates where detailed image was collected. (B) Details of the razor-striped Coscinodiscus laminae and silt grains from Figure 7A. (C) Sec-ondary electron micrograph of a Dictyocha speculum assemblagefrom sample M-BP. The clean, pristine nature of the silicoflagellatesindicates that they likely avoided grazing and were rapidly sedimented.(D) Backscattered electron micrograph of a silicoflagellate (s) assem-blage from sample 2–8 (Fig. 5A, arrow 2).

7). Some irregular, sand-sized aggregates composed ofpristine, unfragmented D. speculum are evident in BSEMimages (cf. fig. 2E in Grimm et al., 1997). Preservation offrustules in laminae with a diverse assemblage is gener-ally poor to moderate, whereas monospecific diatom andsilicoflagellate assemblages display moderate to excellentpreservation.

Thick, Continuous Diatomaceous Laminae

These laminae are white colored in outcrop and slabbedsamples, range in thickness from 1–3 mm, and are promi-nent in diatom-dominated, high-bimodality couplets (Fig.4C). Light and electron microscope analyses reveal thatseveral genera produce thick, continuous diatomaceouslaminae (Table 3). Monospecific to near-monospecific as-semblages of Coscinodiscus sp., Thalassiothrix longissi-ma, Rouxia californica, Thalassiosira sp., and Chaetocerosresting spores are common (Fig. 8). Sand-sized flocs ofChaetoceros resting spores commonly occur scatteredthroughout our samples (Fig. 8D). Thick, continuous dia-tomaceous laminae containing Denticulopsis sp. (cf. fig. 5in Chang and Grimm, in press) and Rhizosolenia sp. arerare.

Thick, Discontinuous Diatomaceous Laminae

Thick, discontinuous diatomaceous laminae are con-spicuous features, being white in outcrop and slabbedsample, irregular in thickness, and lensoid in profile.Upon closer inspection, some laminae are actually com-posed of centimeter-sized flocs that have been amalgamat-ed and flattened by compaction, producing an overall clot-

ted texture (Fig. 9). Most thick, discontinuous diatoma-ceous laminae are located within the opal-rich DE-1 andDE-2 zones (Figs. 2 and 9A). The thickest and mostextensive lamina, however, occurred amongst subequal tomud-dominated pin-striped laminae in Unit 2D of ourmeasured section (Figs. 2 and 9B).

Microscopic analyses of thick, discontinuous diatoma-ceous laminae reveal pristine, low-diversity diatom as-semblages. Smear-slide and SEM analyses of a thick, dis-continuous diatomaceous lamina from a sample (M-29) inthe opal-rich zones reveal mud-free, monospecific assem-blages of Thalassiothrix longissima (Fig. 10). Preservationof the frustules is excellent and no other diatom taxa arepresent. The extensive lamina from sample 2–8 (Fig. 9B)is composed of setae from Chaetoceros vegetative cells,with a minor component of Chaetoceros resting-spore bod-ies and other diatoms (Fig. 11). Preservation of the setaeand other microfossils is excellent.

Macerated Biosilica Laminae

These laminae occur at irregular intervals throughoutour samples, are commonly light olive-gray (2.5 Y 7/2) tolight brown (10YR 7/1), and range in thickness from 1–7


TABLE 3—Paleoecological analyses of diatomaceous laminae.

Occurrence

Thinbiosili-ceous

laminae

Thickcontin-uous

diatom-aceous

laminae

Thickdiscon-tinuousdiatom-aceous

laminae

Mono-generic

floc Comments and Paleoecological Interpretation

A. Major diatom taxa*Thalassiosira antiquaChaetoceros

Thalassiothrix longissimaChaetoceros setaeNitzschiaCoscinodiscus marginatusActinoptychus undulatusEthmodiscusRouxia californicaStephanopyxis turrisRhaphoneisHemiaulusRhizosolenia styliformisDenticulopsis

YY

YYYYYYYYYYY

YY

YY

Y

Y

YY

YY

Y

Y

Y

Y

cold-water diatom (1); ubiquitousresting spore; post-bloom biologically mediated sedimentation(3)cold-water, upwelling dominant (1); forms diatom matsbroken off from vegetative cellsubiquitous, but not in great numberscold-water diatom; ubiquitouscoastal neritic diatom (2); ubiquitouslarge diatom, whole frustule never observed; ubiquitouscold-water diatom (1); ubiquitousrobust; resting spores commonly observedbackground primary productivity; usually in detrital laminaebackground primary productivityrare; only ‘‘stylus’’ portion observedcold-water diatom (1); rare as monogeneric laminae and flocs

B. Minor diatom taxaHemidiscus cuniformisAsteromphalusActinocyclusbenthic floraLeptocylindrus

YYYYY

background primary productivitybackground primary productivitybackground primary productivityreworked from benthic environments in the photic zoneresting spore; rarely observed

C. Other microfossilsSilicoflagellatesRadiolariaCoccoliths

YY

Y ubiquitous; monospecific assemblages rarerare; background primary productivityoccurred as a thick, continuous monogeneric lamina

D. Other grainssponge spiculespollendetrital silt grains

YYY

common background debrisrare background debris; derived from landoccurs as background debris in variable amounts

* Groups are listed in order of decreasing abundance, from semi-quantitative smear/strewn slide counts (Appendix 2).References: (1) Barron and Keller, 1983; (2) Pike and Kemp, 1996; (3) Grimm et al., 1996, 1997.

mm with constant lateral thickness (Fig. 5A). Most ofthese laminae occur within diatom-rich intervals.Electronmicroscope analyses show that these laminae are com-posed entirely of highly fragmented and closely packedbiosilica (Fig. 12). The highly fragmented state of the frus-tules prevents positive identification of most diatom taxa;however, a diverse assemblage of biotic components withdistinct morphologies is present.

INTERPRETATIONS OF BIOLOGICAL, ECOLOGICALAND SEDIMENTARY PROCESSES

Laminated diatomites from Celite Quarry consist ofmm-scale couplets and laminae from either mud-dominat-ed, diatom-dominated or subequal intervals. Laminatedsediments are event-stratified because they preserve dis-crete sedimentary events in a relatively continuous series.Where subseasonal events, such as diatom blooms, arepreserved, a high-resolution record of paleoenvironmentand diatom paleoecology is available for study (Kemp,1995; Grimm et al., 1996, 1997).

Detrital and diatomaceous laminae result from environ-

mental conditions that were present at the respectivetimes of production and deposition. By analogy to laminat-ed sediments forming in modern environments, we inferthat diatomaceous laminae from the Monterey Formationwere produced during seasons when increased irradiance,upwelling, and nutrient delivery to surface waters favoreddiatom proliferation. Detrital laminae were likely depos-ited during seasons of increased continental rainfall and/or storm activity, and lower diatom production. Becausewe are uncertain as to which time of year the respectivelamina types were produced, our interpretations in the fol-lowing discussions outline only the environmental condi-tions responsible for production and deposition, and notthe particular seasons.

Interpretation of Couplet Styles

The thickness of a diatomaceous lamina, with respect toa detrital lamina, in a given low 151, moderate- or high-bimodality couplet is a function of a combination of diatomflux variability, the amount of terrigenous influx, and ep-isodicity (i.e., intensity and duration) of continental rain-

448 CHANG ET AL.

FIGURE 8—Images of thick, continuous diatomaceous laminae. (A) Secondary electron micrograph of a Coscinodiscus lamina from sampleM-6 (Fig. 5B, arrow 2). (B) Strewn slide photomicrograph of a lamina rich in Rouxia californica. Scale bar 5 60 mm. (C) Backscattered electronmicrograph of a Thalassiothrix lamina from sample 2–8. Cross sections of abundant Thalassiothrix frustules (T) are visible. Arrow denotesstratigraphic up direction. (D) Backscattered electron micrograph of a Chaetoceros resting spore floc. Arrow denotes stratigraphic up direction.

fall (Soutar and Crill, 1977; Baumgartner et al., 1991).Laminated intervals with low-bimodality couplets repre-sent low heterogeneities in sediment flux, perhaps reflect-ing weaker seasonality. Diatomaceous laminae depositedduring these conditions often contain increased silt con-tent, suggesting increased continental rainfall and runoffduring the diatom-producing season, where the increaseddetrital input partially diluted the diatom lamina (Sancet-ta, 1996; Grimm et al., 1996). Conversely, high-bimodalitycouplets represent high sediment heterogeneity, whichmay reflect strong seasonality (Grimm et al., 1996). Dur-ing these conditions, silt-rich detrital laminae are pro-duced exclusively during the rainy season and diatoma-ceous laminae exclusively during the dry season; no dilu-tion of diatomaceous laminae occurs.

Many of the couplets we observed probably representtrue, annually-deposited varves that record subannual cli-matic and oceanographic oscillations (cf. Anderson, 1986).However, we possess insufficient data to test whether cou-plets from the Monterey diatomites are true varves. Nev-ertheless, varve assessment is not essential for interpret-ing the origins of individual lamina types.

Origins of Detrital Laminae

Detrital grains were derived from the continent andfringing shelves and delivered to offshore basins and

slopes by diverse mechanisms. In modern marginal ma-rine basins, the main delivery agent is fluvial; fine terrig-enous sediments from the California Borderland aretransported offshore during the rainy seasons via smallcoastal drainage systems (Gorsline et al., 1984). Mud andsilt are transported across the continental shelf and downthe slope by turbid-layer flows (Stow and Bowen, 1978;Gorsline et al., 1984). Suspended sediments may travelfurther out onto the shelf via nepheloid plumes dischargedby rivers; the uniform thickness, lateral continuity, andlack of erosional bases suggest that most detrital laminawere likely deposited from suspension. Eolian transport ofdetritus from continental deserts, the reworking of shelfsediments during severe storms, or slope failure may alsocontribute to detrital deposition (Baumgartner et al.,1991; Sancetta, 1995).

The thickness of detrital laminae may reflect the inten-sity and duration of continental rainfall events. Soutarand Crill (1977) and Soutar et al. (1981) observed the cor-relation of thick detrital laminae and muddy turbidites tohistorical intense rainfall or flooding events in the SantaBarbara Basin. Muddy, low- to moderate-bimodality inter-vals within our section, such as those from samples M-3and M-6, may record periods of increased continental rain-fall and consequent high-discharge events which producedabove-average contributions of terrigenous detritus (cf.Calvert, 1966).


FIGURE 9—Outcrop photographs of thick, discontinuous diatoma-ceous laminae. (A) A hybrid fault/vein structure, or intrastratal micro-fractured zone (IMZ; Grimm and Orange, 1997), with three mat lami-nae (m) composed of Thalassiothrix longissima, and a bloom lamina(b). Note the numerous diatomaceous flocs (f) throughout the photo-graph. Coin 5 2.4 cm. (B) Conspicuous lamina from sample 2–8 com-posed of unfragmented and pristine Chaetoceros setae. Coin 5 1.8cm. Arrow denotes where high-magnification images were collected(cf. Fig. 11).

FIGURE 10—Scanning electron images of a Thalassiothrix longissimaraft. (A) Composite image showing the entangled, hairlike nature ofthe frustules. White box indicates where detailed image was taken.(B) Magnified image of the left side of the raft showing clean, unalteredcondition of the frustules. Rafts such as these may be the buildingblocks of T. longissima mats and mat laminae.

Diatoms in detrital laminae commonly include well-pre-served, heavily silicified resting spores of various species(Chaetoceros sp., Leptocylindrus danicus, Stephanopyxisturris), and benthic taxa and coastal neritic taxa like Ac-tinoptychus and Arachnodiscus (Cleve-Euler, 1968). Thefragmented remains of lightly silicified planktic diatomsare common in detrital laminae. The generally high de-gree of fragmentation and poor preservation of planktic di-atoms is likely due to zooplankton grazing, dissolution,and/or the physical reworking of previously deposited sed-iments (Sancetta, 1989).

Origins of Thin Biosiliceous Laminae

Our observations of thin biosiliceous laminae indicatethat there are two disparate types: laminae containing di-verse assemblages of poorly to moderately preserved dia-toms, and laminae with well preserved monogeneric as-semblages. We interpret that thin biosiliceous laminaewith high-diversity and highly fragmented diatom assem-blages reflect intermediate levels of primary productivity

conditions (Sautter and Sancetta, 1992; Kemp, 1995). Thediverse diatom assemblages indicate moderate mixingand nutrient levels, perhaps reflecting background condi-tions, that were sufficient to support a diverse phyto-plankton community but not abundant enough to producea bloom. The poor degree of diatom preservation indicateszooplankton stocks that were in phase with diatom stocks,resulting in efficient grazing (Fig. 13A; Peinert et al.,1989). We term these laminae background diatomaceouslaminae, since they likely recorded typical background lev-els of primary productivity and heterotrophic grazingabove Monterey depocenters.

The occurrence of monospecific Coscinodiscus margina-

450 CHANG ET AL.

FIGURE 11—High-magnification images of the Chaetoceros setaelamina from sample 2–8 (cf. Figs. 5A and 9B). (A) Photomicrographof a strewn slide. Setae (s) are knobby and curved, whereas Thal-assiothrix (T) are smooth, hyaline and straight. Some centric diatoms(c) and three Hemiaulus valves (H) are also present. Scale bar 5 60mm. (B) Scanning electron micrograph of Chaetoceros setae (s) anda Thalassiothrix frustule (T). (C) Backscattered electron micrograph ofChaetoceros setae in both cross-sectional (s) and longitudinal (s’) an-gles, Thalassiothrix frustules (T), and a large Coscinodiscus valve (C).

FIGURE 12—Macerated biosilica laminae (cf. Fig. 5A). (A) Scanningelectron micrograph of highly fragmented and closely packed diatomfrustules. (B) Backscattered electron micrograph of a moderate-bi-modality couplet. Compare the closely packed frustules within themacerated biosilica lamina (MBL) to the porous diatomaceous laminaabove.

tus in some laminae (Fig. 7A, B) may reflect strong disso-lution that allowed better preservation of robust frustulesover weakly silicified taxa (Roelofs, 1983; White and Al-exandrovich, 1992). However, we see little evidence of par-tially dissolved C. marginatus because of the excellentpreservation of some of the frustules. As an alternative, C.marginatus laminae may reflect ancient bloom assem-blages in an extant taxon that is ubiquitous but no longerblooms today. The low degree of fragmentation of C. mar-ginatus frustules may indicate production and depositionduring a time when zooplankton were not grazing (Peinertet al., 1989).

Origins of Thick, Continuous Diatomaceous Laminae

These conspicuous laminae are composed of pristinemonospecific to near-monospecific assemblages of mainlyrobust diatom taxa. The formation of thick, continuous di-atomaceous laminae is attributable to several possiblemechanisms which include diatom blooming, followed by

biologically induced aggregation (Alldredge and Got-schalk, 1989), concentration of frustules by dissolution,and concentration by sedimentary processes.

True-blooming diatom species—those exhibiting highdoubling rates and rapid growth—respond to pulses of in-creased nutrient supply after the onset of increased up-welling and irradiance. Bloom assemblages varying fromlamina to lamina were likely due to the different growthprocesses of each diatom group, their seeding strategies inresponse to certain oceanographic conditions, and which-ever taxa were present when conditions were favorable(Fig. 13B; Smetacek, 1985; Pitcher, 1990). Modern diatomtaxa which bloom and rapidly sediment as monogenericassemblages include Chaetoceros (Hargraves and French,1983; Pitcher, 1990). By analogy with modern taxa, theirMiocene counterparts may have behaved similarly to pro-duce thick laminae. We term these bloom laminae. Con-versely, there may also have been diatoms, such as Cosci-nodiscus and Denticulopsis, from the Miocene thatbloomed and created thick laminae, but whose moderncounterparts no longer employ this strategy.

The excellent preservation of frustules in monogenericand monospecific bloom laminae and flocs may be attrib-utable to biologically mediated aggregation and accelerat-ed sedimentation. Alldredge and Gotschalk (1989) andPassow et al. (1994) outline this phenomenon in modernupwelling environments, whereby diatoms produce stickygels, flocculate and rapidly sink in large masses. Grimm etal. (1996, 1997) discovered unique monospecific diatom as-semblages in laminated Holocene sediments from theSanta Barbara Basin and attributed their formation tothis mechanism, terming it self-sedimentation. Efficientexport via self-sedimentation allows diatoms to bypasszooplankton grazing, and results in the delivery of unal-tered frustules to the seafloor. The efficient flux of largemasses of unaltered diatoms delivers opal and labile or-ganic carbon from the surface waters directly to the sedi-ments, thereby enriching the carbon and opal content ofsediments accumulating directly beneath highly produc-


FIGURE 13—Interpretative cartoons summarizing the origin of individ-ual lamina types; see text for details. (A) Formation of thin biosiliceouslaminae containing a high-diversity assemblage, and recording mod-erate to high fragmentation (‘‘background diatomaceous laminae’’). (B)Formation of thick, continuous diatomaceous laminae containing true-blooming taxa (‘‘bloom lamina’’). Modified from Grimm et al. (1996).(C) Formation of Thalassiothrix ‘‘mat laminae’’ in either oceanic frontdevelopment or stratified water conditions. (D) Formation of Chaeto-ceros setae laminae; this mechanism is highly uncertain. (E) Forma-tion of macerated biosilica laminae. High relative rates of zooplanktongrazing and/or dissolution result in the export of fragmented frustulesto the seafloor (Peinert et al., 1989; Grimm, 1992).

tive waters (Sancetta et al., 1991; Archer et al., 1993).Grimm et al. (1997) discuss the ecological and evolution-ary significance of the self-sedimentation phenomenon.

The formation of thick, continuous diatomaceous lami-nae by concentration of frustules via dissolution seems un-likely because frustules examined with light and electronmicroscopy show little or no alteration. Thus, we discountdissolution as a possible mechanism for formation. A plau-sible explanation for the formation of thick laminae con-taining elongate diatoms such as Thalassiothrix, diatomswith long projections, or colonial chains is concentrationby tangling and rapid sedimentation (Alldredge and Got-schalk, 1989). The effects of tangling are described morefully below.

Origins of Thick, Discontinuous Diatomaceous Laminae

Two types of laminae fall within this category: thoseconsisting of pristine monospecific assemblages of the pen-nate diatom Thalassiothrix longissima, and those com-posed of entangled and unfragmented Chaetoceros setae.

Origins of Thalassiothrix longissima Laminae

Thalassiothrix longissima laminae possess a macro-scopically crinkled appearance and contain pristine, inter-twined frustules (Figs. 9A and 10). These characteristicsare similar to laminae rich in T. longissima described byKemp and Baldauf (1993) and Boden and Backman(1996), and are attributed to diatom mats. Thus, we pos-tulate that T. longissima also congregated within mats,and interpret thick, discontinuous T. longissima laminaeas mat laminae. The interpreted mechanisms of mat for-mation are outlined below.

Specific requirements of frustule morphology, verticalmigration, and oceanographic setting are necessary toform mat laminae. Thalassiothrix longissima are open-ocean diatoms with hairlike frustules, measuring between1.5–5.0 mm wide and up to 4 mm long, that interlock andmesh to form buoyant, colonial bundles, patches, and mats(Kemp et al., 1995). The meshwork produced by interlock-ing frustules results in a mat that is uningestible to graz-ing zooplankton and impenetrable to bioturbating benthicfauna (Paffenhofer et al., 1980; Boden and Backman,1996).

Modern rhizosolenid mats have been used as an analogto describe the formation of T. longissima mat laminae(Kemp and Baldauf, 1993; Sancetta, 1994; Kemp et al.,1995). Rhizosolenia is an elongate oceanic diatom that canintertwine to form mats up to 30 cm in diameter (Villarealand Carpenter, 1989). These mats can aggregate into ex-tensive sheets that can extend for several square kilome-ters (Yoder et al., 1994).

Rhizosolenia mats can form and accumulate during oce-anic front development or strongly stratified water condi-tions (Sancetta, 1994; Yoder et al., 1994). Oceanic frontsdevelop when cold surface waters subduct beneath warm-er surface waters. Rhizosolenid mats were observed ingreat abundance following the 1992 El Nino event as nor-mal cold-water conditions (anti-El Nino or La Nina) re-turned to the equatorial Pacific (Yoder et al., 1994). Coldwater subducted beneath residual warm water, a sharpfront developed, and mats congregated at shallow depths

452 CHANG ET AL.

in sunlit waters on the warm side of the front (Yoder et al.1994). Mats appear to favor the warm side of the front be-cause downwelling velocities are lower than those on thecold side (Yoder et al., 1994; Fig. 13C).

Buoyancy regulation and vertical migration in Rhizoso-lenia is essential in stratified waters where light and nu-trients are spatially separated (Villareal et al., 1993).When nitrogen availability in surface waters is depleted,rhizosolenid mats sink to the nutricline in the subphoticzone to replenish nitrate and subsequently return to thesunlit surface (1–2 m depth) to photosynthesize (Fig. 13C).By returning to the surface, the mats deliver largeamounts of new nitrogen to the surface (50% of the new ni-trogen in the North Pacific gyre; Villareal and Carpenter,1989; Villareal et al., 1993).

Nutrient depletion, breakdown of the stratified watercolumn, excessive mat size, senescent cells and physiolog-ical loss of buoyancy are causes for rhizosolenid mat mor-tality (Sancetta, 1994). Their large size permits rapidsinking from surface waters, and sedimentation of ungraz-ed and unfragmented frustules clustered as primary ag-gregates (Sancetta et al., 1991). Similar biological and en-vironmental factors may have caused the formation andsedimentation of T. longissima mats in the Monterey For-mation. Broken mat fragments are preserved as sand tosmall-pebble sized aggregates; depositional amalgama-tion of such mat fragments formed the irregular, clottedfabric observed in some mat laminae.

Thalassiothrix longissima mat laminae are very com-mon in the DE-1 and DE-2 zones of our measured section(Fig. 2); their abundance records a unique environmentalinterval in the Monterey Formation. We suggest thatstratified-water conditions may have persisted for longdurations, or that El Nino events, and the subsequent de-velopment of oceanic fronts, occurred more frequently tocause the formation of abundant mat laminae. During thisprolonged interval of water stratification and/or frontaldevelopment, abundant mat laminae and other thick dia-tomaceous laminae were deposited, resulting in the DE-1and DE-2 zones being the most richly diatomaceous in thequarry (Celite Corporation, unpublished data; Johnson,1998). These zones are also abundant in hybrid fault/veinstructures termed intrastratal microfractured zones(Grimm and Orange, 1997; Fig. 9A). We suggest that theelevated opal content of laminated diatomites in the DE-1and DE-2 zones influenced their physical properties, per-mitting higher susceptibility to failure, and the conse-quent formation of intrastratal microfractured zones (cf.Grimm and Orange, 1997; Chang and Grimm, in press).The lateral continuity of the mat-rich horizons beyond theCelite Quarry has not yet been determined. Correlation ofmat-rich horizons in the Monterey Formation to equiva-lent horizons that may be present around the Pacific Rimcan determine whether broad-scale signatures of waterstratification and/or El Nino conditions are recorded bymat laminae in the Monterey Formation.

Origins of Chaetoceros Setae Laminae

The formation of thick, discontinuous diatomaceouslaminae containing well-preserved Chaetoceros setae isproblematic to interpret, particularly because they arecomposed of vegetative cell setae but vegetative cells are

absent. We hypothesize that for large volumes of setae tobe generated, a Chaetoceros bloom occurred. The preser-vation of setae and the absence of vegetative cells may beattributed to the relative robustness of the setae and,therefore, higher survivability of grazing and/or dissolu-tion as compared to weakly silicified vegetative cells (Bulland Kemp, 1995; Pike and Kemp, 1996). One may alsosuggest that zooplankton grazed exclusively on the vege-tative cells and discarded the setae, which then fell outfrom the surface waters, became tangled and concentrat-ed, and were rapidly sedimented (Fig. 13D; Alldredge andGotschalk, 1989; Bull and Kemp, 1995).

Near-monospecific setae laminae have been describedby Pike and Kemp (1996) in laminated Holocene sedi-ments from the Gulf of California. Bull and Kemp (1995)described both laminae and sand-sized masses of Chaeto-ceros radicans setae in laminated diatom ooze of Pleisto-cene age from the Santa Barbara Basin. These authors donot consider fecal pellet packaging of setae masses a suf-ficient mechanism of formation, and attribute the absenceof vegetative cells in setae laminae to dissolution.

Modern assemblages of concentrated Chaetoceros setaehave been collected from the gills of farm-raised salmon inSechelt Inlet, British Columbia (K. Grimm, unpublisheddata, 1997). In this setting, episodic Chaetoceros bloomscause high mortality of penned salmon, as large numbersof setae become lodged in the gills, resulting in hemor-rhaging and asphyxiation. We have observed several ‘‘fishkill’’ horizons in Monterey Formation diatomites and pres-ent this mechanism as a plausible and testable hypothe-sis. We are unaware of concentrated setae in modern sed-iments or sediment traps.

Origins of Macerated Biosilica Laminae

Macerated biosilica laminae, consisting of fragmentedfrustules from a diverse assemblage, occur within diatom-rich intervals. Most macerated biosilica laminae occurwithin moderate- to high-bimodality couplets, apparentlysubstituting for either background diatomaceous or bloomlaminae in these couplets. If macerated laminae are simi-lar in microfossil composition to high-diversity back-ground diatomaceous laminae, then similar interpreta-tions regarding ecology and oceanography can be postulat-ed, although higher relative intensity of zooplankton graz-ing must be invoked to account for extensive maceration ofdiatom frustules (Sancetta, 1989; Fig. 13E). However, itseems unlikely that zooplankton grazing alone could ac-count for large volumes of macerated biosilica observed inlaminae that are up to 7 mm thick. A possible combinationof grazing, dissolution, and physical reworking during ver-tical flux and/or lateral transport could result in thick lam-inae of macerated biosilica (Sancetta, 1989; White and Al-exandrovich, 1992). Fragmentation of frustules into small-er fragments also results in greater dissolution rates be-cause of increased surface area.

Monospecific Silicoflagellate Assemblages

Pristine Dictyocha speculum assemblages occur as thinlaminae and discrete flocs, such as those observed in sam-ples M-BP and 2–8 (Fig. 7D). In modern oceans, silicofla-gellates are a common yet minor component in living phy-


toplankton communities, and have not been observed tooccur in monospecific or monogeneric life assemblages.Grimm et al (1996) reported silicoflagellate laminae andflocs from the Santa Barbara Basin and attributed theirformation to biologically mediated aggregation and rapidsedimentation of a low-diversity phytoplankton assem-blage. As discussed by Grimm et al. (1996), these assem-blages may suggest exceptions to strictly uniformitarianecologies, where phytoplankton taxa that are nearly ubiq-uitous but in low abundance in modern environments mayhave thrived as low-diversity or bloom assemblages in therecent geological past.

CONCLUSIONS

Laminated diatomaceous sediments from the MioceneMonterey Formation at Celite Quarry in Lompoc, SantaBarbara County, California, were described from fieldstudies, hand sample observations, x-radiography, lightmicroscopy, and scanning electron microscopy. A new lam-ina-style classification scheme was developed to describevariations among couplets and individual laminae. We as-sessed biologically mediated sedimentary processes anddiatom paleoecology from our observations. The majorfindings and interpretations are summarized below:

(1) Mud-rich and diatom-rich intervals at Celite Quar-ry are attributed to allocyclic and autocyclic mechanismsthat governed the relative rate of accumulation of terrige-nous detritus and biosilica.

(2) Non-laminated intervals include massive muddybeds, bioturbated muddy beds and speckled beds. Thesebeds are sharply based, abruptly overlain by undisturbedlaminated sediments, and represent episodic events(Chang and Grimm, in press).

(3) Laminae and couplets were classified by bimodality,thickness, compositional domination, spacing, and cyclici-ty (Table 2). Our classification provides a clear foundationfor consistent description and comparison of laminatedsediments in many different settings.

(4) Detrital laminae are composed of clay, detrital siltgrains, macerated biosilica, and a diverse assemblage ofdiatom groups, including robust resting spores and coastalneritic taxa such as Actinoptychus and Arachnodiscus. Weinterpret that deposition occurred during seasons of in-creased rainfall or storm activity which delivered detritusfrom the continent and/or shelf to the seafloor. Some thick-er detrital laminae are attributable to high-dischargeevents (i.e., floods).

(5) Thin biosiliceous laminae are generally #1 mmthick. Two types were described: high diversity laminaeand monogeneric laminae. Laminae with a diverse assem-blage were produced by moderate primary productivity,which we consider as background conditions, where zoo-plankton stocks were in phase with slowly growing diatomstocks (Peinert et al., 1989). Fragmentation of frustules isattributable to zooplankton grazing. Laminae containingpristine monospecific assemblages of Coscinodiscus mar-ginatus and the silicoflagellate Dictyocha speculum are in-terpreted as biologically aggregated assemblages.

(6) Thick, continuous diatomaceous laminae containmonospecific to near-monospecific diatom assemblages.Some of these laminae were deposited from blooms, where

preservation of pristine frustules is attributed to biologi-cally mediated aggregation and efficient sedimentationthat bypassed heterotrophic grazing (i.e., self-sedimenta-tion; Grimm et al., 1997).

(7) Thick, discontinuous diatomaceous laminae aresubdivided into Thalassiothrix longissima mat laminaeand Chaetoceros setae laminae. T. longissima mat laminaerecord the presence of high nutrient levels, stratified wa-ter conditions and/or oceanic front development. Theselaminae may be analogous to thick mat laminae discov-ered in oxygenated waters of the equatorial Pacific (Kempand Baldauf, 1993) and North Atlantic (Boden and Back-man, 1996). Like modern rhizosolenid mats, T. longissimamats may have photosynthesized in surface waters andmigrated to deeper waters to take up nitrate. The matsrapidly sank when nutrients were exhausted and/or whenthe stratified water column deteriorated. The abundanceof mat laminae within the opal-rich DE-1 and DE-2 zonesin Celite Quarry may record prolonged periods of strati-fied-water conditions, or frequent El Nino conditions andconsequent oceanic front development. Chaetoceros setaelaminae are dominated by unfragmented setae derivedfrom vegetative cells; these laminae are problematic. Un-derstanding the sedimentology of setae laminae requiresfurther study of modern-day Chaetoceros ecology.

(8) Macerated biosilica laminae range in thicknessfrom 1–7 mm, occur in diatom-dominated intervals, andare composed of highly fragmented biosilica attributableto relatively high intensity of zooplankton grazing, disso-lution, and/or physical reworking of diatom frustules.

(9) Our data are consistent with an interpretation thatthe diatom/detritus couplets in Monterey Formation diat-omites are probably seasonal rhythmites and most likelytrue varves (Anderson, 1986). We base this conclusion onsedimentary and paleontological similarities to youngerdiatomaceous laminites from DSDP and ODP samples,and on the close similarity of many diatom laminae to sub-annual and subseasonal sedimentary events in modernenvironments (Soutar et al., 1981; Baumgartner et al.,1985; Pike and Kemp, 1996; Grimm et al., 1996).

The specific paleoenvironmental reconstructions inter-preted for different lamina types provide a foundation fora detailed time-series of paleoceanographic conditionsthrough time in Monterey Formation diatomites and othercorrelative units in the Pacific Rim (cf. Ingle, 1981; Whiteet al., 1992). Future microstratigraphic studies that em-ploy lamina-scale geochemical assessment should furtherrefine interpretations of paleoenvironment and deposi-tional settings. Such comprehensive, high-resolution stud-ies are prerequisite to linking rates and trajectories of pastenvironmental change, environmental and ecological vari-ability of modern environments, and probabilistic fore-casts of future change.

ACKNOWLEDGMENTS

Thanks go to Max Taylor, Rick Behl, and John Barronand for helpful discussions. Constance Sancetta, Tom Al-geo, and an anonymous reviewer provided critical reviewsof earlier drafts of the manuscript. We are grateful to TonySumner and Eric Morlan from the Celite Quarry for theircourtesy and generosity. This research was supported bygrants from the National Science and Engineering Re-

454 CHANG ET AL.

search Council (Canada), ARCO, and Exxon ProductionResearch. This is a contribution to IGCP 374: Paleoclima-tology and Paleoceanography from Laminated Sediments.

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ACCEPTED FEBRUARY 26, 1998

456 CHANG ET AL.

APPENDIX 1Diatom taxa and abundances derived from a strewn slide sample of a detrital lamina in Sample 2-8 (Fig. 5A, arrow 1).

Abundance Paleoecological Interpretations

I. Diatom floraActinoptychus

A. splendensA. undulatus

benthicsChaetoceros

C. cinctumC. debilisC. diademaC. radicansC. vanheurkerii

Coscinodiscus marginatusEthmodiscusHemiaulusHemidiscus cuniformisNitzschiaRhaphoneisRouxia californicaStephanopyxis turrisThalassiothrix longissima

RCR

FRRRCAFRRFFRCF

coastal neritic diatom washed off from shelf (1)

e.g. Arachnodiscus and Navicula; reworked from shelfresting spore survival strategy

ubiquitous and robust; dissolution assemblagebackground primary productivitybackground primary productivitybackground primary productivitybackground primary productivitybackground primary productivityfragments reworked from previously deposited sedimentsresting sporefragments reworked from previously deposited sediments

II. Other microfossilsSilicoflagellates F ubiquitous

III. PreservationLightly silicifiedHeavily silicified

PM–G

poor preservation due to grazing and dissolutionrobust and dissolution-resistant (1)

Abundance: A 5 abundant; C 5 common; F 5 few; R 5 rare.Preservation: G 5 good; M 5 moderate; P 5 poor.Reference: (1) Pike and Kemp, 1996.


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