Modern Sediments and Facies Model for a Microtidal Coastal ...

M O D E R N S E D I M E N T S A N D F A C I E S M O D E L F O R A M I C R O T I D A L C O A S T A L P L A I N E S T U A R Y , T H E J A M E S E S T U A R Y , V I R G I N I A l

M A Y N A R D M. N I C H O L S School of Marine Science, Virginia Institute of Marine Science

College of William and Mary Gloucester Point, Virgqnia 23062

G E R A L D H. J O H N S O N Department of Geology

College of William and Mary Williamsburg, Virginia 23185

AND

P A M E L A C. PEEBLES Virginia Department of Transportation

Colonial Heights, Virginia 23834

AaSTRACr: Modern sediments of the James River estuary have been studied to characterize the lithofacies and to relate the facies pattern to estuary morphology and the energy regime. The estuary was formed by Holocene drowning of a river valley incised in Cenozoic coastal plain deposits. Morphologic analysis of sinuosity, width-depth ratios, and convergence characteristics reveals three compartments: 1) bay-mouth, 2) estuary funnel and 3) meander zone. Each compartment exhibits a characteristic lithofacies reflecting different proportions of wave, tidal and fluvial energy. These lithofacies form a longitudinal tripartite pattern, i.e., sand-mud--sand, with coarse-grained sediment at the energetic ends of the system. The seaward facies boundary is transitional as a result o f mixing fluvial and marine sediment. In contrast, the landward boundary is abrupt as a result of a rapid seaward decrease in the river flood bedload which is partly attenuated by the tide. The tripartite facies develop in a transgressive wedge filling the path o f the pre- Holocene fluvial drainage. A model is proposed for recognizing the sequence of ancient transgressive estuarine facies. The sequence fines upward except at the mouth and reflects the seaward evolution of environments from fluvial to fluvial estuarine, estuarine and marine estuarine.

INTRODUCTION

Estuarine facies are affected by variable energy conditions and multiple sediment sources. In microtidal systems, tidal signatures are often obliterated by bioturbation, and the sediments are mixed and reworked by river floods and storm action. Consequently, estuarine facies are commonly complex and difficult to define and rec- ognize (Frey and Howard 1986). Only a few facies models are available from which to generalize and to compare facies relationships. These are mainly for macrotidal and mesotidal systems in temperate zones, e.g., the Gironde estuary (Allen et al. 1973; Allen and Truilhe 1988); Wil- lapa Bay (Clifton 1982, 1983); Cobequid Bay-Salmon River (Dalrymple et al. 1990); and the Georgia coast estuaries (Howard and Frey 1985). Facies models of microtidal systems are limited except for the Delaware estuary (Knebel et al. 1988).

This paper delineates and characterizes the modern lithofacies of a coastal plain estuary from morphologic and lithologic features. It relates the resultant facies pattern to the energy regime and synthesizes the results in a facies model.

DATA SOURCES

Facies delineation in the James estuary is facilitated by a substantial background of hydrodynamic and sedimentologic data. Hydrodynamics of the estuary are known from numerous field surveys (e.g., Pritchard 1952; Shidler

Manuscript received 6 March 1990; revised 6 January 1o91.

and Maclntyre 1967; Fang et al. 1973); from a 180-m- long hydraulic model (Nichols 1972a); and from numer- ical models (Fang et al. 1973; Officer and Nichols 1980; Cerco 1982). The gross lithology of surface sediments is known from more than 300 grab and core samples (Mort- cure and Nichols 1968; Nichols 1972b; Shidler 1975; Trotman and Nichols 1978; Lunsford et at. 1980, 1982; Cutshall et al. 1981; Nichols and Cutshall 1981; Byrne et al. 1982; Brush and Fleischer 1985; Schaffner et al. 1987; Nichols 1990). Minor structures are known from peels of 14 can cores and x-ray radiographs of 2 to 6-cm- thick slabs from 45 box cores (Cutshall et al. 1981; Schaff- ner et al. 1987) and from side-scan sonargrams (Wright et al. 1987). Sediment pathways and sinks have been traced by Kepone contamination (Nichols 1990) and by clay mineralogy (Feuillet and Fleischer 1980). Strati- graphic and lithologic data on the Holocene fill come from boring records ofbridge, tunnel, and power line crossings of the Virginia Department of Transportation, the Vir- ginia Electric and Power Company, and the U.S. Army Corps of Engineers. These are augmented by seismic reflection profiles, including those of DeAlteris (1988) and Colman et al. (1988) in the Chesapeake Bay mouth. All longitudinal distances herein are referred to 0 (km) landward or seaward of Hampton Roads mouth.

COASTAL SETTING

The James River estuary is one of many drowned river valley tributaries to Chesapeake Bay that form a dendritic system of waterways along the U.S. mid-Atlantic coast (Fig. 1). Like other major estuaries in the region, as well as their Pleistocene analogues, the James has a fluvial

JOU~_NAL OF SvJ>~i~rr~atv PFrROLOGV, VOL. 61, NO. 6, NOV., 1991, P. 883-899 Copyright © 1991, SEPM (Society for Sedimentary Geology) 0022-4472/91/0061-883/$03.00

884 M A Y N A R D M. NICHOLS, GEtL4LD H. JOHNSON, AND PAMELA C. P E E B L E S

FIG. ! .--Location of the James River estuary in Chesapeake Bay region and its setting on the Atlantic Coastal Plain, in relation to the Fall Zone and Piedmont physiographic provinces. Drainage basin, ha- chured; James estuary proper and Chesapeake Bay mouth, opaque.

source to the west, and it leads into a major bay or sound that is connected to the ocean by a passage through a barrier. The study area extends to the ocean shore to include the ancestral course of the James and the full spectrum oflithotopes. The Fall Zone separates the coastal plain and Piedmont provinces (Fig. 1). This zone is marked by a steep gradient where the rivers drop 4 m/km to the head of normal tide in the estuary. Conse- quently, under normal conditions of river discharge the change from fluvial to estuarine tidal conditions is relatively abrupt.

The estuary experiences a humid sub-tropical to temperate climate with winter and spring wet seasons. It receives most of its fluvial sediment load, an estimated 2.4 × 106 tons annually, in pulses during the wet season, January to April. An estimated 90% of the suspended sediment load, mainly silt and clay, is delivered to the estuary in only 11% of the time. Table 1 summarizes the physical and hydrologic characteristics of the estuary.

MORPHOLOGY

The morphologic framework of the estuary is deter- mined by a valley system inherited from erosion of coastal plain deposits of Pleistocene and Tertiary age. Although the stratigraphic units vary in thickness, each unit consists

TABLE 1 .--Physical and hydrodynamic characteristics of the James River estuary

Fluvial and estuafine drainage area 26,360 km 2 Surface area of estuary 611 km 2 Precipitation 1079 mm/yr Length 161 krn Width, average 5.1 km Depth, average 5.8 m Depth/width ratio 0.0011 Volumetric capacity, MLW 2.5 km 3 Freshwater inflow, average, Richmond 213 mVs

Low inflow 28 m3/s High inflow 322 mVs Flood > 1500 m3/s

Tidal prism 0.28 x 109 m 3 Tide range, average 70 cm Flow ratio, average ~ 0.10

Low inflow 0.03 Mean hydraulic residence time 2 219 days

High inflow 5 days Mean freshwater fraction residence time 138 days Suspended sediment load in

turbidity maximum 100-270 mg/l

Proportion of freshwater entering during a tidal cycle to the tidal prism.

Volumetric capacity divided by average river inflow.

of coarse sandy, gravelly basal deposits which fine upward to silt and clay. The surficial stratigraphic units underlie relatively flat terraces separated by scarps which decrease in elevation seaward and toward the estuary (Johnson et at. 1980; Peebles et al. 1984). Along the estuary funnel, Tertiary sediments crop out in bluffs, while both Tertiary and Cretaceous are exposed in bluffs along the meander zone. However, none of the older units crop out in the bay mouth.

From the ocean entrance of Chesapeake Bay to the Fall Zone at Richmond, the morphology varies from that of a bay-mouth shoal complex to that o f a meandering tidal river (Fig. 2). Three morphologic compartments are rec- ognized: 1) bay-mouth, 2) estuary funnel, and 3) meander z o n e .

The bay-mouth zone, extending 26 km seaward from the James estuary mouth (0 km) to the ocean, is a near- marine zone with a relatively straight axial channel, Thimble Shoals Channel, bordered by wide banks and shoals, < 10 m deep (Fig. 2). Bay-mouth morphology is broadly shaped by interdigitate ebb and flood tidal channels alternating with shoals having prominent sand waves 2 to 4 m high on their flanks (Ludwick 1975). Whereas the waves face both ebb and flood directions (north- westerly and southeasterly), the deposits exhibit an overall southward progradation (Colman and Hobbs 1987).

The estuary funnel extending from Hampton Roads landward to Jordan Point reflects drowning of a river valley incised in late Cenozoic fluvial-estuarine deposits. Meanders are broad, the axial channel is sinuous, and the shoals, which likely are drowned flood plains, are wide. Moreover, margins are indented by branching tributary creeks that lead into modern fluvial streams. Shores of the funnel are fnnged by bluffs (5 to 18 m high) inter- rupted by cliff-faced marshes (~ 1 m high), narrow beach- es, and a few sand spits. The shore is eroding at an average

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rate of 0.4 m/yr, and thus the open water area of the funnel is enlarging with time. Despite shore modifications and the drowned configuration, morphology of the funnel is approximately adjusted to the tidal dynamics. This is indicated by the longitudinal trends of dimensionless width, cross-sectional area and tidal prism as a function of dimensionless distance that conform to an exponential rate with minimal deviation (Table 2). Of note, the width convergence coefficient (~w) is within the range of coef- ficients described for meso- and macrotidal estuary channels by Wright et al. (1973) and Kostaschuk (1985). Chan- nel morphology and tidal dynamics likely co-adjust through sedimentation on the channel floor.

The meander zone, between Jordan Point and the Fall Zone, is characterized by pronounced meanders including artificial cutoffs, abandoned meander loops, and an ox- bow. In contrast to the estuary funnel, the sinuosity ratio and convergence coefficient (ra) are relatively high: 1.73 and 0.07, respectively (Table 2, Fig. 2). Seaward parts of the channel are flanked by marshes, a swampy floodplain, and narrow tidal flats. Additionally, necks of the incised meanders are capped by Holocene swamp and point bar deposits. Drowning of the Pleistocene valley however, has not been sufficient to submerge entire meanders, the tributaries, or the floodplains significantly.

TIDAL PROCESSES AND CIRCULATION

The tide range is relatively low and varies within narrow limits from 85 cm at Chesapeake Bay mouth to 54

TABLE 2 . - Channel morphology and tidal discharge parameters for the meandering zone and estuary funnel. For distribution of cross-sec-

tional areas and tidal prism, see Figure 2A and B

M e a n d e r i n g Z o n e F_stuary F u n n e l

Convergence coefficient' (~w) Correlation coefficient (r)

Cross-sectional 2 area

Convergence coefficient (~a) Correlation coefficient (r)

Tidal prism ~ coefficient OP) Correlation coefficient (r)

-9.96 -2.93 -0.85 -0.86

-0.07 -0 .03 -0.97 -0 .96

-0 .10 -0 .03 -0.89 -0 .87

' From the relation: Wx/Wo = e -~ ' '~ (Wright et al. 1973), where ~w is the horizontal convergence coefficient, Wx is the width at distance X landward of the mouth, l is the landward distance and wo is the width at the mouth.

2 From the relation: Ax/Ao = e -~(~> (Wright et al. 1973) where ~a is the horizontal convergence coefficient, Ax is the cross-sectional area at distance X landward of the mouth, 1 is the landward distance and Ao is the area at the mouth.

3 From the relation: Px/Po = e -~j'~ (Wright et al. 1973) where 6p is the tidal prism coefficient, Px is the tidal prism in a section x landward of the mouth, l is the landward distance and Po is the tidal prism at the seawardmost section near the mouth.

cm in the estuary funnel and 97 cm at Richmond (Fig. 3A). The tidal characteristics, summarized in Figures 2B and 3A, reflect progressive wave tendencies in the seaward part and both progressive and standing wave tendencies in the landward part.

Average peak strength of ebb and flood currents varies between 38 to 59 cm/s in the funnel and mouth zones and diminishes to less than 19 cm/s in the meander zone (Fig. 3B). These variations, which reflect landward mod- ification of the tide wave, are accompanied by a change in time-velocity asymmetry, i.e., from nearly symmetrical in seaward areas (e.g., 31 km) to asymmetrical in landward areas (e.g., 111 and 140 km) (Fig. 3D). At 158 km landward of the estuary mouth, the flood is overcome by river flow, and ebb dominates throughout the tidal cycle. As a consequence, sediment movement is seaward through the meander zone. Besides the tidal currents, river floods produce current strengths greater than 280 cm/s in the meander zone, while in the bay mouth meteorologically induced flows superimposed on tidal currents produce speeds up to 100 cm/s (Chesapeake Bay Institute 1953).

The James River estuary is a classic (type B) coastal plain estuary. From extensive field data, Pritchard (1952, 1954) tested the relationship between the net non-tidal circulation and the salt flux. Freshened water flows seaward through the upper layer, while more salty water flows landward through the lower layer. Near-bottom net current, therefore, changes direction in the funnel zone (Fig. 3C). Consequently, the landward flow can transport a marine-derived suspended load landward through the estuary funnel to the inner limit of salty water (Nichols and Biggs 1985). The landward limit of salty water shifts 63 km along the estuary in response to seasonal changes of river inflow (Fig. 3C). Across the estuary, salinity decreases toward the southwest shore in response to Coriolis effects on the density flow. Consequently, the level of no- net-motion separating the upper and lower layers varies

886 AIAYPv~IRD M. NICHOLS , G E R A L D H. JOHNSON, AND P A M E L A C. P E E B L E S

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depth across the estuary and is deeper on the southwest side than on the northeast side.

FACIES CHARACTERIZATION AND DELINEATION

T e x t u r e

Surficial texture changes with distance landward from coarse to fine and then coarse. Clean sand is abundant in the bay-mouth zone, with fine well-sorted sand on shoals and medium to coarse sand and gravel patches in channels and along the south margin (Fig. 4B). Sand abundance, i.e., greater than 75% of the total sample, extends landward 15 km from the bay-mouth boundary into the estuary funnel at Hampton Roads. Within the estuary funnel, sand is common along margins, where it is supplied to the estuary mainly by shoreline wave erosion of Pleis- tocene deposits. In the meander zone, sand (interspersed with gravel and admixtures of silt and clay) is abundant on the channel floor where it is supplied by river floods and by current scour of Pleistocene and Tertiary deposits on the floor and banks.

Clay is abundant in the main axial channel of the es-

tuary funnel and in quiescent embayments and tributary creek mouths (Fig. 4C). The broad pattern is parallel, or subparallel, to the channel isobaths. In Hampton Roads, a clay zone, i.e., clay greater than 50% of total sample, extends seaward along the southeast side of the channel and on adjacent shoals. In the meander zone, clay is limited to small patches in abandoned meander loops, swamps and tidal fiats. Although the percentages of sand, silt, and clay vary greatly throughout the estuary, samples from the funnel zone contain higher proportions of clay than silt (approximately 2:1) than do those of the meander zone (approximately 1:2) (Fig. 4A).

A scatter diagram of mean grain size (Fig. 4A') reveals decreasing size from the bay mouth into the funnel as well as through the meander zone into the funnel. Mean size is minimal in the central funnel zone. Standard deviation values assume a similar trend, with very poor sorting in the funnel.

In terms of sand : mud ratios along the axial channel, high ratios (greater than 70%) occur at opposite ends of the system, the bay-mouth, and meander zones (Fig. 4D). The textural change between the mouth and funnel zones is relatively gradual, whereas between meander and funnel zones, it is steep (Fig. 4D). The first transition, which occurs in a zone where tidal velocity (Fig. 3B) and wave action diminish landward, is accompanied by a landward gradational change in size sorting, from well-sorted fine sand to poorly-sorted silty clay. These trends, interpreted with the aid of cumulative frequency curves (Nichols 1972b), represent a transition between coarse and fine sediment whereby two log normal end-member popula- tions, unimodal sand and bimodal silty clay, are mixed and deposited in the presence of a weak or partially ef- fective sorting agent. Sand with an intermediate size mode is likely moved landward as bedload along the channel, whereas the silty clay is likely deposited from the tidal and fluvial suspended load, mainly from river floods. The sand : mud ratios thus gradually diminish landward between the bay-mouth and funnel zones.

In contrast, the second transition of sand : mud ratios, between the funnel and meander zones, occurs where discharge of river floods rapidly diminishes seaward through enlarged cross sections, i.e., as it passes from the confines of the partially channelized meander zone to the broader funnel zone (Fig. 2A). Transport competency of near-bottom flow is also reduced by flood tidal currents, which attenuate the river current. Through the zone from 140 km to 111 km flood tide becomes more prevalent and time-velocity curves more symmetrical (Fig. 3D). As a result, the fluvial load (which consists of a mixture of sand and gravel bedload and fines in suspension introduced mainly during floods) is partly fractionated and "fined downstream", since the coarse material is unable to move far. Sand : mud ratios, therefore, drop within a short distance seaward.

From the change of sand : mud ratios along the channel, textural facies are differentiated by specifying a boundary at a sand : mud ratio of 70% (Fig. 4D). The resultant facies exhibit a threefold zonation or tripartite arrangement: sand-mud-sand. These facies boundaries approximately


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FIG. 4. --A) Distribution of sand, silt and clay pereenmges for different lithofacies based on the classification of Shepard (1954); sand in meander zone not plotted. A') Scatter diagram of mean grain size and standard deviation based on relations of Inman (1952). B) Distribution of sand abundance greater than 75% of the total sample. C) Distribution of clay abundance greater than 50%, dense oyster shell and marsh, paludal sediments. Lines A-A' and B-B' indicate locations of lithologic sections (Figs. 5, ] 0). D) Variation of sand:mud ratio with distance landward along the estuary. Zones where sand-mud ratios exceed 70% are close to the boundaries of morphologic zones, i.e., bay-mouth, estuary funnel and meander zones (Fig. 2).

888 MA YNARD M. NICHOLS, GERALD H. JOHNSON, AND PAMELA C. PEEBLES

A A'sw NE A

d

E

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FIG. 5.--A) Cross-estuary variations of sand-mud ratio and sediment accumulation-scour rates in the estuary funnel, section A-A' (Fig. 4C). B) Corresponding bathymetric variations, and marginal sub-facies, marsh and shoal, as well as channel. Level of no-net-motion at different river inflow stages: 1) ~ 120 mVs (Pritchard 1952), 2) ~ 30 m3/s (Wood and Hargis 1971). View landward.

correspond to the boundaries of the morphological compartments (Fig. 2).

Several component subfacies on margins of the channel give the composite estuarine sequence additional character. These are defined by lateral morphological, textural, and biogenic variations with depth and distance across the estuary funnel. Farthest landward is the marsh subfacies, consisting of salt marsh and paludal sediments that occupy the upper intertidal range of creek banks and low- lying antecedant point bars (Fig. 4C). The marsh deposits mainly consist of organic-rich mud but also contain ap- preciable quantities of sand near estuary shores. Intertidal fiats are of limited extent; however, a few narrow deposits of silty clay or sandy clay occur along banks and point bars in the meander zone. The shoal subfacies, extending from the intertidal zone to about the 6 m depth, has relatively high sand : mud ratios (greater than 70%) and low accumulation rates where depths are less than about 2 m (Fig. 5). Across the shoals, sand passes channelward into sandy clay, or admixtures of sand-silt--clay. Oyster biostromes reach 3 m in thickness (DeAlteris 1988) and extend 37 km through the estuary funnel where bottom salinity ranges between 5 and 20%0. The biostromes rise 0.5 to 1.5 m above the broad shoal surface, thus contrib- uting shell fragments and variable textural patterns.

The channel subfacies at water depths between 6 and 12 m consists of mud on the southwest side and sandy sediment on the northeast side (Fig. 5A). The textural asymmetry develops because the net non-tidal transport changes across the channel and the channel receives sediment from contrasting sources. Marine input is indicated by marine foraminifera and by coal contaminants from seaward sources (Nichols 1972b). Fluvial input is indicated by Kepone contamination (Nichols 1990) and by clay mineralogy of river sediment (Feuillet and Fleischer 1980). Near-bottom current measurements (Pritchard 1952, 1954; Nichols 1972a) reveal that net non-tidal den-

sity flow is directed landward and at a greater strength along the northeast side than along the southwest side. Rapid mud accumulation on the southwest side is asso- ciated with the lateral intersection of the level of no-net- motion and the bottom (Fig. 5B). In this zone, the net density flow approaches zero and thus encourages sediment entrapment. Similar asymmetrical textural and accumulation patterns also occur at the bay-mouth facies boundary (Fig. 4B, C).

Sediment Accumulat ion and Budget

Sediment accumulates in the various lithotopes at different rates and in diverse patterns. In the bay-mouth sand facies, rates derived from historic bathymetnc changes range from >~ 1.0 cm/yr of fill to ,~ 0.5 cm/yr of scour with an overall average net accumulation of approximately 0.7 × 10 -2 tons/m2/yr (Byrne et al. 1982). According to Colman et al. (1988), the pattern exhibits alternating erosion and deposition, suggesting migration of shoals and channels southwestward across the mouth. Progradation is indicated by seismic-reflection profiles showing a wedge-shaped package of beds 5 to 35 m thick dipping southward and bayward (< 2*). Such a trend implies a sediment source from the northeast along the coast (Colman et al. 1988).

Accumulation rates in the funnel mud facies, which were derived from historic bathymetric changes, reach approximately 11 cm/yr or 4.4 x 10 -2 tons/mZ/yr in Burwell Bay (30-38 km). This depocenter is close to the inner salt limit during high river inflow (e.g., > 1500 m3/s at Richmond), a time when the bulk of the fluvial sediment is delivered. Rapid accumulation is favored by entrapment of high concentrations of fine suspended material in the net non-tidal current null zone and by diminished tidal current strength (Fig. 3B, C). Generally, rates are higher in less energetic zones, e.g., the main channel, basins, tributary creek mouths and reentrants, than on shoals and more energetic zones of the channel. The resultant pattern is extremely heterogeneous (Nichols 1990). In the meander zone, sand facies accumulation rates, which are estimated from U.S. Army Corps of En- gineers' maintenance dredging records, are approximately 1.4 cm/yr or about 1.1 x 10 -2 tons/m2/yr of fill on the average. The fill, which is localized on channel shoals, is probably offset by scour, because historic bathyrnetry changes in non-dredged areas show little net change.

When the net mass accumulation rates are compared with sediment input rates, an approximate mass balance is derived (Table 3). Fluvial input is derived from an average of daily suspended sediment discharge measurements by the U.S. Geological Survey at Scottsville, Vir- ginia and Carterville, Virginia for nine years (U.S. Geo- logical Survey 1951-56, 1977-81). These data are extrapolated to the estuary funnel head by area-weighting the sediment yield of the upper basin over the lower drainage basin. Input of fine sediment from shores and banks is derived from net erosion rates over about 100 years (Byrne and Anderson 1977), assuming 70% of the

ESTUARINE SEDIMENTS 889

sediment is silt and clay. Net exchange of suspended sediment through the estuary mouth via the lower estuarine layer is derived from box model estimates of suspended sediment flux at moderate and high river inflow levels (207 and 1082 m3/s, respectively, at Richmond; Officer and Nichols 1980). Summing up, the total input of fine sediment amounts to an estimated 3.2 to 3.4 x 106 tons/ yr (Table 3).

Accumulation in marshes and swamps is derived from radiocarbon chronology of basal peat deposits in the middle estuary (Ellison and Nichols 1976). For the estuary floor, accumulation rates are derived from historic bathymetric changes and from the depth of Kepone contamination in more than 100 cores taken between 1966 and 1978 (Nichols 1990). Sediment volume is converted to mass from measurements of average dry weight sediment density of cores. The resultant accumulation rates from bathymetric changes range 1.5 to 1.8 x 106 tons/yr, while from Kepone contamination they average 2.6 x l06 tons/ yr. Additionally, maintenance dredging records of the U.S. Army Corps of Engineers between 1965 and 1981 indicate that 0.4 x 106 tons/yr accumulate in the shipping channels.

The apparent disparity whereby input exceeds accumulation by 0.3 to 1.4 x 106 tons/yr can be caused by escape of fluvial input through the mouth during river floods, a flux not accounted for in the box model exchange rates. Nonetheless, the budget reveals the relative im- portance of source terms with a dominance of fluvial input over shore and marine influx. It is estimated that 45 to 92% of the fluvial input is retained within the estuary funnel zone (Nichols 1990). Accumulation amounts to an average annual layer about 3.8 m m thick, which is less than the total submergence, 4.3 mm/yr, derived from 40 to 60 years of tide gage measurements at Hampton Roads (Gornitz and Lebedeff 1987).

Sedimentary Structures

Individual sedimentary structures occur in more than one lithofacies (Table 4), so that a single structure is not a valid criterion for discriminating lithofacies. Instead, assemblages of structures found within a lithofacies are deemed characteristic and therefore constitute the useful discriminating criteria.

The strong energy of tidal currents and storm waves in the bay mouth results in extensive surface bedforms, including multidirectional small-scale ripples, sand waves and large scale dunes (e.g., Fig. 6A). Beneath the surface (0 to 40 cm), however, stratification is largely destroyed by biogenic reworking with only a few well-maintained biogenic structures and crossbeds preserved. The deposits are mainly homogeneous and massive sand. Erosion-deposition surfaces are present locally, and shell lag accumulates on the channel floor.

In the funnel zone where energy conditions are relatively low but diverse, laminated mud bedding prevails. It is common in the freshened upper sector (74 to 113 km landward) and in the middle sector (37 to 57 kin),

TABLE 3 . - - Comparison of fine sediment input and accumulation rates in the estuary funnel zone, metric tons per year

Mass/Year

Sources

J a m e s R i v e r 2.4 x 106 tons Shore e ros ion 0.5 x 106 tons M o u t h 0.27--.0.5 x 106 tons

To ta l i n p u t 3 . 2 -3 .4 x 106 tons

A c c u m u l a t i o n

M a r s h a n d s w a m p 0.1 x l06 tons Estuary floor (natural channels) 1.5-2.6 x 10 6 tons Shipping channel 0.4 x 10 6 tons Total accumulation 2.0-3.1 x 10 6 tons

Difference, source input minus accumulation 0.3-1.4 x l0 6 tons

particularly at sites where the sediment accumulation rates are relatively fast, > 3 cm/yr (Schaffner et al. 1987). The laminated beds, which are often silty or sandy and exhibit wavy or lenticular laminations, alternate with thick mud layers that are often bioturbated irregular or homogeneous layers (Fig. 7). The change indicates rapid short- term accumulation such as fiver floods or storm events followed by slow long-term accumulation under tidal in- fluences in which biogenic activity exceeds physical accumulation and reworking. Episodic events are evidenced by truncated biogenic reworked beds and erosional contacts overlain by interlaminated bundles of silt or sand. Disrupted mud clasts occur locally. In the funnel zone (0 to 35 km), accumulation is relatively slow, < 1 cm/yr; structures are irregular layered, mottled or homogeneous; and biogenic reworking prevails. Shells are common inclusions as fragments or aggregates, with some individuals buried in the living position (Fig. 8). Sand laminae are discontinuous, cross-laminated, and exhibit initial ripple building from a traction load. The shoal subfacies of the funnel zone (10 to 45 km) is marked by a broad range of composite structures. Irregular and mottled beds are common, with thick mud and thin sandy beds. Post-depositional alterations of interlaminations are common, with physical structures disrupted by burrows. Microcross- laminations and incipient ripples occur.

River flooding in the meander zone channel results in numerous surface bedforms, including small-scale ripples and sand waves (Fig. 6B), besides local scour troughs and bank slumps. The channel fill is mainly coarse sand and gravel with discontinuous silt interbeds of variable thickness. Whereas the coarse material is introduced inter- mittently during floods, the fine material is likely deposited from suspension during low river stage when tidal flow prevails. However, the fine-grained deposits may be eroded by subsequent floods except locally where com- pacted. The gravel beds have erosional contacts at the base and are crudely layered and weakly graded. The sand and gravelly sand commonly exhibit indistinct large-scale cross-bedding with trough and planar sets; laminations are often enhanced by quartz pebbles or sub-rounded wood fragments. Clay pebbles and disrupted mud clasts

890 M A Y N A R D M. NICHOLS, GERALD H. JOHNSON, AND PAMELA C. P E E B L E S

TABLE 4 . - Occurrence of sedimentary structures in principal lithofacies of the James estuary channel. Tabulated from side-scan sonar imagery and x-ray radiographs or lacquer peels of box cores

RI o

t~

.J

z

I,U P- Z D

STRUCTURE TYPE

Smooth

Current Rippled

Wave Rippled

Dunes

Slumps

Scour Troughs

Massive

Parallel to Subparallel Laminations

Lenticular Laminations

Wavy Laminations

Distorted Layering and Laminations

Cross Laminations, Planar

Cross Laminations, Trough

Scour and Fill

Discontinuities and Truncation

Clay Inclusions or Clasts

Shell Fragments

PlanUWood Debris

Tree Stumps

Tubes

Burrows

Shell Lag

Shell Aggregations

MEANDER ZONE SAND

O

LITHOFACIES ESTUARY

FUNNEL MUD

O

O

O

O

O

BAY MOUTH

SAND

O

O

Gas/Water Fissures O O

Bioturbation < 10% < 10% to 99% 70 to 99%

Abundant Common Rate O Absent or not observed

are also observed. Marginal shoals and intertidal flats have interbedded sand, silt, or clay layers with several varieties of sandy cross-laminations, e.g., discontinuous wavy and lenticular bedding. Also observed are scattered clay and wood inclusions, and erosional contacts reflect-

ing variable current activity. The beds are commonly disrupted by burrows and biogenic activity.

When the broad distribution of structures is compared through the range of lithofacies, it is evident that the structures are most complex and variable in the meander


A B A Y - M O U T H

SW 18

A

~E

20 I - O.. w 22 ¢:3

24

C H A N N E L

LANDWARD NE

I I I I I I

0 20 40 60 80 lOOm

B M E A N D E R Z O N E C H A N N E L SEAWARD

5 0

E

0 t i i

0 50 lOOm FIO. 6.--A) Fathogram of sand dunes from the bay-mouth sand facies in the main channel near the estuary funnel entrance off Old Point

Comfort ( - 2 km) (Nichols 1972b). Dunes are displayed in a near vertical profile with ~ 1.0 m amplitude, 15 m wave length; many display landward (flood, NE to SW) asymmetry in central parts. The dunes form on a medium to coarse sand bed and are largely straight-crested in plan view. B) Side-scan sonar image of sinuous-crested sand waves in the meander zone channel (142 km landward). Amplitude ranges ~ 0.3 to 0.7 m, wave length ~ 10 to 20 m. The waves form on a fine to coarse sand bed at 9.0 to 11.0 m water depth and exhibit seaward (ebb, NW to SE) asymmetry.

zone sand facies (Table 4). Changes of energy and sediment supply in this facies are greatest and the resultant deposition is discontinuous. In contrast, bedding and laminations are best developed and preserved in the funnel mud facies. In this facies, total energy is lower than elsewhere, energy fluctuations have diminished, and the

rate of sediment accumulation is relatively high. Al- though conditions in this zone are favorable for organ- isms, they are less able to rework the sediment because accumulation is fast (Schaffner et al. 1987). Therefore, the prospect of preserving laminations is better here than elsewhere. In the bay-mouth sand facies, laminations are

892 / v IAYNARD M. N I C H O L S , G E R A L D H. JOHNSOIV; AND P A M E L A C. P E E B L E S

0

5

CM

I0

15

20

- 2 5

" 5 0

FIG. 7.--X-ray radiograph of box core 2-cm-thick slab from the estuary funnel m u d facies (48 km landward) displaying laminated mud with bundles of silty sand laminae (light-toned) fining upward between 18 to 23 cm depth. This sequence is likely caused by rapid sedimentation of Hurricane Camille flooding, 1969. It overlies a muddy biogenic reworked zone (B) having truncated erosion surface (C), a zone repre- senting relatively slow long-term accumulation rates. Additionally, the m u d layer at 13 to 18 cm depth is likely a product o f slow, year to year, accumulation with interbedded discontinuous sandy laminae caused by occasional floods, freshets or storms. Another erosion surface (A) occurs at 8 cm reflecting a high energy event, possibly Tropical Storm Agnes, 1972. Radiograph from Schaffner et al. (1987).

5

CM

I0

15

2 0

2 5

3 0

3 5

FIG. 8.--X-ray radiograph of box core from the estuary funnel m u d facies (68 kin) displaying laminated mud with layer of Rangia clams at 20 cm depth buried in life position. Mortality is likely caused by rapid sedimentation of Hurricane Camille flooding, 1969. Also displayed is a bioturbated layer between 20 to 35 cm depth, bundles of silty sand displayed in an upward fining sequence at 14 to 20 cm depth, indistinct erosion surface E, with disarticulate Rangia shells, R (likely caused by Tropical Storm Agnes, 1972) at 8 cm depth and discominuous laminae of silty sand in upper 8 cm.

scarce due to bioturbation, despite high energy and substantial accumulation.

ENERGY DISTRIBUTION

The principal sources of energy that affect facies development are the river inflow, tides, and surface waves. The relative intensity of different energy components, however, varies with location and water depth. To relate the tripartite facies pattern to the energy distribution, the different types of energy are incorporated into a common parameter, energy density, E (Komar 1976; Wright et al. 1980). This is the total specific energy including potential and kinetic energy, expressed as: E = p v 2 where p is the water density and v is the velocity near the bed. Fluvial energy density is derived from high river discharge, 1500 mVs at Richmond, a level reached about five times a year. Discharges of this magnitude transport the bulk of

the fluvial sediment load into the estuary. Wave energy density is derived for storm waves: 1) at the bay mouth ( - 2 6 km seaward) based on orbital velocity estimated from linear wave theory (Ludwick 1975); 2) on-site wave gage and current measurements of peak values attained about five times annually at Thimble Shoals ( - 1 2 kin) (Boon et at. 1990); and 3) orbital velocities derived from linear wave theory and hindcasted for 50 mph storm winds at 5 to 8 m water depths in the funnel zone. Tidal energy density is derived from the tidal prism (Fig. 2B) for spring tide range.

The distribution of total energy density and various components at 22 locations is illustrated in Figure 9. Of note, wave energy, which includes ocean waves and waves generated within the bay and estuary, drops an order of magnitude with distance landward through the bay-mouth zone ( - 2 6 km to 0 kin). The ratio of wave to tidal energy approaches unity in the estuary funnel, 38 krn landward. In contrast, fluvial energy drops two orders of magnitude,

E S T U A R I N E S E D L ~ E N T S 8 9 3

lo' A ,,.._SAND _1_ M-D FA,'JE~ J . . . . . . . I- -= Ill, . ,T,s T 3 . . . . . . .,

/ l l l l f l l l l l l l t N TOTAL ENERGY ~ q i i I

I l t l I IIIIA i ~--~=. ..,...:~i!!::: ....... ::::: ~_~=_-L~:-:<<

101 I ] I I I I I I I ' I

160 120 80 40 0 -20Km. 4 DISTANCE LANDWARD

FIG. 9.--Distribution of total energy density with distance landward including components, fluvial, tidal and waves in relation to tripartite facies boundaries; compiled from various sources (see text).

between 160-116 km, or 44 km seaward from the fall zone; a trend corresponding to the rapid seaward enlarge- ment of cross-sectional area (Fig. 2A). Of note, the seaward gradient of total energy density levels off near the sand-mud facies boundary (116 km) (Fig. 9). This location is a zone where river floods are attenuated by the tide, and it is the seaward limit of bedload transport in- ferred from sandy bedforms. In the funnel zone, the total energy density is relatively weak and varies within narrow limits. In summary, the tripartite facies pattern broadly reflects the energy distribution. Sediments from the ends of the system, where river and wave energy are intense during floods and storms, are coarser-grained and better sorted than sediments from the central, less energetic sector dominated by weak tides.

SEA-LEVEL CHANGE

The modern lithofacies develop in an estuafine system undergoing long-term submergence, as evidenced by a composite age-depth trend (Fig. 10). Submergence prevails overall, but rates have slowed from ~ 12.5 mm/yr in an early phase (7000 to 9000 YBP) to ~ 1.6 mm/yr in the last 4000 yr. The continuity of marsh deposits to the 8-m depth in several cores from the funnel zone (E1- lison and Nichols 1976) indicates marsh accretion has kept pace with sea-level rise in the last five millennia. In the channel, however, accretion has lagged sea-level rise inasmuch as the channel is not filled to capacity today.

FACIES MODEL

When the modern tripartite facies is considered in the context of a deposit "trapped" and fossilized in an evolv- ing deposifional system, the vertical relationship of various component facies can be conceptualized (Peebles 1984). The rationale for constructing the model is that the estuary is in a state of transgression; it assumes that the vertical sequence is similar to the longitudinal sequence of modern sediments. Therefore, the model is

12

1'C AGE, YEARS BP X 10 3

10 8 6 I , i ,p I

SEA LEVEL CHANGE

/ /

/o / o/

O O o / / o o

Jo; REGIONAL /

/ o'~/ / o °~ o ° /i o _ _

/ i o % / I

f

~ ° i

4

/ ~ o q l y A

. / ......~ Z. . ~ ,~ I" LOCAL

q i i

• J A M E S E S T U A R Y

R A P P A H A N N O C K ! E S T U A R Y

o U.S. EA./r COAST Ml l f lm~ & Era~3j. 11ml Di l lon & Oldaka, l g 7 8

0 0 MLW

10

DEPTH rn

2O

30

40

50 Fno. 10.--Composite relative sea-level and submergence trend during

the past 11,000 years. Age-depth relations in upper part of envelope (< 8 m) derived from radiocarbon-dated samples of marsh deposits from upper James and Rappahannock estuaries (Ellison and Nichols 1976; Nichols 1972a). Lower part (> 15 m) is a regional trend derived from a portion of an envelope of Milliman and Emery (1968) and adjusted by Dillon and Oldale (1978).

constructed by projecting the longitudinal sequence into a vertical sequence assuming onlap development. The axial channel of the funnel zone is relatively stable. It fills by prograding channelward and accreting vertically (Fig. 5A) rather than by lateral migration with time as in some meso and macrotidal estuaries (Clifton 1982; Terwindt 1988). A state of transgression is indicated by: l) enlarge- ment of the bay-mouth and funnel zones through shore recession with modern rates averaging 40 m/century in the funnel, 38 m/century in the bay mouth (Byrne and Anderson 1977) and 310 m/century on the ocean coast (Dolan et al. 1990). 2) A relict fluvial drainage system in the bay mouth thinly covered in the southern half by Holocene sediments. These sediments truncate laterally continuous Pleistocene deposits older than the last gla- ciation along north and south banks (Colman and Hobbs 1987). 3) Evidence from the drowned river valley configuration previously described in the morphology section. 4) Occurrence of old brackish marsh peat deposits (10,340 to 15,280 YBP) in the bay mouth at greater depths (20 to 29 m) than young peat (880 to 4880 YBP) in the funnel zone, 100 km landward of the mouth at depths of 1.5 to 7.3 m (Ellison and Nichols 1976). Additionally, the relatively young peat in the funnel zone tributaries disconformably overlays, and onlaps, older formations and migrates landward and upward with sea-level rise (Johnson and Berquist 1989). 5) A fining upward sequence with silt and clay above coarse sand and gravel in the funnel zone (Fig. 11B) except for coarsening in upper parts near the bay mouth (0 kin) (Fig. 1 IC). This is likely a result of reworking and barrier progradation, a

894 M A Y N A R D M. NICHOLS, GERALD H. JOHNSON, AND P A M E LA C. P E E B L E S

A B B A' / SHOALS A

HANNEL m ::::::::::::::::::::

M U D FACIES

LITHOLOGY VIEW LANDWARD ~ ~ Grave, ~ ~;'.'~':.::~"~::.~::~.~

1 km ~ SaN & Gravel ~ ~ Sh¢l ~: ::~~:~" ;: :"~/ SAND . . . . . . . . . . FACIES S a n d ~ , . , . , , j - Silty sand ~ Peat layer

C

MaTh / / j . . . . . _ 'X::.,,,.., . . . . .

..:.~., :;.:.'::!: :: :- ::~...~

M E A N D E R Z O N E E S T U A R Y F U N N E L B A Y M O U T H

• . .

°° 'o" , " , "° % ' ° ° , ° m

Marine.. : ,":,':, ~:::::~::~ Estuarine

~ . ~ ' " ~ Estuarine

:.:.:~.==:.:.. ~ ' - luvlal B~ : ; . : ,..~ : .., .~.

FIG. 11.--A) Lithologic cross-section A-A' in the funnel zone (Fig. 4C) showing Holoccne fill overlying Pleistocene and Tcrdary deposits (P! and Tr) in axial channel (right, center) and in a tributary (leR). Section view landward; based on bridge borings. B) Lithologic cross-section B-B' at the funnel mouth (Fig. 4C) showing Holocene fill in axial channel; view landward; based on bridge borings. C) Lithologic longitudinal section along channel axis from bay mouth (right) to meander zone (leR) showing Holocene elastic wedge and corresponding vertical sequence oflithofacies; schematic.


COMPOSITE FACIES MODEL

T H I C K N E S S t m L ITHOLOGY \ , Clay ,Si Fs.Ms,Cs,Gr

:...: ..:.: :!..:~

BAY MOUTH I - [i '~'~'-'; i7'~ ".':''~::: ' :" ~':

SAND FACIES 4 1 ~ ~ ' ~ ' " " ' ~ ~..:-:~: ::.:.:,

:f[-_-t_

ESTUARY FUNNEL I _~_.-~=--_.~

MUD FACIES ' --1.: -_

:.:" .:-,,.¢: ..t '-~...- ."~'-,~--:.?

MEANDER ZONEsAND FACIES Z'5[! ! ~ ! ; ~ i ~ i t

: :" " % " Gravel ~...o : ; ~ Laminations

:':i~!.:.i: ,sand -~_-~ Si~orbeds

Massive fine so nO; scour and fill, shell lag and fragments, local x-beds, burrows and tubes, grave~ beds

Laminated mud bedding; interbedded with bioturbated layers; shell aggregations, tubes, burrowss, erosional contacts and discontinuities.

Laminated beds with silty. sandy laminae fining upward.

Medium to coarse sand with silt and gravel, x-beds, massive; silt interbeds, discontinuities, wood, clay inclusions

Gravel

Unconformity

Crees be0ding

~ , ~ Ripples

,Moor and fill - - - - " Mud, silty ~ Eroaiortsurlaco

~ ~ Si'lelt ~ ! ~ I BtJrrow$ ~ 1 ~ Biotumbte£t

FIG. 12.--Schematic fades model of composite transgressive vertical sequence in a microtidal estuary.

trend similar to transgressive sequences reported for Del- aware Bay (Knebel et al. 1988). The modern lithotopes, therefore, have Holocene analogues that are part of a transgressive wedge filling the path of the pre-Holocene fluvial drainage (Fig. 11C). The sequence reflects the re- treat and seaward evolution of environments from fluvial to fluvial estuarine, estuarine and marine estuarine.

Figure 12 illustrates an ideal vertical sequence as it would be viewed in a vertical section of a single channel fill about 15 m thick. A thicker sequence would be produced by long-term submergence, e.g., Holocene fill in the James is 37 to 46 m thick (Fig. 1 IA, B). The sequence is bounded at the base by a pre-transgressive erosional unconformity produced by fluvial erosion at lowered sea level. The lowermost unit consists of fluvial gravel that represents sediment likely deposited by the last major river flood. The fluvial gravel is overlain by cross-bedded or massive medium to coarse sand and gravel deposited in a fluvial influenced estuarine environment, e.g., corresponding to the modern meander zone. Upper parts may show remnants of interbedded clay and silt with scattered burrows indicative of shoals or intertidal deposits. Little information is available, however, to dif-

ferentiate fluvial and fluvial estuarine deposition in the channel deposits. Consequently, the nature of the boundary between these deposits is unknown. The lower sand facies grades abruptly upsection into the laminated mud facies, an analogue of the modern estuary funnel channel fill (Fig. 7). The change corresponds to the relatively abrupt lithologic transition observed between the meander zone sand facies and funnel mud facies (Fig. 4D); however, some interfingering may occur. Bioturbated mud layers alternate with laminated beds and become thicker and more prevalent upward. Flanks of the fill may locally preserve marginal subfacies with oyster biostromes and thin peat layers. The mud or shale unit is the most extensive and occupies over one-half of the total section. This unit grades upsection into massive fine sand likely to contain a few cross beds, shell lag and and biogenic structures indicative of the bay-mouth sand facies. It likely makes up less than 20% of the total section thickness, with variations depending on the extent o f subsequent marine erosion. Overall there is a trend of fining upward except in the uppermost unit.

FACIES SUMMARY AND COMPARISONS

The extensive studies of modern sediments in the James River estuary reveal distinct morphological compartments, a change in energy intensity, variations in sand: mud ratios, and different minor structures that give the facies a distinctive character. Figure 13 summarizes the salient characteristics of each principal lithofacies and shows their spatial relationship to each other. These characteristics, and the assemblage of component subfacies, serve to distinguish estuarine facies from other types of coastal sediments. They are likely useful for discriminating facies in other modern microtidal systems, and they allow comparison and contrast with meso and macrotidal estuarine facies.

Comparison of the James microtidal facies with com- parable facies in a macrotidal system such as the Gironde Estuary, France (Allen et al. 1973; Allen and Truilhe 1988) highlights the facies characteristics. Whereas the mud-dominated channel fill of the James preserves laminated mud and bioturbated interbeds produced by alternate river floods and biogentic activity under weak tides, the funnel facies of the Gironde preserves rhythmic sand-mud alternations produced by strong tidal currents. The deposits are non-bioturbated and fluvial flood sedimentation is absent as floods are damped out by the tide. Tidal dominance in the Gironde is also evidenced by a tidal sand bar complex with linear ebb-flood lobes ex- hibiting cross beds with tidal bundles, erosional reacti- vation surfaces and occasional clay drapes. Whereas the James channel is bordered by thin subtidal shoal deposits containing oyster biostromes, the Gironde has a belt of thick intertidal laminated mud. Sediment reworking is dominantly biogenic in the James but physical in the Gironde.

8 9 6 .~IAYNARD M. NICHOLS, GERALD H. JOHNSON, AND PAMELA C. P E E B L E S

C H A R A C T E R I S T I C

MICRO TIDAL FACIES SUMMARY . ~ / // Tide Limit , .1 2 " "'":'::::i:i!i!::::iii!iii!iii~CE~lNL

. ~ , u u _ Tide Limit Salt Limit ~ ~!:::':':':':':':':':':':':':':':'~

~ , " ~ . , ~ ¢ ' ~ ~ ~ = J ~-_---_--~ ..# Salt Limit . ~ ! i ! i i i ! i i i i i ! i ! i ! i i i i i ! i ! i ! i ~

MORPHOLOGIC ZONE

• AREAL DISTRIBUTION • SURFACE RELIEF

MEANDER ZONE ESTUARY FUNNEL BAY MOUTH

5% 50% 44% ROUGH, STEEP SMOOTH, UNEVEN ROUGH, UNEVEN

FLUVIAL MARINE E N V I R O N M E N T ESTUARINE EBTUARINE ESTUARINE

LITHOLOGY

• SDtMUD, MEAN, RANGE

MUD

LOCALLY SHEU.

24, 3-72

SAND

LOCALLY GRAVEL, MUD

70, 8-97

SAND

LOCALLY GRAVEL

80, 48-100

• FACIES BOUNDARY --ABRUPT . . . . TRANSITIONAL--

SEDIMENT SOURCE FLUVIAL & BANK I FLUVIAL>MARINE>SHORE I MARINE & BARRIER

ENERGY DISTRIBUTION

ACCUMULATION Mean, ¢m/yr tons/ m2/yrxlO "2

- EROS'N-DEPOS'N STATUS

BEDDING

• BIOTURBATION

.TOTAL .<.~.::~1:~'~ / ~ . . . . . . .~ . . . . . . . . . . . .

. . . . . . . . . . . . . ~ . ~ = ~ = = . > = = ~ = ~ . = = ~ ~ ? ~

I

1.4 3.1 I ; 1 . 0 1.1o 1.2B ~ I 0.7

i

DEPOS'N ~ EROSION DEPOS'N >> EROSION DEPOS'N > EROSION

I l l l ] l l l i l l l l J I t l H ] l [ H I I l l l H i I I I I I ]11 E t H I ] r N ~ I I ~ip=,:.,~,,~'lr_

~ PHYSJCALX-BEDLAYERED ~ i i : ~ " ~ I ~ N C ' . ' . ' . ' . ' - ' : ' : ' : ' ' : ' : ' " ~ I ] I I ] I N [ I N I N m l W T T r T i T T . . . . . . " . - ' - ' - ' . ' . ' . • . ' . ' . ' 2 "

< 10% 10-90% "'"'"<'""<':':':'"'':2':'70-99o~ " ' ~ "

STRUCTURES

o TYPE

- INCLUSIONS

:::::::.~ .~:~!:

".'oU'. k' . ' .": ' -

MASSIVE TO X-BEDDED ED

SAND CLAY r WOOD

LAMINATED MUD INTERBEDDED WITH BIOTURBATED LAYERS

SHELL

i iii!ii~i~!!ii!ii~i~ii!!i!i~ili

MASSIVE SAND WITH X-BEDS & BIOT'D

SHELL


Although microtidal and macrotidal deposits exhibit contrasts, they are similar in having a longitudinal tripartite facies pattern. This pattern is reported in microtidal drowned river valley estuaries of New South Wales, Australia (Roy 1984), in several mesotidal estuaries of the Georgia coast (Howard 1975), and in the macrotidal Gironde Estuary, France (Allen et al. 1973; Allen and Truilhe 1988) as well as in fossil meso-macrotidal estuaries of Alberta, Canada (Rahmani 1988). This occurrence suggests these systems have a relatively high energy level at opposite ends of the system. Alternately it reflects the convergence of two opposing sediment sources, fluvial and marine. Length of the mud facies likely varies in different estuaries with the relative influence of tides to river inflow or to wave action.

CONCLUSIONS

The Specific conclusions and generalizations from the findings of this study arc:

I) Estuary morphology forms three distinct zones: I) bay- mouth, 2) estuary funnel, and 3) meander zone. Each zone exhibits a characteristic lithofacies that reflects varying proportions of wave, tidal and fluvial energy.

2) The lithofacies form a longitudinal tripartite pattern, sand-mud-sand, with coarse-grained sediment at energetic ends of the system and fine-grained sediment in the less energetic central sector.

3) The seaward facies boundary, mouth to funnel (sand to mud), is transitional as a result of mixing fine- grained fluvial and coarse-grained marine sediment in a zone where total energy gradually decreases landward. In contrast, the landward boundary, meander zone to funnel (sand to mud), is abrupt as a result of a rapid seaward decrease in the bcdload transport of fiver floods which is partly attenuated by the tide.

4) Accumulation rates are fastest in the estuary funnel mud facies, a less energetic zone where fine-sediment is entrapped in the estuarine circulation. The fill preserves laminated mud interbcdded with bioturbatcd layers, a bedding that signifies changes from fast to slow accumulation indicative of contrasting episodic river flooding and normal tidal conditions.

5) Lateral changes in accumulation rate and texture of channel fill exhibit an asymmetry reflecting marked lateral gradients of morphology and the classic estuarine circulation.

6) The tripartite facies evolve in a transgressive system as a wedge filling the path of pre-Holoccnc drainage. The sequence fines upward except at the mouth and reflects the seaward evolution of environments from

fluvial to fluvial estuarine (meander zone), estuarine (funnel) and mar ine estuarine (bay-mouth).

The next step in advancing our knowledge is to verify the proposed model with new informat ion from other modern, as well as ancient, microtidal systems focusing on the differentiation of fluvial and fluvial-estuarine facies.

ACKNOWLEDGMENTS

The authors are indebted to the numerous investigators whose previous data have provided a foundat ion for advancing analyses and interpretations. We thank J.R. Boersma and J.H.J. Terwindt of the Univers i ty of Utrecht for discussions and for lacquer peels that facilitated the study. K. Stubblefield drafted the figures and B. Marshall typed the manuscript . This is Virginia Insti tute of Marine Science contr ibut ion n u m b e r 1638.

REFERENCES

AtJ..mq, G.P., AND TRtnLnE, G., 1988, Stratigraphie and facies model of a transgressing estuarine valley fill in the Gironde estuary (France) (abstract), in James, D.P., and Leckie, D.A., eds., Sequences, Stratig- raphy, Sedimentology: Surface and Subsurface: Proceedings of the Canadian Society of Petroleum Geology Technical Meeting, Memoir 15, Calgary, p. 575.

ALLEN, G.P., Bouo-mr, J.M. CnaROm,mL, P., CnsrnaqG, P., Gnvgr, J., GONTHmR, E., JOUA~.AU, J.M., KJ.,n~tO~mL, A., LnTouo-m, C., LE- OIGAN, P., OoeRON, C., PuJos, M., TESSON, M., AND VERbmrr~, G., 1973, Environments and sedimentary processes of the North Aqui- taine coast: Guidebook, Institut de Geologie du Bassin d'Aquitaine, 183 p.

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