Palaeosols and sequence stratigraphy of the Lower Permian Abo Member, south-central New Mexico, USA GREG H. MACK*, NEIL J. TABOR and HENRY J. ZOLLINGER à *Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003, USA (E-mail: [email protected]) Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA àHess Corporation, 500 Dallas St., Houston, TX 77002, USA Associate Editor: Stephen Lokier ABSTRACT The relationship between palaeosols and sequence stratigraphy is tested in the Lower Permian Abo Member, south-central New Mexico, by comparing interfluve and fluvial-terrace palaeosols with palaeosols that developed within lowstand-fluvial deposits. Interfluve and fluvial-terrace palaeosols consist of primary pedogenic features, including vertical root traces, vertic structures, Stage II and III pedogenic calcite and translocated clay (argillans), which are cross-cut or replaced by low-aluminium goethite, gley colour mottling, sparry calcite veins and ankerite. The polygenetic character of the palaeosols is consistent with initial development for several thousand to tens of thousands of years on well-drained interfluves or fluvial terraces, followed by waterlogging due to invasion by a rising water table that locally may have been brackish. In contrast, lowstand-fluvial sediment that filled incised valleys contains only rooted and vertic palaeosols, whose immaturity resulted from high aggradation rates. Palaeosols similar to those in the Abo Member have been recognized in other ancient strata and, when combined with high- resolution correlation, provide evidence for interpretation of sequence- stratigraphic surfaces and systems tracts. Keywords Incised valley, New Mexico, palaeosols, Permian, sequence strati- graphy. INTRODUCTION Palaeosols are potentially important tools in deciphering sequence-stratigraphic relationships in mixed terrestrial and marine basins. Particu- larly instructive are those palaeosols that form on interfluves and on fluvial terraces associated with incised valleys (Fig. 1). As sea-level falls below the depositional-shoreline break, rivers on the coastal plain erode their floodplains, creating incised valleys separated by interfluves (Fig. 1A; Van Wagoner et al., 1988). Although an interfluve may be eroded locally by widening of the incised valley, much of its surface remains a stable landscape and the site of soil formation (Daniels et al., 1971). The erosional floor of the incised valley constitutes a type 1 sequence boundary that is correlative to the adjacent interfluves and the soils developed upon them (Van Wagoner et al., 1988). The cutting of incised valleys may be episodic, producing inset fluvial terraces (Fig. 1B; Wright, 1992; Blum, 1993; Blum & Tornqvist, 2000; Strong & Paola, 2006). Under these condi- tions, initial erosion is followed by partial back- filling of the valley with fluvial sediment which, in turn, is followed by renewed incision to a lower topographic level. As is the case with interfluves, abandoned terraces will be sites of soil formation until removed by erosion or buried by younger sediment. Palaeosols that develop on interfluves were predicted in the model of Wright & Marriott Sedimentology (2010) 57, 1566–1583 doi: 10.1111/j.1365-3091.2010.01156.x 1566 Ó 2010 The Authors. Journal compilation Ó 2010 International Association of Sedimentologists
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Palaeosols and sequence stratigraphy of the Lower Permian AboMember, south-central New Mexico, USA
GREG H. MACK*, NEIL J. TABOR� and HENRY J. ZOLLINGER�*Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003, USA(E-mail: [email protected])�Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA�Hess Corporation, 500 Dallas St., Houston, TX 77002, USA
Associate Editor: Stephen Lokier
ABSTRACT
The relationship between palaeosols and sequence stratigraphy is tested in the
Lower Permian Abo Member, south-central New Mexico, by comparing
interfluve and fluvial-terrace palaeosols with palaeosols that developed
within lowstand-fluvial deposits. Interfluve and fluvial-terrace palaeosols
consist of primary pedogenic features, including vertical root traces, vertic
structures, Stage II and III pedogenic calcite and translocated clay (argillans),
which are cross-cut or replaced by low-aluminium goethite, gley colour
mottling, sparry calcite veins and ankerite. The polygenetic character of the
palaeosols is consistent with initial development for several thousand to tens
of thousands of years on well-drained interfluves or fluvial terraces, followed
by waterlogging due to invasion by a rising water table that locally may have
been brackish. In contrast, lowstand-fluvial sediment that filled incised valleys
contains only rooted and vertic palaeosols, whose immaturity resulted from
high aggradation rates. Palaeosols similar to those in the Abo Member have
been recognized in other ancient strata and, when combined with high-
resolution correlation, provide evidence for interpretation of sequence-
stratigraphic surfaces and systems tracts.
Keywords Incised valley, New Mexico, palaeosols, Permian, sequence strati-graphy.
INTRODUCTION
Palaeosols are potentially important tools indeciphering sequence-stratigraphic relationshipsin mixed terrestrial and marine basins. Particu-larly instructive are those palaeosols that form oninterfluves and on fluvial terraces associated withincised valleys (Fig. 1). As sea-level falls belowthe depositional-shoreline break, rivers on thecoastal plain erode their floodplains, creatingincised valleys separated by interfluves (Fig. 1A;Van Wagoner et al., 1988). Although an interfluvemay be eroded locally by widening of the incisedvalley, much of its surface remains a stablelandscape and the site of soil formation (Danielset al., 1971). The erosional floor of the incised
valley constitutes a type 1 sequence boundarythat is correlative to the adjacent interfluves andthe soils developed upon them (Van Wagoneret al., 1988). The cutting of incised valleys may beepisodic, producing inset fluvial terraces (Fig. 1B;Wright, 1992; Blum, 1993; Blum & Tornqvist,2000; Strong & Paola, 2006). Under these condi-tions, initial erosion is followed by partial back-filling of the valley with fluvial sediment which,in turn, is followed by renewed incision to alower topographic level. As is the case withinterfluves, abandoned terraces will be sites ofsoil formation until removed by erosion or buriedby younger sediment.
Palaeosols that develop on interfluves werepredicted in the model of Wright & Marriott
1566 � 2010 The Authors. Journal compilation � 2010 International Association of Sedimentologists
(1993) and have been documented in the strati-graphic record (Gibling & Bird, 1994; Aitken &Flint, 1996; O’Byrne & Flint, 1996; McCarthy &Plint, 1998; McCarthy et al., 1999; Plint et al.,2001; Feldman et al., 2005). Lower Permianstrata in south-central New Mexico provide anexcellent opportunity to test currently evolvingideas about palaeosols and sequence strati-graphy, because sequence boundaries associatedwith incised valley floors can be correlated inoutcrop over distances of metres to kilometresto coeval interfluves and, in a few cases, fluvialterraces can be recognized within the incisedvalley fill. In addition, palaeosols that areunrelated to interfluves or fluvial terraces arepresent within lowstand-fluvial deposits. Thefocus of this study is to compare interfluve andfluvial-terrace palaeosols with lowstand-fluvialpalaeosols and to define how their differencesrelate to sequence stratigraphy.
GEOLOGICAL SETTING
Lower Permian strata in south-central NewMexico display a regional facies change fromfluvial red beds of the Abo Formation in the northto marine limestones and shales of the HuecoGroup in the south (Figs 2 and 3; Kottlowski,1963). In a narrow transitional zone betweenmarine and terrestrial palaeoenvironments nearLas Cruces, ca 100 m of siliciclastic red beds, greyshales and carbonate rocks are interbedded on thescale of a few metres within the middle to upperWolfcampian (Sakmarian) Abo Member of theHueco Formation (Kottlowski, 1963; Seager et al.,1976, 2008; Kietzke & Lucas, 1995; Kozur &LeMone, 1995; Kues, 1995). These strata areinterpreted to have been deposited in fluvial,estuarine and shallow-marine to peritidal envi-ronments (Mack & James, 1986; Krainer & Lucas,1995; Mack et al., 2003a,b; Mack, 2007).
Mack (2007) recognized 16 sequence bound-aries and associated sequences in the AboMember in the Robledo Mountains (Figs 4and 5) and 18 sequence boundaries and associ-ated sequences in the Dona Ana Mountains(Figs 6 and 7). Based on dividing the number ofsequences by the biostratigraphically determinedduration of deposition of the Abo Member, eachsequence was estimated to be on the scale of105 years, corresponding to fourth-order sea-levelcycles of Van Wagoner et al. (1990). The fourth-order sequences in the Abo Member are interpretedto have been influenced strongly by glacial-eustaticsea-level changes related to eccentricity-drivenMilankovitch cycles (Mack et al., 2003a; Mack,2007), probably during late Phase III (299 to291 Ma) of Gondwanan glaciation in Antarcticaand Australia and during the early part of a youngerphase in Australia (287 to 280 Ma) (Isbell et al.,2003; Montanez et al., 2007).
Palaeosols in the Abo Member developed with-in lowstand-fluvial sediment that was depositedin incised valleys, on fluvial terraces withinincised valleys and on interfluves betweenincised valleys (Figs 4 to 7). The transgressivesystems tract includes estuarine sediment andassociated ravinement surfaces within incisedvalleys and shallow-marine sediment and itsbasal ravinement surface above interfluves. High-stand systems tracts are represented withinestuarine successions by progradation of bayheaddelta and marsh facies over mudstones of theestuarine central basin (Mack et al., 2003a; Mack,2007). With the exception of scattered root traces,bayhead-delta and marsh deposits in the Abo
A
B
Interfluve
Incisedvalley
Interfluve sequence boundary
Valley floor sequence boundary
soil soil
1. Initial erosion of incised valley
2. Partial backfilling of initial incised valley
3. Renewed erosion of incised valley
4. Partial backfilling of deepened incised valley
terrace with soil
5. Deposition of estuarine sediment during transgression
Interfluve soil Interfluve soil
River
Inter-fluve
Fig. 1. (A) Sequence boundary associated with thefloor of an incised valley and adjacent interfluves. (B)Fluvial terraces produced by episodic cutting andpartial backfilling of an incised valley.
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Member lack pedogenic features and will not bediscussed further. Coastal-plain fluvial sedimentdeposited during highstand is not preserved inthe Abo Member, because of deep erosion asso-ciated with the cutting of incised valleys. As aresult, it is common for lowstand-fluvial sedimentto directly overlie marine limestone or shale ofthe transgressive systems tract (Mack, 2007).
METHODS
Field and petrographic methods
Each of 17 interfluve and fluvial-terrace palaeo-sols and 35 lowstand-fluvial palaeosols wastraced laterally between the logged sections
(Figs 3 to 7), and the best-exposed examples weredescribed in the field in terms of horizons andtheir thickness, colour and pedogenic features. Inaddition, 10 interfluve and fluvial-terrace palaeo-sols and five lowstand-fluvial palaeosols weresampled for petrographic examination (40 thinsections total), using the concepts and termino-logy of Brewer (1964).
Laboratory methods
Whole-rock palaeosol matrix and nodule sampleswere ground in a corundum mortar and pestle andloaded as randomly oriented powder mounts forX-ray diffraction. Continuous scan analyses wereperformed on all samples with a Rigaku Ultima IIIX-ray diffractometer (Rigaku Corporation, Tokyo,
N
200 km Equator
Equator
10°N
10°N
Aeolian erg
Aeolian erg
Arizona New Mexico
Utah Colorado
Alluvial plain
Abo Fm
Studyarea
Shallow sea
Shallow sea
Hueco Fm
USANew Mexico Robledo
Mts
Dona Ana Mts
10 km
N
LASCRUCES
I-10
Rio Grande
32° 33 N
106° 45 WB
A
Pedernal MtsFluvial red beds
Marine limestone and shale
I-25
?
?
?
Fig. 2. (A) Early Permian (Wolf-campian) palaeogeographic map,adapted from Peterson (1988) andZiegler et al. (1997), superimposedon the modern ‘Four Corner’ statesof New Mexico, Arizona, Utah andColorado. The study area is shownby a rectangle. (B) Location map ofstudy area in southern New Mexico.The grey shading shows the studyareas in the Robledo and Dona AnaMountains, which are shown inmore detail in Fig. 3. I-10 and I-25refer to interstate highways that passthrough the city of Las Cruces, NewMexico.
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Japan) using Cu-Ka radiation with a scanningspeed of 1� 2h min)1. For powder diffractionanalyses of bulk samples, scans were run between2� and 40� or 70� 2h. For eight samples of goethitefrom five palaeosols (Figs 4 to 7), the amount ofaluminium (Al+3) substituted for iron (Fe+3) in thecrystal lattice was determined by the X-raydiffraction method of Schulze (1984), with ananalytical uncertainty of ±3 mol.%.
Calcite nodules were collected from six palaeo-sols in the Dona Ana Mountains and three palaeo-sols in the Robledo Mountains, with two to sixsamples analysed from each palaeosol for carbonand oxygen isotopes (27 total analyses) (Figs 4to 7). Isotopic analyses were also performed onankerite from two palaeosols in the Dona AnaMountains (eight analyses) and one palaeosol inthe Robledo Mountains (one analysis), and onesparry calcite vein from the Dona Ana Mountainswas analysed (one analysis) (Figs 4 to 7). Wheremultiple samples were analysed, the mean value ispresented here. In each case, the 1r standarddeviation is <0Æ8& for carbon and oxygen.
Representative carbonate samples from eachpalaeosol were examined petrographically andthose without evidence of diagenetic recrystalli-zation were drilled directly from thin sections,
matching billets, and polished slabs using a table-top drill system with 100 lm diamond bits on anx, y and z stage. Between 5 and 20 mg of powdersthat contained carbonate minerals were loadedinto reaction vessels, and atmospheric gases wereevacuated. Samples were then dissolved in 100%H3PO4 in vacuo at 25�C for ca 16 h to produceCO2. The CO2 samples were cryogenically puri-fied and analysed for carbon and oxygen isotopiccompositions using a Finnigan MAT 252 isotoperatio mass spectrometer (Thermo-Fisher Scien-tific Corporation, Waltham, MA, USA) at South-ern Methodist University. Isotope values arereported in standard delta notation:
d13Cðor18OÞ ¼ ðRsample=Rstandard�1Þ � 1000
where R is the ratio of heavy to light stableisotope present in the sample or standard. Theisotopic ratios (d values) are reported relative tothe Vienna Peedee Belemnite standard (VPDB;Craig, 1957) for both carbon and oxygen.
Interfluve and fluvial-terrace palaeosols
Interfluve and fluvial-terrace palaeosols wererecognized in the Abo Member at eight strati-
N
N
A
B
1
1
5
26
7
4
3
8
1 km
1 km
32 22 30 No
o
o
o106 55 W
....
.. ..
.. ..
...
..
......
....
....
..
..
....
. . . ...
. . . ... . .
..
..
..
.. . . . . ....
. . ... . .. . .
..
.... ..
..
..
..
. ...
. ...
. . . ... . . ..
. .
..
...
... . . .
...
. . . . ..
. ..
.
.
... ..
.
. .. .
..
..
....
. ...
. ...
..
. ...
..
....
.. ...
..
..
..
. ...
. . . ..
...
..
..
. .
..
..
..
..
... .
..
..
..
Apache Canyon
2
3
4
5
632 30 N
106 52 30 W
Lucero
Arro
yo
Robledo Mountains
Dona Ana Mountains
Fig. 3. Location of logged sectionsin the Robledo Mountains (A) andDona Ana Mountains (B), southernNew Mexico, adapted from Mack(2007). Normal faults are indicatedby bold lines with a ball on thedown side. Dashed and dottedlines indicate ephemeral streamsflowing to the east or south-east inthe Robledo Mountains and to thesouth-west in the Dona AnaMountains. The logged sections inFigs 4 to 7 are projected onto aneast–west line.
Permian palaeosols and sequence stratigraphy, New Mexico 1569
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graphic positions in the Robledo Mountains andnine stratigraphic positions in the Dona AnaMountains (Mack, 2007). Those palaeosols thatoccupied interfluves developed within clay-richshallow-marine or estuarine parent sediment(Figs 8 to 11), although in a few cases pedogenicfeatures extend into underlying fluvial siltstones(Figs 8 to 11). In all cases, the interfluve palaeo-sols display sharp, erosional upper contactsbeneath marine ravinement surfaces, and are
overlain by shallow-marine facies. Palaeosols thatoccupied fluvial terraces developed in fluvialsediment, including sandy, silty and clay-richparent material (Fig. 11). The fluvial-terracepalaeosol in the Dona Ana Mountains (Fig. 11,DA-2i) has a conformable upper contact beneathestuarine-marsh deposits, whereas the fluvial-terrace palaeosol of DA-10i has a sharp, probablyerosional upper contact (Fig. 11). The interfluveand fluvial-terrace palaeosols are divisible into
"first yellow"
ss-1ss-1ss-1ss-1
ss-1ass-1a
ss-1a
ss-2a
ss-2a
ss-2a
ss-2bss-2b
ss-3
ss-3ss-3ss-3
ss-4ss-4
ss-5
ss-5
ss-5
SB1
SB2
SB3
SB4
SB4SB5
SB6SB6
SB7
SB8
SB9
SB9
SB10
0
5
Metres
LST
LST
LST
LST
LST
LSTLST
TST-est
TST-est
TST-est
TST-est
TST-est
TST-est
TST-marine
TST-marine
TST-marine
HST-est
HST-est
HST-est
TST-marine
TST-marine
LST
LST
LST
TST-est
TST-est
TST-est
TST-marine
TST-marine
EW0·2 km 1·0 km 1·5 km
Fluvial-channel
Fluvial-floodplain
Estuarine
Marine limestone
Marine shale-siltstone
Lagoonal to peri-tidal dolomicrite
Sequenceboundary
Inferredincisedvalley
Robledo Mountains-lower
Covered
RM-1a
RM-2
RM-3, 2c, 1g
RM-4, 2g
RM-5, 2c
1.
2.
3.4.
(includes shale-siltstone partings)
ss-3 = sandstone-siltstone interval
SB1
LST = lowstand systems tract
TST = transgressive systems tract
HST = highstand systems tract
V Palaeosol in LST
V
V
VInterfluve or fluvial-terrace palaeosol
RM-3, 2c, 1g = palaeosolsample number followedby number and type of sample for laboratoryanalysis.
Fig. 4. Logged sections of the lower part of the Lower Permian Abo member in the Robledo Mountains, adapted fromMack (2007), illustrating sequence-stratigraphic relationships. Here, and in Figs 5 to 7, the palaeosols examined inthis study are designated RM-1, etc., or DA-1, etc., with RM = Robledo Mountains and DA = Dona Ana Mountains.Samples collected for laboratory analysis are listed after the sample number (for example, RM-3, 2c, 1g), with thenumber referring to the number of samples collected and the letters referring to the type of sample and analysis (c, a,v = stable carbon and oxygen isotopic analysis of calcite nodules, ankerite or sparry calcite vein, respectively;g = X-ray diffraction analysis of goethite).
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multiple horizons, based on the vertical distribu-tion of pedogenic features and colours describedbelow (Figs 8 to 11).
A Horizons
A horizons are rare in interfluve and fluvial-terrace palaeosols, and are distinguished by adarker grey colour and/or by a higher concentra-tion of root traces than exist in associatedsub-surface horizons (Fig. 11, DA-2i). Root tracesvary from a few centimetres to 20 cm long andexist as moulds or clay-filled or silt-filled casts;they are sparse to moderately abundant, with the
smaller root traces generally concentrated in theupper 20 cm of the profiles. The paucity ofrecognizable A horizons may be the result ofoxidation of much of the organic matter duringburial diagenesis, making them difficult to dis-tinguish based on colour and/or erosion of theupper part of the palaeosol profile during marinetransgression (O’Byrne & Flint, 1996; Yang, 2007).
B Horizons
A B horizon is defined as an originallysub-surface horizon in which pedogenic pro-cesses have eliminated the primary sedimentary
1·8 km 1·0 km 1·5 km
Upper Hueco
ss-6ss-6
ss-6
ss-6
"middlelimestone"
ss-7ss-7ss-7
ss-8
ss-8 ss-8 ss-8
"slope-forming interval" ~25 m thick
EW
SB11
SB11
SB12SB12
SB13
SB13
SB14SB14
SB15SB15
SB16
TST-marine
LST
LST
LST
LST
TST-est
TST-est
TST-est
TST-est
HST-est
HST-est
TST-marine
TST-marine
TST-marine
TST-marine
LST
LST
TST-est
TST-est
TST-marine
TST-marine
0
5
Metres
SB16
RobledoMountains- upper
ss-9
ss-9 ss-9ss-9
5678
V
V
VV
V
RM6, 2g
RM-7
RM8, 2c, 1a
Fig. 5. Logged sections of the upper part of the Lower Permian Abo Member in the Robledo Mountains, adapted fromMack (2007). See Fig. 4 for legend.
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layering, such as bedding or current-generatedsedimentary structures. In addition, B horizonsmay have evidence of downward movement ofpedogenic material through the profile (trans-location) and in situ precipitation of mineralsfrom soil solutions. The B horizons of interfluvepalaeosols of the Abo Member display drab greyand green colours, which may have been inheritedfrom the originally organic-rich marine or estua-rine parent sediment (Figs 8 to 11 and 12A).However, interfluve palaeosols display lightershades than pedogenically unmodified estuarineand marine mudstones, suggesting that somedegree of oxidation of the original organic matteroccurred during soil formation. Fluvial-terracepalaeosols display reddish colours consistent withdeposition and subsequent pedogenesis underoxidizing conditions (Fig. 11). B horizons gener-
ally contain root traces similar to, but less abun-dant than, those in A horizons, as well as blockypeds. Six types of B horizons were distinguishedby the presence of a single pedogenic feature (Bt,Bg or Bss) or a combination of pedogenic features(Bgss, Bkgss and Btgss), including vertic features(subordinate descriptor ‘ss’), calcite nodules (sub-ordinate descriptor ‘k’), gley features (subordinatedescriptor ‘g’) and translocated clay (subordinatedescriptor ‘t’) (Figs 8 to 11).
Vertic featuresVertic features are produced by the shrinking andswelling of expandable clays (Ahmad, 1983) andare especially well-developed in the clay-richinterfluve palaeosols. Vertic features evident inoutcrop include pedogenic slickensides, wedge-shaped peds and desiccation cracks up to 50 cm
ss-1 ss-1
ss-2 ss-2ss-2
ss-2
ss-3
ss-3
ss-3 ss-3
ss-4 ss-4 ss-4
terrace
"first yellow"
0
5
Metres
WSW ENE
SB1
SB2SB2SB2
SB3
SB4
SB4
SB5
SB6
SB7
SB8
LST
TST-est
TST-marine
LST
TST-estHST-est
TST-est
TST-est
TST-est
LST
LST
LST
TST-marine
TST-marine
TST-marine
TST-marine
Dona Ana Mts -lower
SB4
DA-2,2c
DA-3
DA-2i, 6a
LST
12
3
4SB8
0·3 km0·6 km0·2 km
V
SB7
SB6DA-4, 2c, 1a
DA-5
Fig. 6. Logged sections of the lowerpart of the Lower Permian AboMember in the Dona Ana Moun-tains, adapted from Mack (2007).See Fig. 4 for legend.
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deep and 3 cm wide (Fig. 12A and B; Driese &Foreman, 1992; Joeckel, 1994; Driese & Ober,2005; Driese et al., 2005). Petrographically, avertic fabric is manifested by centimetre-scalepatches of clay aligned in a single direction(masepic plasma fabric) or in an orthogonal
pattern (lattisepic plasma fabric) (Brewer, 1964;Driese & Ober, 2005). A pedogenic origin for thevertic structures is supported by their absence inpedogenically unmodified shales of the AboMember.
"middlelimestone"TST-marine
TST-marine
TST-marine
TST-marine
TST-marine
TST-marine
LST
LST
LST
LST
LST
LST
LST
SB9
TST-est
TST-est
TST-est
TST-est
TST-est
TST-est
HST-est
HST-est
HST-est
SB10SB11
ss-6ss-6
ss-7
LST
LST
LST
TST-est
TST-est
TST-est
TST-marine
TST-marine
TST-marine
terrace
SB12
ss-8ss-8
SB14
SB15
SB16SB17
SB18
ss-9
ss-10
ss-11
Unconformable top
"secondyellow"
"second rhizo-corallium
0
5
Metres
SB13
Dona AnaMountains- upper
0·6 km
300 m
5
6WNW
ESE
DA-10, 5c, 2g
DA-10i, 6c
DA-7, 3c, 1v
DA-6, 4c, 1g
V
V
V
V
V
V
V V
ss-7
ss-12
Fig. 7. Logged sections of the upper part of the LowerPermian Abo Member in the Dona Ana Mountains,adapted from Mack (2007). See Fig. 4 for legend.
Palaeosol RM-3 interfluve, estuarine and fluvial parent sediment
Bkgss
ss
ss
ss
ss
gc
m
m Bg
Bgss
Roottraces
Desiccationcracks
Blockypeds
Wedge-shapedpeds
Calcitenodules
Goethitenodules
Ripples
Fissile
Colour
Colour
Glossifungites burrows
Unmodified sed.
Unmodifiedfluvial sed.
Bss
10YR 5/4
10YR 5/4
10R 4/2
10R 2/2
Veins and lensesof sparry calcite
5Y 4/1
5GY 4/1
m Red and green or grey mottlingss Slickensidesgc Goethite ped coatsA Ankerite
Horizon subordinate descriptors: k = calcite nodules;g = gley; t = translocated clay; ss = slickensides
Fig. 8. Profiles of representative interfluve palaeosolsof the lower Abo Member in the Robledo Mountains.Depth refers to the position below the top of thepalaeosol.
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Calcite nodulesPresent in each of the interfluve and fluvial-terracepalaeosols are calcite nodules, which range indiameter from a few millimetres to 3 cm. Thesenodules commonly are dispersed widely withintheir horizon, making them equivalent to Stage IImorphology of Gile et al. (1966) and Machette(1985) (Fig. 12A and C). Less commonly, pedo-genic nodules coalesced into thin (ca 30 cm) bedswith irregular upper and lower contacts, corre-sponding to incipient Stage III morphology of Gileet al. (1966) and Machette (1985). The thicker bedsof pedogenic calcite are designated K horizons,following Gile et al. (1965) (Fig. 11, DA-10). Wherea single or multiple horizons overlie a calcichorizon, it ranges from 20 to 40 cm beneath thetop of the profile. The nodules and thin beds aregenerally composed of homogeneous, micro-crystalline, low-magnesium calcite, although afew display microfractures, brecciation and aclotted, peloidal texture (Fig. 12D). The d13C val-ues of calcite nodules range from )3Æ1& to )7Æ2&,whereas d18O values range from )4Æ1& to )8Æ9&
(Fig. 13).
Gley featuresGley features are manifested in interfluve andfluvial-terrace palaeosols of the Abo Member asgoethite and colour mottling. Goethite is present
as discrete nodules, coatings and partial replace-ment of calcite nodules. X-ray diffraction analy-ses of the goethite (111) and (110) d-spacingsindicate <5% Al+3 substitution for Fe+3 in thegoethite crystal lattice (mean = 2Æ1%; range = 0%to 5%; n = 8). Goethite nodules range from 1 mmto 2 cm in diameter and are scattered widelywithin the finer matrix (redox concentrations;Vepraskas, 1999; Fig. 12C). Thin (lm to milli-metre-scale) coats of goethite (sesquans) surroundcalcite nodules (Fig. 12D) and line wedge-shapedpeds, root traces and the fractures within brecci-ated calcite nodules (channel ferrans) (Fig. 12D;Venneman et al., 1976; Venneman & Bodine,1982). In a few cases, spherical goethite nodulesca 1 mm in diameter are positioned entirelywithin calcite nodules. Herein referred to as‘corona structures’, they have an opaque core ofgoethite that is rimmed by microferrules of goe-thite or hematite that become less concentratedoutward from the core (Fig. 12E). In a few cases,
Palaeosol RM-8 interfluve,estuarine and fluvial parent sediment
Colour
Colour
Horizons
Horizons
Fig. 9. Profiles of representative interfluve palaeosolsof the upper Abo Member in the Robledo Mountains.See Fig. 8 for symbols. Depth refers to the positionbelow the top of the palaeosol.
Depth (cm)
0
50
100
Clay Silt SandHorizonsColour
Depth (cm)
0
50
100
Clay Silt SandHorizonsColour
Palaeosol RM-4 interfluve, estuarine and fluvial parent sediment
Fig. 10. Profiles of representative interfluve palaeosolsof the Abo Member in the Robledo and Dona AnaMountains. See Fig. 8 for symbols. Depth refers to theposition below the top of the palaeosol.
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crude alignment of the microferrules imparts aradial fabric to the rim, and the core may bedetached from the rim (disorthic), with the voidfilled with coarse calcite. X-ray diffraction anal-
ysis establishes the existence of calcite within thecorona structures which, along with their spher-ical shape, suggest that they represent partialreplacement of calcite nodules by goethite.
Gleization of interfluve and fluvial-terracepalaeosols of the Abo Member is also representedby red and grey or green colour mottling, producedby alternating oxidation and reduction of iron(Fig. 12F). In some cases, gley mottling occupies aposition as the lowest horizon (Bg) in the palaeo-sol, beneath a calcic B horizon (Fig. 8, RM-3). Inthis case, gley mottling may have occurred con-temporaneously with development of the initialsoil, perhaps representing a zone of fluctuatingwater table directly beneath the well-drainedprofile. In most cases, however, the zone of gleymottling has an irregular contact or a sharp butoblique contact with vertic and calcic horizons.
ArgillansArgillans (clay coats) result from downward move-ment (translocation) of clay through the palaeosolprofile. Argillans coat peds, grains and calcitenodules (Fig. 12G), as well as line root traces andfractures in brecciated calcite nodules. Argillansare common but not abundant within the interfluveand fluvial-terrace palaeosols of the Abo Member.In only a few cases were they considered pervasiveenough to warrant designation of an argillic hori-zon (Bt), based on a petrographic concentration of>1% (Fig. 8, RM-1a; Fig. 10, DA-7; Soil SurveyStaff, 1975). The paucity of recognizable argillansmay be the result of their modification or destruc-tion by vertic processes (Gile et al., 1981).
C Horizons
C horizons are defined as subsurface horizons inwhich pedogenic processes have not destroyedevidence of original sedimentary layering, such asbedding or current-generated sedimentary struc-tures, and which lack evidence of translocation orin situ precipitation of soil material present inoverlying B horizons. C horizons are underlain bypedogenically unmodified sediment. Relativelyrare in interfluve and fluvial-terrace palaeosols ofthe Abo Member, C horizons display some combi-nation of pedogenic colours or colour mottling,poorly developed blocky peds and a few root traces(Figs 9 to 11).
Additional features
Three features that are not considered within thecontext of horizon designations are present with-
Fig. 11. Profiles of representative interfluve and flu-vial-terrace palaeosols from the Dona Ana Mountains.See Fig. 8 for symbols. Depth refers to the positionbelow the top of the palaeosol.
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C
B
D
E
G
A
Bss
Bkgss
Bgss
Bg
F
H
Fig. 12. (A) Field photograph ofinterfluve palaeosol RM-3, overlainby ledge-forming shallow-marinelimestone. The Bkgss horizondisplays light-coloured calcic nod-ules and curved fractures that rep-resent wedge-shaped peds. The Bgsshorizon has light grey and red mot-tling. Hammer is 25 cm long.(B) Wedge-shaped peds lined withslickensides in interfluve palaeosolRM-4. The arrow points to slicken-sided underside of wedge-shapedped. Hammer is 25 cm long.(C) Calcite and goethite nodules inthe Bkgss horizon of interfluvepalaeosol DA-10. The large calcitenodule near the centre of thephotograph is coated with goethite.Scale is in centimetres. (D) Photo-micrograph under crossed nicols ofa brecciated calcite nodule (lightbrown) lined by dark red goethite.Bar scale is 0Æ5 mm. (E) Photomi-crograph under uncrossed nicols ofa corona structure replacing a calcitenodule. The corona structure iscomposed of a dense core of goe-thite, which grades outward to morewidely space goethite microferrules.The outer ring is detached and filledwith sparry calcite. Bar scale is0.5 mm. (F) Field photograph of Bkghorizon of fluvial-terrace palaeosolDA-2i, showing red and light greymottling (gley) and light-colouredcalcite nodules (arrow). Hammer is25 cm long. (G) Photomicrographunder crossed nicols of the Bt hori-zon of interfluve palaeosol RM-1a.Yellowish clay coats dark clay-richpeds (ped argillans) and a few sandand silt grains (embedded grainargillans). Bar scale is 0.5 mm. (H)Field photograph of lowstand-fluvial palaeosol showing desicca-tion cracks, wedge-shaped andblocky peds and sparry calcite vein(arrows). The palaeosol is overlainby ledge-forming crevasse-splaysiltstone. Hammer is 25 cm long.
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in some interfluve and fluvial-terrace palaeosols.Less than 25% of the palaeosols contain thin(<3 cm) veins and discontinuous lenses of sparrycalcite (Figs 10 and 11). The veins and lenses arecommonly parallel to bedding, but they may alsocross-cut palaeosol horizons at a low angle or mayfollow wedge-shaped peds. The carbon isotopicvalue of one sparry calcite vein ()1Æ7&) isconsiderably more positive than the values forcalcite nodules, although the oxygen isotopicvalue of the sparry calcite vein ()6Æ1&) fallswithin the range of the calcite nodules (Fig. 13).Also present in the interfluve and fluvial-terracepalaeosols is ankerite [Ca(Mg, Fe+2, Mn)(CO3)2]which was identified in three palaeosols (Fig. 11,DA-2i).
Petrographically, ankerite is expressed as irreg-ular zones and veins with high refractive indexwithin calcite nodules, and as thin (millimetre-scale) coats around calcite nodules. Both thecarbon and oxygen isotopic values of ankerites,which range from )0Æ6& to )2Æ0& and )1Æ7& to)3Æ7&, respectively, are considerably morepositive than those of calcite nodules (Fig. 13).
Marine Diplocraterion and Skolithos burrowsare present in four interfluve palaeosols (Figs 10and 11). These dwelling and escape burrowsextend downward for up to 20 cm from the top ofthe palaeosol profile, are positioned directlybeneath marine ravinement surfaces and werepassively filled with sediment from the overlyingmarine facies. The marine burrows developedduring sea-level rise on muddy sediment whichwas made firm by desiccation during pedogenesisand, as such, belong to the Glossifungites ichno-facies (Pemberton & Frey, 1985; Driese & Fore-man, 1991; Mack et al., 2003a).
Interpretation of interfluve and fluvial-terracepalaeosols
The interfluve and fluvial-terrace palaeosols ofthe Abo Member are unusual because of thecoexistence in the same horizon of pedogenicfeatures that generally form in different soilenvironments. For example, calcite nodules,argillans, vertical root traces and vertic featuresare created in well-drained soils within thevadose zone (Buol et al., 1997; Driese & Ober,2005). This observation is consistent with thecarbon and oxygen isotopic values of nodularcalcite from the Abo Member, which are similarto those of Lower Permian palaeosol calcitecollected from other fully terrestrial basins (Macket al., 1991; Mora et al., 1996; Ekart et al., 1999;
Tabor et al., 2004; Montanez et al., 2007). How-ever, the range of isotopic values in calcitenodules of the Abo Member is highly variableand the lowest oxygen values approach that ofankerite, suggesting that some alteration of pedo-genic calcite may have occurred in contact withmarine or brackish water. The relationship be-tween the depth to the top of the Bk horizon andmean annual precipitation noted by Retallack(2005) is not applicable to Abo palaeosols,because all but one have erosional upper surfaces.However, the relatively thick (50 to 80 cm),diffuse Bk horizons in Abo palaeosols are similarto those in modern areas with monsoonal, trop-ical climate (Retallack, 2005), supporting theinterpretation based on vertic features for sea-sonal precipitation.
In contrast to the vadose features describedabove, gley features are produced in waterloggedsoils, where translocation of soil material andvertical root growth are rare (Boersma et al., 1972;Vepraskas & Wilding, 1983; PiPujol & Bermann,1994; Buol et al., 1997; Vepraskas, 1999; Driese &Ober, 2005). The low Al+3 substitution values inAbo Member goethites are similar to those ingoethite nodules precipitated from ground waterdeposits, rather than goethite nodules that grow asa result of high Al-activity in soil solutions fromweathering of aluminosilicates (Siehl & Thein,1989). The source of iron in goethite may have beenweathering of iron-bearing minerals in a poorly
–10 –8 –6 –4 –2 0
0
–2
–4
–6
–8
δ
δ
13
18
C
O
PDB
PDB
Calcite nodules,Dona Ana Mts
Calcite nodules,Robledo Mts
Ankerite, DonaAna Mts
Ankerite,Robledo Mts
Sparry calcitevein, Dona AnaMts
Ankerite
Calcite nodules
Sparrycalcitevein
Fig. 13. Stable carbon and oxygen isotopes of calcitenodules, ankerite and a sparry calcite vein from AboMember palaeosols. Each data point represents themean of two to seven samples from each site, with theexception of the sparry calcite vein which represents asingle analysis. Standard deviations for each of themultisample analyses are <0Æ8&.
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drained profile. Goethite crystallization probablyresulted from migration of ferrous iron-bearingfluids to better oxidized areas within the profilethat were permissive of ferric iron as the dominantvalence state, and/or to fluctuations in redox statein the profile which led to in situ oxidation offerrous iron-bearing minerals such as pyrite.
Sparry calcite veins probably also precipitatedbelow the water table or in a narrow zone offluctuating water table, based on horizontal tosub-horizontal orientations, coarse crystal sizeand sharp upper and lower contacts (Mack et al.,2000). A locally reducing environment in awaterlogged soil is also suggested for the forma-tion of ankerite, because it contains ferrous iron(Tucker & Wright, 1990).
Field and petrographic evidence suggest thatthose pedogenic features associated with water-logged conditions developed late in the history ofdevelopment of the interfluve and fluvial-terracepalaeosols. For example, sparry calcite veins andgley colour mottling cross-cut, and thus post-date,calcic and vertic horizons. Moreover, a secondaryorigin for goethite is indicated, because it coatspre-existing wedge-shaped and blocky peds,calcite nodules and root traces, and it partiallyreplaces calcite nodules (corona structures).Ankerite also is interpreted as being secondaryin origin, because it cross-cuts and coats calcitenodules.
The relationships described above can beexplained if the interfluve and fluvial-terracepalaeosols of the Abo Member are polygenetic,produced by changing soil conditions throughtime. Unlike the definition of a polygeneticpalaeosol by Marriott & Wright (1993), in whichan original soil is modified from the top down-ward by a new soil created by changing environ-mental conditions, the Abo palaeosols weremodified from the bottom upward by a risingwater table. The primary soil features, such asvertical root traces, vertic features, argillans andcalcic nodules, formed when the interfluves orfluvial terraces occupied a position above thewater table. Stage II and incipient Stage IIImorphology of the calcic horizons suggests thatthe palaeosols developed in the vadose zone forthousands to tens of thousands of years (Gileet al., 1966, 1981; Machette, 1985). The palaeo-sols were subsequently invaded by the water tableduring sea-level rise and infilling of the incisedvalleys, which produced gley features, sparrycalcite veins and ankerite. A local marine contri-bution to the ground water due to mixing ofmeteoric water and sea water is consistent with
the carbon isotopic composition of the ankerites(Fig. 13), which are similar to values interpretedfor Early Permian sea water (Veizer et al., 1999).Similarly, the carbon isotopic value of the sparrycalcite vein is consistent with mixing of meteoricand sea water. However, its oxygen isotopic valueis similar to that of the calcite nodules and to thatof Pliocene–Pleistocene ground water calcites(Fig. 13; Hall et al., 2004), raising the possibilitythat the vein could have experienced diagenesisby meteoric water. Finally, the presence ofGlossifungites burrows indicates that some ofthe interfluve palaeosols were colonized bymarine burrowers following final marine inunda-tion of the interfluves. These vertical dwellingburrows are different from the feeding burrows inlowstand-fluvial palaeosols described below andmay be unique to interfluves and fluvial terraces.
PALAEOSOLS IN LOWSTAND-FLUVIALDEPOSITS
Pedogenic features and horizons
Two types of palaeosols are common withinlowstand-fluvial deposits of the Abo Member(Fig. 14). One type occupies the upper fewcentimetres of crevasse-splay siltstone beds or,less commonly, the uppermost beds of fluvial-channel deposits. These palaeosols are red incolour and display vertical root traces and/orsmall (<20 cm wide) in situ tree moulds. In a fewcases, insect feeding traces of the Scoyeniaichnofacies are present (Mack et al., 2003a).These rooted horizons are designated A horizonsif the original sedimentary layering has beensignificantly modified and C horizons if most ofthe sedimentary layering remains, although bothmay be present within the same profile (Fig. 14).
Clay Silt
Depth(cm)
0
50
100
Colour Horizons
Ripples 10R 4/2AC
Bssss
ss
ss
5RP 4/2
Lowstand-fluvial palaeosols
Fig. 14. Profiles of representative palaeosols devel-oped in lowstand-fluvial deposits. See Fig. 8 for key tosymbols. Depth refers to the position below the top ofthe palaeosol.
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The second type of palaeosol within the low-stand-fluvial deposits is restricted to red overbankmudstone and generally consists of a single vertichorizon (Bss), 20 to 100 cm thick, that is charac-terized by some combination of wedge-shapedand blocky peds, slickensides and desiccationcracks up to 70 cm deep (Fig. 12H). The verticfeatures gradually diminish in abundance down-ward into pedogenically unmodified mudstone,while the tops of the palaeosols are commonlytruncated by crevasse-splay or channel deposits.In a few cases, an A horizon may be distinguishedby a slightly higher concentration of fine roottraces than exists in the underlying Bss horizon.Less than a third of these palaeosols also havegley colour mottling and sparry calcite veinssimilar to those described in interfluve andfluvial-terrace palaeosols. However, there is nofield or petrographic evidence in the lowstand-fluvial palaeosols of calcite nodules, argillans,goethite nodules or goethite sesquans.
Interpretation of palaeosols in lowstand-fluvial deposits
The presence of vertical root traces is consistentwith development of the lowstand-fluvial palaeo-sols in the vadose zone, while red colour probablyformed in response to alteration during burial ofiron oxides and hydroxides to hematite (Retal-lack, 1991). The palaeosols with rooted A androoted A and C horizons could have developed inas little as one growing season, although the smalltrees would probably have required a decade ormore of growth. Similarly, vertic structures canform within a few hundred years, although theymay also exist on much older landscapes(Ahmad, 1983; Buol et al., 1997). The palaeosolsin the lowstand-fluvial deposits of the Abo Mem-ber most probably owe their immaturity to highsediment aggradation rates, which inhibited for-mation of the more mature features diagnostic ofthe interfluve/fluvial-terrace palaeosols, such asdevelopment of multiple horizons, Stage II calcitenodules and argillans. The presence of gleycolour mottling and sparry calcite veins in a fewof the vertic palaeosols suggests invasion by thewater table late in the history of their develop-ment (Sheldon, 2005).
DISCUSSION
Evidence is accumulating from studies of ancientstrata that palaeosols provide important clues for
the recognition of systems tracts and sequence-stratigraphic surfaces (Gibling & Bird, 1994;Aitken & Flint, 1996; O’Byrne & Flint, 1996;McCarthy & Plint, 1998; McCarthy et al., 1999;Plint et al., 2001; Unfar et al., 2001 Olszewski &Patzkowsky, 2003; Atchley et al., 2004). Duringthe creation of type 1 sequences, like those in theAbo Member, soils are predicted to developduring lowstand and early transgression, as wellas during highstand (Fig. 15).
Pedogenesis is a dominant process on theinterfluves, because of their position above theactive sites of incision and subsequent sedimen-tation. Following their abandonment, soilprocesses on fluvial terraces within incised val-leys should be similar to those on adjacentinterfluves, which is the case in the Abo Member.Interfluve and fluvial-terrace soils should be well-developed, because their history spans the timerequired to cut and subsequently backfill theincised valleys. Although the time required to dothis may vary significantly depending upon thedriving force behind sea-level change, it probablywill be at least in the order of several thousandyears or more. Interfluve/fluvial-terrace soils arealso expected to have several generations ofpedogenic features, because late in their historythey are likely to be invaded by a rising watertable in response to marine transgression anddeposition within the incised valley. The phre-atic water may be meteoric or a mix of meteoricand marine, and the soil may reside below thewater table long enough to develop a variety ofgley features. This history is evident in thepolygenetic interfluve and fluvial-terrace palaeo-sols of the Abo Member, as well as in previouslydescribed Carboniferous and Cretaceous inter-fluve palaeosols (Aitken & Flint, 1996; O’Byrne &Flint, 1996; McCarthy & Plint, 1998; McCarthyet al., 1999; Plint et al., 2001). However, in UpperCarboniferous strata described by Feldman et al.(2005), the interfluve palaeosols are not poly-genetic, and thin coals directly overlying theinterfluve palaeosols are interpreted as evidenceof a rising or perched water table associated withtransgression.
Soils may also form during lowstand and earlytransgression in an environment of net aggradationduring backfilling of incised valleys (Fig. 15A).The resultant soils will exist in floodplain sedi-ment or on the tops of abandoned channels andwill be part of the lowstand or lower transgressivesystems tracts, depending on the definition used indesignating systems tracts (Van Wagoner et al.,1990; Allen & Posamentier, 1993, 1994; Plint et al.,
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2001). Soils of the lowstand/early transgressivesystems tract are expected to be relatively im-mature, because of high sediment aggradation rates(Wright & Marriott, 1993). These soils may displaygley features if they originally developed in areas ofhigh water table or if they were invaded by a risingwater table during sea-level rise. The latter processmay occur episodically as a result of parasequence-level sea-level fluctuations (Unfar et al., 2001).These predictions are borne out in the AboMember, where lowstand-fluvial deposits consistof immature, red palaeosols, some of which dis-play gley colour mottling and sparry calcite veins.
Although not testable in the present study,Wright & Marriott (1993) predicted that duringhighstand, when the rate of creation of accom-modation space and rate of aggradation are low,palaeosols might develop on the coastal plain, butcould be eroded by meandering rivers. In somecases, however, relatively mature, welded palaeo-sols have been recognized in the uppermosthighstand systems tract (McCarthy et al., 1999;Plint et al., 2001).
Finally, polygenetic palaeosols similar to theinterfluve and fluvial-terrace palaeosols of theAbo Member may result from climate change
lowstand fluvial
Interfluve soil
Fluv
ial t
erra
ce so
il
incised valley
Transgressive marine
Progradingcoastal plain
floodplain soils
Lowstand and early transgressive systems tractsA
B Highstand systems tract
Incised valley
Aggradingfloodplain
soils
Aggrading
Palaeosols: (1) mature, polygenetic palaeosols on interfluves and fluvialterraces (2) immature, palaeosols on floodplain of aggrading incised valley.
Palaeosols: (3) maturity and survivability of palaeosols may depend onsediment aggradation rate and the rate at which rivers migrate laterallyacross their floodplain, while colour depends on depth to water table.
Rising water table
Lowstand fluvial2
3
Incised valleys
1
1 Lower transgressive systems tract
Estuary
Fig. 15. Schematic model predicting the location, relative maturity and history of soils that develop during thecreation of type 1 sequences. (A) Palaeosols predicted to develop in the lowstand and early transgressive systemstracts. (B) Palaeosols predicted to develop in the highstand systems tract on a prograding coastal plain.
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independent of sequence-stratigraphic position(Driese & Ober, 2005). Moreover, partial orcomplete erosion of interfluve and fluvial-terracepalaeosols before final burial might mitigate theiruse as indicators of sequence boundaries (O’By-rne & Flint, 1996), and intermittent depositionmight prevent an interfluve or fluvial-terracepalaeosol from attaining its full maturity and/ormight not allow the primary, well-drainedfeatures to develop fully (Aitken & Flint, 1996).For these reasons, it is advisable to use palaeosolsin conjunction with high-resolution correlation tointerpret sequence-stratigraphic relationships.
CONCLUSIONS
The Lower Permian Abo Member, south-centralNew Mexico, provides a test of currently evolvingideas concerning the role of palaeosols in theinterpretation of sequence stratigraphy. In the AboMember, sequence boundaries associated withincised valley floors can be correlated in outcropwith coeval interfluves and fluvial terraces.Mature, polygenetic interfluve and fluvial-terracepalaeosols consist of primary features that devel-oped in the vadose zone, such as vertical roottraces, vertic features, Stage II and III pedogeniccalcite and argillans. During subsequent sea-levelrise and backfilling of the incised valleys, theinterfluve and fluvial-terrace palaeosols wereinvaded by a meteoric to locally brackish watertable, producing low-aluminium goethite, gleycolour mottling, sparry calcite veins and ankerite.In contrast, lowstand-fluvial palaeosols createdduring backfilling of the incised valleys are redand display only root traces and vertic structures;their immaturity is a response to high aggrada-tion rates. Although there is a clear distinctionbetween interfluve/fluvial-terrace and lowstand-fluvial palaeosols in the Abo Member, palaeosolsshould be used in conjunction with high-resolution correlation to interpret sequence strati-graphy.
ACKNOWLEDGEMENTS
This research was supported in part by theGeological Society of America, the Rocky Moun-tain Section of SEPM, and the Clemons andWemlinger Scholarships at New Mexico StateUniversity. Steve Driese, Mary Kraus, CynthiaStyles, Paul McCarthy, Colin North, V. PaulWright, Stephen Lokier and an anonymous
reviewer read an earlier version of this articleand made helpful suggestions for its improvement.
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