Mannvile Sedimentology-Stratigraphy Article

Sedimentology (1991) 38,913-934

Significance of molluscan shell beds in sequence stratigraphy: an example from the Lower Cretaceous Mannville Group of Canada’

I N D R A N I L B A N E R J E E * and S U S A N M . K I D W E L L ?

*Institute of Sedimentary and Petroleum Geology, Calgary, Alberta, Canada ?Department of Geological Sciences, University of Chicago, Chicago, Illinois, USA

ABSTRACT

Detailed study of marine shales (the Ostracod zone) within a Cretaceous, third-order transgressive- regressive sequence in the Alberta Foreland Basin reveals a systematic association between shell beds and parasequence-scale flooding surfaces, including surfaces of maximum flooding. The Ostracod zone (a subsurface lithostratigraphic unit known as the Calcareous Member in outcrop) consists of 10-20 m of black shale and bioturbated sandstones with many thin, fossiliferouslimestones. Parasequences (shallowing- up cycles 2-3 m thick) were delineated within this transgressive unit based on lithology, sedimentary structures, degree of bioturbation, dinoflagellate diversity, total organic carbon and carbon/sulphur ratios ; many flooding surfaces are firmgrounds or hardgrounds.

Shell-rich limestones occur in three different positions relative to these flooding surfaces, and each has a distinctive bioclastic fabric and origin. (i) Base-of-parasequence shell beds (BOPS) lie on or just above flooding surfaces in the deepest water part of a parasequence; they are thin (up to a few centimetres), graded or amalgamated skeletal packstones/wackestones composed of well-sorted granular shell, and are interpreted as hydraulic event concentrations of exotic shell debris. (ii) Top-of-parasequence shell beds (TOPS) are capped by flooding surfaces at the top, shallowest water part of a parasequence; they typically are several decimetres thick, are physically amalgamated packstones/grainstones or bioturbated wackestones, and contain abundant whole as well as comminuted shells; these are composite, multiple- event concentrations of local shells. (iii) Mid-sequence shell beds rest on as well as are capped by firmgrounds or hardgrounds, and are intercalated between parasequences in the deepest water part of the larger sequence; they are laterally extensive lime mudstones a few decirnetres thick, with sparse shells in various states of dissolution, recrystallization and replacement; these beds are terrigenous-starved hiatal concentrations and record maximum flooding within the Ostracod zone. Offshore sections of the Ostracod zone typically contain several starved mid-sequence shell beds, underscoring the difficulty of identifying a single ‘maximum flooding surface’ within a third-order sequence.

INTRODUCTION

Sedimentologists have had a long-standing interest in the post-mortem histories (taphonomy) of shells, particularly for reconstructing palaeocurrent and diagenetic regimes (see Brett & Baird, 1986; Goldring, 1990, for reviews). Shell concentrations, on the other hand, have been less fully studied, even though coquinas are common and highly distinctive rock types. Shell-rich deposits range from rapidly generated storm and mass-mortality event concentrations, to

’ Geological Survey of Canada Contribution No. 12191.

accretionary biostromes and bioclastic facies, highly condensed hiatal concentrations, and erosional and corrosional lags (Fig. 1). Each of these types have different stratigraphic utility given the spectrum of time-scales over which they form and the variety of biological and physical processes that can be involved (see Kidwell, 1991).

The purpose of this investigation is to demonstrate the practical value of coquinas in subsurface sequence analysis, using a setting where the basic sequence stratigraphy has already been established on the basis

913

914

ecobgically brief episode of shell concentration

&

I . Banerjee and S . M . Kidwell

amabamation or r r r i ; e ; ; multiple

\ EVENT

average or mndensed CONCENTRATION

expanded section

preservation COMPOSITE CONCENTRATION

\

truncation of significant section by erosion/corrosion, strong taphonomic culling of bioclasts

Fig. 1. General, qualitative types of shell concentrations, based on inferred histories of accumulation (figure simplified from Kidwell, 1991).

of conventional lithological and geophysical logging methods (Banerjee, 1990a). The Ostracod zone of the Lower Cretaceous Mannville Group of Canada contains a series of thin, mollusc-bearing limestones and comprises the central, marine core of a 100-m- thick, third-order transgressive-regressive siliciclastic sequence in the Alberta Foreland Basin. Taphonomic analysis enabled us to differentiate several types of shell beds within these deposits based on biostrati- nomic features (e.g. degree of fragmentation, close- packing, orientation). Systematic patterns in the occurrence of these distinctive types, (i) at the base, or (ii) at the top of small-scale shallowing-up cycles (parasequences) or (iii) at the culmination of deepening-upwards trends among stacked parasequences, suggest that shell beds might be used to identify events of flooding and maximum transgression in these types of sedimentary basins.

STRATIGRAPHIC FRAMEWORK

Lower Cretaceous strata in southern Alberta, Canada, constitute a NE-tapering wedge of clastic sedimentary rocks in a foreland basin that received synorogenic detritus from the emerging Cordillera during the Columbian Orogeny (Porter et al., 1982). The lower- most unit in this sequence is known as the Mannville Group in the subsurface of the Interior Plains (equivalent to the Blairmore Group in the outcrop),

and ranges in age from Barremian to Middle Albian (Banerjee, 1990b). The Mannville Group extends eastward to the Cordilleran disturbed belt to the edge of the Canadian Shield in NE Alberta, Saskatchewan and Manitoba, where it crops out in a narrow, NW- trending belt that marks the erosional edge of the sedimentary prism.

Major lithologies, stratigraphic subdivisions and regional facies variation of the Mannville Group are summarized in Fig. 2. Although the stratigraphy of the Mannville Group has been formally recognized, its various subdivisions have not yet been formalized. Consequently, all lithostratigraphic names used in this study are informal.

Lithologically, the Mannville Group consists chiefly of interbedded shales and sandstones, many of which are horizons for producing oil and gas. Carbonate rocks are limited to a few thin, more or less extensive, fossiliferous limestone horizons that, together with black or dark grey shales, constitute the Ostracod zone (known in outcrop as the Calcareous member). Although there is a biostratigraphic connotation to the term ‘Ostracod zone’ (see Loranger, 1951), it has mostly been used in a lithostratigraphic sense (Hunt, 1950; Farshori, 1983; Finger, 1983; Banerjee & Davies, 1988). The Ostracod zone is recognized widely in Alberta, where it has been used as a marker unit in correlation.

Earlier authors believed that the Mannville Group is entirely or predominantly of non-marine origin

Molluscan shell beds in sequence stratigraphy 915

m :la SOUTH

Metres ' 0

c:y) EDMONTON J i j

r - . - - - . I ALBERTA I

SUNBURST

THICKER ( > 2 m ) SANDSTONE

THINNER l.2m) SANDSTONE.SHALE,COAL

L I M E S T O N E . S H E L L D E B R I S , BLACK SHALE

C H E R T BRECCIA

C O A L

Valleyfi l l I

t loo

I

200

300

I N D E X M A P

a Fig. 2. Stratigraphic cross-section of the Mannville Group along a line across southern Alberta and Montana.

(Glaister, 1959; Mellon, 1967; McLean & Wall, 1981). More recently, workers have recognized considerable marine influences in these sediments (Hopkins, 1981 ; Hopkinset al., 1982; Finger, 1983;Hradsky&Griffin, 1984; Jackson, 1985; Wanklyn, 1985; Banerjee & Davies, 1988) and have related the Mannville Group to the Clearwater transgression that straddles the Aptian-Albian boundary. According to available sea- level curves of the Cretaceous (Haq et al., 1987), this transgression is part of a third-order global transgression-regression cycle.

Sequence stratigraphy

Based on core and log studies by the first author, including detailed palynological studies, facies patterns in the Mannville Group can be cast in terms of a sequence stratigraphic model for the southern Interior Plains of Alberta (Fig. 3 ; depositional systems tract terminology follows Van Wagoner et al., 1988). The Mannville Group is bounded by two unconformities, of which the basal subcretaceous unconformity

is regional in extent. The upper unconformity is also regionally extensive, and is overlain either by the Joli Fou Shale or by the locally developed Basal Colorado Sandstone. Incised palaeovalleys filled with tidal sands have been identified in the upper unconformity, but this unconformity commonly is subtle lithologically. However, conspicuous changes in geochemistry, mineralogy, palynology, and micropalaeontology have been recorded across it (Banerjee, 1991).

The maximum flooding interval (MFI) within the Mannville sequence lies somewhere within the shaly Ostracod zone which constitutes the transgressive systems tract (TST in Fig. 3). The MFI is indicated either by an interval of thin (< 1 m) limestone beds with local submarine hardgrounds, or organic-rich black shale with high-diversity, open-marine dinoflagellates (Banerjee & Davies, 1988). Many cored sequences contain more than one intercalation of limestone or black shale and thus do not present a single, unambiguous maximum flooding surface, and therefore this part of the Ostracod zone is referred to here as the maximum flooding interval.

916 I. Banerjee and S . M . KidweN

t 375 km I

IVF ' \

200111

100

0

I V F incised valley-fill TST transgressive systems tract

MFS maximum flooding surface SB sequence boundary

TS transgressive surface HST highstand systems tract

Fig. 3. Sequence stratigraphy of the Mannville Group showing the upper and lower unconformities, the parasequence sets, and the probable maximum flooding surface within the Ostracod zone.

Sediments lying below the Ostracod zone have been interpreted as incised valley fills, which vary from marginal marine (Ellerslie member) to non-marine (Cutbank and Sunburst members) and contain paralic to freshwater palynomorphs (Banerjee & Davies, 1988; Banerjee, 1990b).

Sediments above the Ostracod zone constitute a two-part highstand systems tract. The lower part comprises a parasequence set (1 in Fig. 3) with northward-prograding downlapping surfaces (Glau- conitic Sandstone) which contains paralic to open- marine dinoflagellates (Banerjee, 1990b). The upper part of the highstand tract is an aggradational parasequence set (2 in Fig. 3) of coal-bearing cycles (Banerjee & Goodarzi, 1990).

Lithostratigraphic definition of the Ostracod zone

As defined in this study, the Ostracod zone is an interval 10-30m thick dominated by dark-grey to black shales and interbedded thin (< 1 m) limestone beds (with or without bioturbated sandstone beds), some of which are rich in molluscan shells. Ostracod remains are rare. The Ostracod zone rests with a sharp base on either a thick (> 5 m), typically wavy-bedded and bioturbated fine-grained sandstone (in core- section FCD 7722, this sandstone is capped by a

1 0 0 k m

Land

S e a O u t c r o p b e l t

- -__ E d g e of t h e S h o r e l ine d i s t u r b e d b e l t

Fig. 4. Schematic palaeogeography at the time of deposition of the Ostracod zone. Location of the four sections studied are shown. FCD (FCD 7722) and 11-8 (11-8-41-24W4) are wells and CR (Crescent Falls) and 541 (Hwy 541) are outcrops.


Skolithos-burrowed firmground), or a greenish or reddish shale (Hwy 541 outcrop section; see Figs 4 & lo). It is sharply overlain by a thick, medium-grained, cross-bedded or cross-laminated sandstone, which may be capped by a coal bed. The sharp base of the Ostracod zone is interpreted as indicating a sudden deepening or inundation event, and the top sandstone is interpreted as recording a shallowing event; the intervening black shale-rich Ostracod zone thus represents the transgressive systems trast of this third- order sequence (Fig. 5).

This lithostratigraphic definition of the Ostracod zone-black shale characterized by thin limestones and bounded by thick sandstones-is practical for use in outcrop as well as subsurface sequences, where both sandstones and limestones are geophysically distinctive. Although most of the limestones are very thin (<0-5 m), they have a characteristic geophysical signature (a low gamma ray, high-density (neutron) sharp-peaked log) and can be correlated over distances of the order of kilometres (Fig. 6) .

In the Crescent Falls outcrop (Fig. 4), this black shale-limestone interval has been named the Lower Moosebar Member (Taylor & Walker, 1984; Rosen- thal, 1988). These strata are included in our study owing to their similar lithological character, strati-

graphic position and age to rocks elsewhere in the basin known as the Ostracod zone.

LITHOLOGICAL CYCLES WITHIN THE OSTRACOD ZONE

The Ostracod zone consists of a series of relatively thin (3-5 m), predominantly asymmetrical, ‘shallowing-up’lithological cycles or parasequences (sensu Van Wagoner et al., 1988). This cyclical nature is illustrated for four sections, two outcropping and two subsurface, in Figs 7(b)-10. The base of each parasequence is generally sharply defined and marked by a thin shell bed, burrowed firmground or siderite horizon and is overlain by a series of lithologies indicative of successively shallower depths. The sand-to-mud ratio, degree of bioturbation, colour, and specific sedimentary structures were all used as depth indicators. The lithological compositions of parasequences vary considerably within each section: some grade from dark shale into wavy-bedded wackestone or hummocky cross-stratified sandstone whereas others encompass only part of this lithological spectrum (Figs 7-10). Although no single lithological composition is typical, each can be readily interpreted as a parasequence bounded at base and top by marine flooding surfaces

Surface (MF

Shallowing Upward Cycles 1,2,3 ’ (Parasequences)

‘ S )

ss a sandstone (or wackestone) (Top-of-Parasequence shell bed)

s h n black shale

1st limestone packstone/grainstone (Base-of-Parasequence shell bed) a lime mudstone (Mid-Sequence shell bed)

Fig. 5. Schematic diagram of the parasequences within the Ostracod zone, and distribution of shell bed (limestone) types. Compare a complex parasequence with the cross-section and photograph shown in Fig. 11.

918 I . Banerjee and S . M . Kidwell

Fig. 6. Correlation of thin (< 1 m) limestone beds within the Ostracod zone between close-spaced wells along a line from well 11-8 which is labelled 1 here (see Fig. 4 for location of well 11-8). All geophysical logs, except the one marked SP, are gamma ray.

(i.e. surface of sharp juxtaposition of deeper over shallower water deposits; see photographic part of Fig. 11).

The lithological compositions of parasequences vary not only within each section but between sections across the study area (Figs 7-10). Much of the between-section variation can be explained in terms of relative distance from the palaeoshoreline, as inferred from proportions of tide- (more onshore) and wave-dominated (more offshore) sedimentary structures. For example, tidal influence during deposition of the Hwy 541 outcrop section is indicated by the dominance of wavy-bedded heterolithic structures in sandstones lying below and above the Ostracod zone and in calcarenites within the zone. In contrast, wave dominance is indicated for the Crescent Falls outcrop section by (i) bimodally orientated gastropods in shell pavements (Fig. 12) and (ii) hummocky cross-stratified sandstones near the top of the section. Indicators of lower water energy and less freshwater influence are encountered in core-sections FCD 7722 and 11-8, which are located further east of the inferred palaeoshoreline (Fig. 4). More extensive bioturbation and higher dinoflagellate diversities indicate a more offshore location for these two core sections relative to the two outcrop sections (Hwy 541 and Crescent Falls).

Although most parasequences are asymmetrical and shallow upward, core section 11-8 contains several

‘deepening-up’ intervals that cannot be resolved into smaller parasequences and also contains one fully symmetrical cycle (‘deepening-up’ followed by ‘shallowing-up’; 5045-5064; Fig. 7).

Distribution of shell beds

As shown in Figs 7-10, shell beds are not distributed randomly with respect to these lithological cycles. Instead, they are closely associated with the flooding surfaces that bound parasequences. Shell beds tend to occur (i) resting on the flooding surface at the base of a parasequence (BOP shell bed), (ii) lying just below the flooding surface that defines the top of a parasequence (TOP shell bed), or (iii) lying at the top of a period of deepening or between two parasequences, and bounded both top and bottom by firmgrounds or hardgrounds (which may represent flooding surfaces). We refer to these last examples as mid- sequence beds, to indicate their position within the deepest water part of the Ostracod zone. As described in the following section, BOP and TOP shell beds in the Ostracod zone are skeletal grainstones, packstones, or wackestones, whereas the mid-sequence shell beds are red, brown, or grey lime mudstones with highly dispersed shells that have been largely obliterated by neomorphic diagenesis.

The relative abundance of shell bed types varies across the study area and is related at least in part to


( 0 ) SHELL BEDS

T Top-of-Parasequence shell bed

pa = physically amalgamated b bioturbated

B Base-of-Parasequence shell bed

p a = physically amalgamated gr =graded g =gastropod-rich

M Mid-Sequence shell bed

b = bioturbated I =laminated s = sideritic

sharp contact - burrowed firmground - H bored hardground - _ _ _ rapidly gradational contact - undulatory contact

sampling level

(scoured? loaded?)

HCS hummocky cross-stratification

palaeogeographical variation in lithological cycles (Figs 7-10). Mid-sequence lime mudstones are most numerous in offshore sections (e.g. core-section 11-8 in Fig. 7b, and cores taken further east in the basin) and are represented in most sections of the Ostracod zone by at least one example. BOP and TOP shell beds become less common southward in the outcrop belt and are poorly represented south of the Hwy 541 section.

The lithology representing the deepest water and most distal position within the Ostracod zone is the lime mudstone. Its distal setting is inferred from its association with fissile black shales and dark terrigenous mudstones, which is corroborated by a high content of total organic carbon (TOC), low carbon-to- sulphur (CjS) ratios and high dinoflagellate diversities

Fig. 7. (a) Key to Figs 7(bt10. (b) Stratigraphic column at location 11-8 (subsurface). TOC = total organic carbon, CjS = carbon-to-sulphur ratio. Note presence of shallowing up cycles (parasequences).

LITHOLOGICAL CYCLES

5010'

5020'

f

5040'-

5050'-

5060'-

5070'-

5080'-

5090- 0 u


WELL DEPTH

3

I1

g

F

E

!

e F

3

F

I I I I I I I I , ! L I , ,

c Deeper Shallower + 5 0 --

Fig. 8. Stratigraphic column at FCD 77-22 (subsurface) (see Fig. 7a for key to symbols).

in these intervals (see section on Marine Indicators and Figs 7-10). The lithologies representative of the shallowest water are the physically amalgamated wackestones/grainstones and the sandstones.

TAPHONOMY OF OSTRACOD ZONE S H E L L BEDS

Shell beds from different stratigraphic positions can be distinguished in core and outcrop on the basis of

(i) bioclastic fabric, i.e. close packing, size sorting and orientation of shells, (ii) microstratigraphic complex- ity, (iii) association with sedimentary structures and with discontinuity surfaces in particular, (iv) physical scale or dimensions, and (v) bed geometry, following the terminology of Kidwell et al. (1986) (Fig. 13). Quality of shell preservation also varies dramatically.

All the shell beds are composed of low-diversity molluscan faunas and are usually dominated by a single bivalve or gastropod taxon. However, identification to the genus level (and in some cases to the

Molluscan shell beds in sequence stratigraphy 92 1

LITHOLOGICAL CYCLES

cis DlNOFl AGFI I ATF

I l l 1 I I I I 1 I I I I I

c Deeper Shallower-,

I I I 1 I I I I

I

u 5 u

I I I I I I I I

I I I It b

I

I 0 u-l

Fig. 9. Stratigraphic column at Crescent Falls (outcrop) (see Fig. 7a for key to symbols).

family level) is not always possible, owing to the incompletely monographed fauna, tightly cemented matrix and requirement of non-destructive analysis of core samples. Poor preservation-highly comminuted debris, diagenetic ghost fabrics, abraded hinge-lines and ornamentation-also limits precise taxonomic assignments in some beds. Taxonomic identifications in this paper are thus provisional.

Base of parasequence (BOP) shell beds

Not all flooding surfaces in the Ostracod zone are mantled by a shell bed (Figs 7-10). However, when a shell bed does occur in this position, it is usually a highly distinctive, well-sorted, densely packed layer

of granular shell debris (fragments < 5 mm, usually 1-3 mm; Fig. 14A) with a matrix of quartz sand and calcareous mud. These BOP shell beds are very thin (0.5-3 cm) and graded, suggesting rapid redeposition from turbulent, probably storm-induced suspension transport (Fig. 14A). At the base of some parasequences, several of these discrete event concentrations may be accreted into thicker accumulations (up to 10 cm); successive 'events' are separated by finer- grained material or have cross-cutting relations (Fig. 14B).

Taxonomic identification of comminuted debris is impossible, other than 'probably bivalve'. The well- sorted nature of the shell debris, however, and its occurrence within intervals of otherwise unfossilifer-


LITHOLOGICAL CYCLES

10- ~

L I 5 0 greenish

C I I I , I I I I l 9 1 I I I I 1 1 1 I , I I

+Deeper Shallower-,

Fig. 10. Stratigraphic column at Hwy 541 (outcrop).

ous black shale (Fig. 7), suggest hydraulic transport from some other habitat. The high fragmentation suggests that, before the final event(s) of storm transport and redeposition, the shells resided in some intervening environment where post-mortem reworking could comminute at least part of the death assemblage. We thus postulate that shell transport was primarily in an offshore direction.

An alternative explanation for these BOPS could be transgressive, erosional reworking of shells from older, truncated deposits (i.e. that the beds are erosional lags; Fig. 1). Several features argue against this interpretation. These include (i) the absence of evidence for erosion associated with the flooding surfaces, (ii) the relatively good mineralogical condi- tion or simple diagenesis/replacement features of the shells, which is inconsistent with exhumation after partial diagenesis, and (iii) the common superposition of BOPS on unfossiliferous facies, which would have been poor sources for shell debris (Fig. 7b).

Top of parasequence (TOP) shell beds

TOP shell beds are the most variable shell bed types in the Ostracod zone (Fig. 13) and include the thickest shell beds (25 cm up to 1 m). Their most distinctive features are that they almost always have complex microstratigraphies and that they include abundant whole shells as well as comminuted debris.

Shells in TOPS may be (i) dispersed or loosely packed in a bioturbated wackestone/floatstone-type fabric (Fig. 1 SA), (ii) densely packed in discrete event- type beds separated by shell-poor sediment and/or burrowed firmgrounds (Fig. lSB), or (iii) densely packed with successive event concentrations tightly accreted to earlier concentrations (Fig. 15C). Shell- sheltered mud (S in Fig. 15C) indicates incomplete hydraulic reworking, but mud pods (e.g. Fig. 16C) suggest burrow-fills.

Shell material is generally poorly sorted, consisting of both whole and broken valves (Figs 15 & 16). Large

auc

PMPZ- 1 P-8- 1 1

EZ6


# N

Apex of conlcel

gastropods 6 n = 4 3

I 1 measurement b (right valve)

n = 2 9

I of pelacypods

0

Fig. 12. Shell pavement in the Crescent Falls section (5.0 m above the base) with palaeocurrent data from shell orientation displaying a strong biomodal pattern. Dominant bivalve is Corbiculu (Leptesthes) aff. C.(L.)fructu (Meek) (E. G . Kaufmann, pen. comm.).

shells and shell fragments in well-bedded concentrations tend to be strongly orientated, either parallel or oblique to bedding (Fig. 1 SC). Stacks of disarticulated bivalves, either convex-up or convex-down, may be produced by bioturbators (e.g. Salazar-Jimenez et al.,

1982; Fiirsich & Kirkland, 1986), but the thick-shelled nature of Ostracod zone examples (e.g. Fig. 16B) more probably indicates deposition by highly turbulent storms (see review by Kidwell & Bosence, 1991).

TOP shell beds vary not only within a single section,


BASE-OF-PAAASEQUENCE SHELL BEDS (BOB) (rest on flooding surface)

HYDRAULIC EVENT- cc"s Tm microgradod scoured base shell hash,

graded shell hash 8 grainstone, scoured base, laminated top

TOP-OF-PARASEQUENCE SHELL BEDS (TOB) (capped by flooding surface)

CGMfXEITE~ENTIWTKNS complex microstratigraphy of hydraulic event- concentrations. burrow-fills. grainstone with mud layers

A

MID-SEQUENCE SHELL BEDS (rest on 8 capped by flooding surfaces in deepest water part of sequence)

HIATAL CONCENlRATKW diagenetic shell ghosts in lime mudstone; more disseminated pyrite, phosphatization +I- hardground

Fig. 13. Shell beds occur in three positions relative to 2-3-m- thick, shallowing-up parasequences within the Ostracod zone; each position is characterized by shell beds of a different genetic type. For each sketch, the scale bar represents 10 cm.

but also between sections. Along the basin margin (outcrop sections at Crescent Falls and Hwy 541 ; Figs 9 & 10) there is little evidence of biogenic disruption but hydraulic fabrics are prominent (e.g. bimodal orientations on bedding planes and edgewise fabrics, as well as shell stacks and size grading; Fig. 12). TOPS are also thickest and have fewest shell-poor intercala- tions along the basin margin (Hwy 541).

TOP shell beds also vary considerably in taxonomic composition, both within and between sections. Some examples consist almost entirely of high-spired rissoid gastropods (e.g. between 7.0 and 8.0 m in outcrop section Hwy 541 ; at 11.7 m from the base in outcrop section Crescent Falls; Fig. 9) whereas others have mixtures of bivalves and subsidiary gastropods (Figs 12, 16B & 17A) or consist almost entirely of bivalves (most examples). Assemblages with abundant bivalves may be dominated by a single species (e.g. by a probable corbulid in Fig. 15A; by a corbiculid bivalve species in Figs 12 & 15C), whereas others have more even representation of different types (Fig. 16C). Gastropod-dominated assemblages are restricted to the most shoreward sections, and thicker-shelled bivalves appear to be more common in the same sections. More detailed faunal trends cannot be determined without finer taxonomic resolution.

The mixed quality of preservation and broad size range of shells observed in most TOPS, along with crude variation in taxonomic composition between examples, argue against extensive lateral transport and homogenization of molluscan death assemblages. Coarse shell material is thus thought to be primarily local in origin with small-scale physical and/or biogenic reworking responsible for disarticulating and

Fig. 14. Examples of hydraulic event concentrations that rest on the basal flooding surfaces of Ostracod zone parasequences (BOPS). (A) Simple graded bed (4262 in core 15-21-47-23W4 located 50 km north of 11-8 in Fig. 4). (B) Example with subtle rnicrostratigraphyof finer and coarser debris, suggesting accretion of multiple events (5036 in core 11-8, Fig. 7b).


Fig. 15. Composite concentrations found at the tops of parasequences show a range of bedding features and bioclastic fabrics, indicating differences in the relative importance of physical and biological reworking. (A) Loosely packed, randomly orientated shells in bioturbated matrix with vague bedding (246 in core FCD77-22). (B) Discrete beds of densely packed, poorly sorted size-graded shells, with shell-poor interbeds and minor burrowed surfaces (5073' in core 11-8). Arrows indicate hydraulic events. (C) Densely packed, predominantly poorly sorted shells in amalgamated layers with either oblique (fair weather) or parallel (storm) orientations (253' in core FCD77-22). S = shell-sheltered mud.


Fig. 16. TOP shell beds in the Ostracod zone are usually composed of poorly sorted whole and broken shells (and see Fig. 15), but shells can be well sorted and/or have highly ordered fabrics. (A) Burrow disrupted layers of very fine, largely well-sorted shell debris (501 1’ in core 11-8). (B) Stacks of densely packed, convex-up and convex-down bivalves; probably stormgenerated (10.1 m from base, Crescent Falls). (C) Densely packed, poorly sorted fabric with pods of shell- poor mud under shell-shelters or as burrow-fills (290 in core FCD77-22).M=mud pods.

rotating specimens from life positions. Finely comminuted shell debris in some TOPS (Figs 15B & 16A) may be derived from exotic sources.

The abundance of comminuted shells, particularly in the thickest shell beds (Fig. lo), suggests that TOP shell beds accumulated in environments closer to

source areas than did BOP shell beds. Hwy 541 might in fact record the environment that served as a temporary reservoir for granular shell debris, lying palaeogeographically between the environment of ‘shell threshing’ and the offshore environments that received deposits of storm-mobilized portions. The


preservation of many whole shells, the sharp-edged fragments and the lack of intense boring, encrustation, or colonization by larger sessile benthos all suggest that shells were neither continuously reworked nor exposed for long periods on the sea floor after death.

We thus interpret the TOP shell beds to be largely within-habitat accumulations of molluscs formed by relatively complex histories of repeated physical reworking, biogenic modification and benthic recolon- ization in habitable marine environments. Death assemblages were reworked and accreted to or amalgamated with earlier assemblages during successive concentration events and then, generally sooner

rather than later, buried by a veneer of siliciclastic sediment that became a substratum for colonization by new bioturbating and/or shell-producing orga- nisms. There is little evidence that the accumulation of dead shells influenced the composition of local benthic communities (i.e. taphonomic feedback).

The variability of TOPS-from predominantly physically stratified to pervasively bioturbated (Fig. 15B)-suggests that they could form across a bathymetric spectrum within the Clearwater sea. The indigenous nature of the shell assemblages and the suite of associated sedimentary structures indicate aerobic environments entirely above average storm

Fig. 17. Mid-sequence shell beds in the Ostracod zone are light-coloured lime mudstones or wackstones with dispersed, randomly orientated ghosts of small, thin shells. (A) Example from core 1 1-8 (5050) has a layer of well-preserved shell plastered on the underside (=remnant of TOP shell bed from underlying parasequence?). H = hardground. (B) Enlarged view of basal firmground.


wave base. None of the TOPS shows evidence for significant stratigraphic condensation; all would be categorized as accretionary or amalgamated composite concentrations (Fig. 1).

Mid-sequence shell beds

These shell-poor, structureless lime mudstones are the least common type of carbonate accumulation in the Ostracod zone. Remarkably uniform in thickness (c. 30 cm; Fig. 7), they are light grey, tan, or reddish depending upon oxidation of incorporated pyrite, and usually have sharp upper and lower contacts (burrowed firmground or bored hardground). They are typically sideritic in outcrop and contain little terrigenous contaminants.

These sparsely fossiliferous 'shell beds' occur either at the top of deepening-up intervals or sandwiched between two shallowing-up parasequences (Figs 7- lo). They are physically distinct from adjacent parasequences, bounded at both top and bottom by burrowed firmgrounds or hardgrounds. They occur in the part of the Ostracod zone where black shales and dark mudstones are most abundant, i.e. in the deepest water 'core' of the zone in individual sections, and in sections located most distally from the palaeoshoreline. They are called mid-sequence shell beds because of this stratigraphic position of maximum water depth and distality within the third-order transgressive- regressive Clearwater sequence (Fig. 5).

Many shells in these concentrations are preserved only as diagenetic ghost fabrics and the fossil assemblage is thus difficult to characterize either taxonomically or taphonomically. Some of the micritic limestones have shell material at their base (e.g. at 5050 in core section 11-8; Figs 7b & 17). This consists of poorly sorted, disarticulated whole and fragmented bivalves similar to features observed in some TOP shell beds (e.g. like parts of 501 1' in core-section 11-8; Fig. 16A). The mid-sequence shell beds themselves appear ' to be structureless rather than microstratigraphically complex. Bioturbation predominates over physical sedimentary structures in locally bioclast- rich parts.

We have no reason to suspect that shells in these shell beds were not produced locally. Although surrounding lithologies are primarily black shales lacking trace fossils, the lime mudstones themselves and the dark mudstones that immediately under- or overlie them (Fig. 7b) are structureless or mottled in ways suggesting bioturbation. In addition, the firm-

grounds and hardgrounds that define the lower and upper edges of the limestones are burrowed or bored, and the limestones themselves are light in colour (Fig. 17A). It thus appears that, at some water depth below the bathymetric zone of black shale deposition, bottom waters were sufficiently aerated to support benthic life (Fig. 18). Dysaerobic levels are suggested for the dark mudstone facies, which are bioturbated but lack shelly body fossils, whereas more fully aerobic conditions are indicated for the light-coloured biomi- critic limestones. These mid-sequence shell beds might have benefited from pelagic carbonate rain as well as benthic shell production, but thiscannot be established with certainty.

Diagenetically, the mid-sequence limestones are complex and distinctive. Shells are found in all stages of dissolution, recrystallization, spar-infilling and replacement by pyrite. Pyrite also occurs as shell coatings. Small (c. 1 mm) phosphatic nodules as well as fish teeth and bones are present in most examples, along with some fragments of lime mudstone (intra- clasts of hardgrounds?). Shells exhibiting brittle deformation are concentrated along some stylolitic horizons, associated with bands and wisps of insoluble material.

Individual mid-sequence limestones are traceable over kilometres at least (Fig. 6), based upon geophysical logs. They show little variation in composition, either laterally or from bed to bed, but do increase in number in an inferred offshore direction. This latter point is consistent with our interpretation of these limestones as terrigenous-starved deposits that inter- finger with the distal edges of backstepping and later downlapping parasequences within the Ostracod zone (Fig. 5). We anticipate that in more offshore parts of the basin than examined here, many individual mid- sequence limestones converge stratigraphically into a single condensed section (sensu Van Wagoner et al., 1988).

The taphonomically and diagenetically complex nature of mid-sequence limestones might serve as a means of distinguishing this type of deposit, which marks the deep-water core of the sequence and the distal limits of backstepping and downlapping parasequences, from the very shell-rich limestones (BOPS and TOPS) that mark shallower water flooding surfaces. Mid-sequence limestones are also distinguished by their lateral continuity and homogeneity, both as individuals and as a group. The coincidence of a palynological biozone boundary (Zone 3/Zone 4 of Banerjee & Davies, 1988) with one of these mid- sequence limestones in core section 11-8 (Fig. 7b) is

9 30 I . Banerjee and S . M . Kidwell

Sea Level

SCHEMATIC LITHOLOGICAL C F W B

Wackestone t

B lack Shale Base-of-Paraseque

Maximum water depth

ence I1 bed

lchnofabr ic (Droser 8 Bott ler. 1966)

-Deep Shallow-

SS Sandstone 7 HCS SH Shale SI Si l ts tone

F W B Fairweather Wave Base Dysaerobic S W B Storm Wave Base HCS Hummocky Cross -s t ra t i f i ca t i on

0 Aerobic ?-.% Flaser Bedding

% Wavy Bedding

Anaerobic Q Lenticular Bedding

Fig. 18. Schematic environmental profile showing distribution of mid-depth anoxic zone and areas of shell bed accumulation. The vertical profile shows positions of shell beds in an ideal section and can be compared with Figs 7-10.

consistent with our characterization of these as a variety of condensed hiatal concentration (Fig. 1).

Other taphonomic observations

Other than the generally thin, physically discrete shell concentrations described here, shell material is ex- tremely rare in the Ostracod zone. There are a few shelly burrow-fills and a few thin intervals of scattered shells in dark mudstones.

At least two explanations are possible for this rather dichotomous, ‘all-or nothing’ distribution of shells. One is that benthic colonization (and/or influx of exotic shells) was itself episodic: perhaps raw shell input tended to be either quite high or quite low, so that the sea floor was either shell-rich or shell-poor even before taphonomic processes came into play. Highly sporadic, opportunistic colonization is typical of many benthos (including Recent Corbula and Corbicula) in highly stressed or variable environments. An alternative or contributing possibility is that shells are more likely to be preserved if they have been concentrated with others (Kidwell, 1986, 1989). This might reflect some form of diagenetic feedback (e.g. self-buffering of porewaters by partial solution of shell; early cementation) or some inhibition of shell- dispersing bioturbators.

MARINE INDICATORS

Both C/S ratios and dinoflagellate diversity were used as independent measures of palaeosalinity. These data are plotted in Figs 7-10 against the lithological columns for comparative purposes.

Geochemical indicators

Twenty to 30 samples in each section were analysed for TOC by Rock-Eva1 pyrolysis. TOC values commonly range from 1 to 5%; type I1 kerogens indicative of marine basins are present in all four sections. Variation of TOC values in the black shales is generally thought to reflect oxygen levels at the time of deposition and ideally is inversely correlated with the degree of bioturbation in the sediments.

Total sulphur contents of black shale samples were determined by the LECO combustion method. Sul- phur values range from 0.1 to 2.7%, but values commonly fall between 0.5 and 1.0%. The samples in the two outcrop sections may have suffered some sulphur loss due to weathering.

The C/S ratio (Berner, 1984; Berner & Raiswell, 1984) has been used extensively as a palaeosalinity indicator, with values less than 5 considered typically marine. The values found in the four sections studied


here, however, fluctuate widely (1-40) (Fig. 7b). Moreover, in many instances these values contradict sedimentological or palynological evidence. For example, the maximum marine influence in core 11-8 (at 5033‘; Fig. 7b) documented by peak dinoflagellate diversity is contradicted by a high, freshwater-type C/S ratio. Such unusual behaviour for the C/S ratio has also been observed in the Western Interior Cretaceous Seaway in Alberta by Bloch (1989) who explained the phenomenon as a result of periods of rapid sedimentation which dampened or shut down the activities of sulphate-reducing bacteria, thus changing the C/S ratios. However, in the present instance, the erratic behaviour of the C/S ratio is most probably due to the high thermal maturity of the shales (T,,,,, of most shales has been found to be > 450OC). Both organic carbon and total sulphur have been affected by higher temperatures thus modifying the original C/S ratios in these shales.

Paly nological evidence

Many samples of the black shales yielded a fair number of dinoflagellates. The palynological analyses were done by Dr E. H. Davies of the Bujak-Davies Group of Calgary. Although the number of species found is not very high ( < 20), the presence of several stenohaline, open marine forms (Cyclonephelium, Oligosphaeridium and Odontochitina) indicates at least an inner shelf environment. The best preservation and occurrence of dinoflagellates were found in core- section 11-8, which on sedimentological grounds has already been interpreted as the most offshore section (Fig.4) of the four described here. Bases of the asymmetrical cycles that we interpret as marine flooding surfaces (parasequence boundaries) commonly coincide with peaks in the dinoflagellate diversity curve (Fig. 7b). The major deepening event in core section 11-8 (at 5057’) is also marked by a peak (about 5’ above it) in the diversity curve. Finally, the tops of all ,four shallowing-up cycles in that same section also coincide with lows or troughs in the dinoflagellate diversity curve (at 5028‘, 5038’, 5070’ and 5080 in core 11-8; Fig. 7).

Molluscan palaeoecology

The apparently low diversity of the molluscan fauna, like the palynological assemblages, is consistent with lower than normal marine salinities. Present-day corbulid and corbiculid bivalves characterize either

brackish or episodically brackish conditions. Abun- dant rissoacean gastropods are consistent with a shallow-water, nearshore or coastal setting.

In his palaeoecological study of the Ostracod zone from further north in Alberta, Wanklyn (1985) also found low-diversity molluscan assemblages dominated by these same taxa, plus other nearshore marine to brackish taxa (Cufhstina = Aphrodina, Modiolus, Cymbophora, Pyrigulifera, Tellina) and freshwater taxa (Sphaerium, Valvata, Pisidium). Similar mixtures of meso/oligohaline and freshwater faunas have been reported from shell beds in Albian black shales of Wyoming (Fiirsich & Kauffman, 1984), again with corbulid (Ursirivus) and corbiculid bivalves (Velori- tina) dominating.

PALAEOGEOGRAPHICAL FRAMEWORK

The low-diversity dinoflagellate and molluscan faunas both suggest variable palaeosalinity in this part of the Clearwater sea. These features have led some authors to conclude that the entire Ostracod zone was deposited in an estuary rather than in an open marine embayment (Finger, 1983). However, the large extent of this shale-limestone unit and the common presence of open marine dinoflagellates throughout the zone (with subsidiary foraminifera; Banerjee & Davies, 1988; Banerjee, 1990b) indicate that the environment was more like a shallow open shelf with episodic freshwater influence than a river mouth or lagoon. We envisage a broad, low-energy marine embayment (Banerjee, 1986) in this area during the Clearwater transgression, with either lateral mixing and/or fluctuating brackish and open marine waters. A large portion of this embayment was aerated, at least partially, including the deepest water portions (areas of dark mudstone and lime mudstone accumulation), but a well-developed oxygen-minimum zone (black fissile shale) existed at an intermediate depth below average storm wave base (Fig. 18).

The Ostracod zone sea has not as yet been mapped. However, the four sections described in detail here demonstrate some general bathymetric trends. Pro- gressively more offshore environments are recorded eastward from outcrop section Crescent Falls to core sections FCD7722 and 11-8; more onshore environments are recorded from Crescent Falls South to Hwy 541 within the outcrop belt. These relative positions roughly conform to the shoreline configuration delin-


eated by Jackson (1985) from regional palaeogeographical considerations, as shown in Fig. 4. Owing to the possibility of tectonic rotation of outcropping strata, absolute palaeocurrent directions cannot be determined, although we know from bioclastic fabrics and other sedimentary structures that palaeocurrent regimes were primarily tide dominated in the most onshore environments and wave dominated in the Crescent Falls area.

CONCLUSIONS: S T R A T I G R A P H I C U T I L I T Y OF O S T R A C O D Z O N E S H E L L

BEDS

The diverse array of shell concentrations in the Ostracod zone-including simple event concentrations of granular shell, complexly amalgamated/ accreted concentrations of whole and fragmented shells and presumably condensed hiatal lime mudstones-have many practical uses for stratigraphic and sedimentological analysis of the Clearwater sequence.

(1) All the shell beds are valuable for palaeoenviron- mental interpretation. In addition to conventional palaeoecological information on palaeosalinity, they provide unique information on palaeocurrent regimes, on sediment-transport directions, and on the short-term dynamics of sediment reworking and deposition.

(2) Virtually all examples of simple, single-event storm concentrations are composed exclusively of finely comminuted, exotic shell debris and lie at the base of shallowing-up cycles (parasequences). These BOP shell beds are valuable cues to flooding surfaces, particularly within the deeper part of the transgressive systems tract where flooding surfaces can otherwise be lithologically obscure (typically shale on silty shale firmgrounds, with or without burrows).

(3) Thicker, microstratigraphically complex shell beds record local reworking of indigenous death assemblages in a variety of relatively shallow- water environments (all above storm wave base and in consistently aerated waters) and mark the tops of many parasequences. These TOP shell beds thus also provide cues to flooding surfaces (which cap them), and are particularly useful in

the shallower part of the transgressive systems tract where base-of-cycle shell beds may be lacking.

(4) Shell-poor, diagenetically complex biomicrites are readily distinguished from the other types of shell beds, occur in the deepest water portion of the zone and are bounded by firmgrounds or hardgrounds. The sharp contacts are presumably parasequence-scale flooding surfaces. These mid- sequence shell-bearing limestones are lithologically and geophysically distinctive, and formed under conditions of sediment starvation in distal, aerated parts of the basin. They are laterally extensive and probably provide excellent chrono- stratigraphic markers.

Onshore sections of the Ostracod zone usually contain only one mid-sequence limestone, but offshore sections contain several such beds, underlining the difficulty of identifying a single ‘maximum flooding surface’ (sensu Van Wagoner etal . , 1988) within a third-order sequence. We thus refer to this deepest water part of the Ostracod zone, defined top and bottom by the stratigraphically highest and lowest deep-water biomicrites, as the maximum flooding interval of the sequence. Although we anticipate that many of these deep-water limestone beds converge stratigraphically into a more profoundly condensed deposit at some greater distance out in the Alberta Basin, it is obvious that stratigraphic subdivision using the concept of a maximum flooding surface rather than unconformities as proposed by Galloway (1989) is not straightforward in all parts of a basin, despite the other advantages of this approach.

Taphonomic analysis of the Ostracod zone thus reveals the sytematic association of distinctive types of shell beds (limestones) with stratigraphically im- portant flooding surfaces within a predominantly terrigenous transgressive systems tract. These beds can be recognized both in outcrop and in core, including using geophysical techniques. Not all im- portant surfaces are marked by a shell/bone bed: the formation of a bioclastic marker depends upon environmental controls on benthic colonization and shell transport (=shell supply), as well as upon terrigenous sedimentation and destructive diagenesis, whichdilute or remove shells. However, when present, shell-rich beds can be valuable tools for stratigraphic and sedimentological analysis, judging from the Ostracod zone and other sequences (Kidwell, 1989, 1991), and probably deserve more general attention from sedimentologists.


ACKNOWLEDGMENTS

The authors are indebted to Drs Jim Haggart and Terry Poulton of the Geological Survey of Canada, and Dr Erle Kaufmann of the University of Colorado for taxonomic advice; Dr Ashton Embry of the Institute of Sedimentary and Petroleum Geology for a review of an earlier version of this paper; Wayne Braunburger for field assistance; Bill Sharman of ISPG for photographic work. Drs M. Fowler and John Bloch of the ISPG advised on the significance of the geochemical data. Susan Kidwell was supported by NSF EAR85-52411 PYI and by a grant from Arco Foundation. Journal reviewers Drs A. G. Plint and R. L. Brenner made many helpful suggestions for improving the manuscript.

REFERENCES

BANERJEE, I. (1986) Lower Mannville sedimentation in the Edmonton “channel” in central Alberta. Pap. Geol. Sur. Can., Current Research Part B, 86-1B, 383-397.

BANERJEE, I. (1990a) Sequence stratigraphy of the Mannville Group, Southern Alberta, Canada. In: SOC. Econ. Paleont. Miner. Research Conference on “Cretaceous Resources, Event and Rhythms”, Program and Abstracts.

BANERJEE, I. (1990b) Some aspects of Lower Mannville sedimentation in southeastern Alberta. Geol. Sur. Pap. Can., 90-11,41 pp.

BANERJEE, I. (1991) Tidal sand sheets of the Late Albian Joli-Fou-Kiowa-Skull Creek marine transgression, West- ern Interior Seaway of North America. In: Clastic Tidal Sedirnentology (Ed. by D. G. Smith, G. E. Reinson, B. A. Zaitlin & R. A. Rahmani), Mem. Can. SOC. petrol. Geol., 16 (in press).

BANERJEE, I. & DAVIES, E.H. (1988) An integrated lithostratigraphic and palynostratigraphic study of the Ostracode zone and adjacent strata in the Edmonton Embayment, central Alberta. In: Sequences, Stratigraphy, Sedimentol- ogy: Surface and Subsurface (Ed. by D. P. James & D. A. Leckie), Mem. Can. SOC. petrol. Geol., 15,261-274.

BANERJEE, I. & GOODARZI, F. (1990) Paleoenvironment and sulfur-boron contents of the Mannville (Lower Creta- ceous) coals of southern Alberta, Canada. Sediment. Geol., 67,297-3 10.

BERNER, R.A. (1984) Sedimentary pyrite formation: an update. Geochim. Cosmochim. Acta, 48,605-61 5 .

BERNER, R.A. & RAISWELL, R. (1984) C/S method for distinguishing freshwater from marine rocks. Geology, 12,

BLOCH, J. (1989) Diagenesis and rock-fluid interaction of Cretaceous Harmon Member (Fort St. John Group) mudstones, AlbertaandBritish Columbia. PhD thesis, University of Calgary, 165 pp.

BRETT, C.E. &BAIRD, G.C. (1986) Comparative taphonomy: a key to paleo-environmental interpretation based on fossil preservation. Palaios, 1,207-227.

365-368.

FARSHORI, M.Z. (1983) Glauconitic sandstone, Countess field “H” pool, southern Alberta. In: Sedimentology of Selected Mesozoic Clastic Sequences (Ed. by J. R. McLean & G. E. Reinson), COREXPO ’83, Can. SOC. petrol. Geol., 2741.

FINGER, K.L. (1983) Observations on the Lower Cretaceous Ostracode zone of Alberta. Bull. Can. petrol. Geol., 31, 32G337.

FURSICH, F.T. & KAUFFMAN, E.G. (1984) Palaeoecology of marginal marine sedimentary cycles in the Albian Bear River Formation of south-western Wyoming. Palaeontol-

FURSICH, F.T. & KIRKLAND, J.I. (1986) Biostratinomy and paleoecology of a Cretaceous brackish lagoon. Palaios, 1,

GALLOWAY, W.E. (1989) Genetic stratigraphy sequences in basin analysis: architecture and genesisof flooding-surface bounded depositional units. Bull. Am. Ass. petrol. Geol.,

GLAISTER, R.P. (1959) Lower Cretaceousof southern Alberta and adjoining areas. Bull. Am. Ass. petrol. Geol., 43, 590- 640.

GOLDRING, R. (1 990) Fossils in the Field: Information Potential and Analogues. Longmans, London.

HAQ, B.U., HARDENBOL, J . & VAIL, P.R. (1987) Chronology of fluctuating sea levels since the Triassic. Science, 235,

HOPKINS, J.C. (1981) Sedimentology of quartzose sandstones of Lower Mannville and associated units, Medicine River area, central Alberta. Bull. Can. petrol. Geol., 29, 12-41.

HOPKINS, J.C., HERMANSON, S.W. & LANTON, D.C. (1982) Morphology of channels and channel-sand bodies in the Glauconitic Sandstone member (Upper Mannville), little Bow area, Alberta. Bull. Can. petrol. Geol., 30, 274-285.

HRADSKY, M. & GRIFFIN, M. (1984) Sandstone body geometry, reservoir quality and hydrocarbon trapping mechanisms in the Lower Cretaceous Mannville Group, Taber/Turin area, southern Alberta. In: The Mesozoic of Middle North America (Ed. by D. F. Stott & D. J. Glass), Mern. Can. SOC. petrol. Geol., 9,401-41 1.

HUNT, C.W. (1950) Preliminary report on Whitemud oil field, Alberta, Canada. Bull. Am. Ass. petrol. Geol., 34,

JACKSON, P.C. (1985) Paleogeography of the Lower Creta- ceous Mannville Group of Western Canada. In: Elm- worth-Case Study of a Deep Basin Gas Field (Ed. by J. A. Masters), Mem. Am. Ass. petrol. Geol., 38,49-77.

KIDWELL, S.M. (1986) Models for fossil concentrations : paleobiological implications. Paleobiology, 12,6-24.

KIDWELL, S.M. (1989) Stratigraphic condensation of marine transgressive records : origin of major shell deposits in the Miocene of Maryland. J. Geol., 97, 1-24.

KIDWELL, S.M. (1991) The stratigraphy of shell concentrations. In : Taphonomy-Releasing Information from the Fossil Record (Ed. by D. E. G. Briggs & P. A. Allison). Plenum Press, New York (in press).

KIDWELL, S.M. & BOSENCE, D.W.J. (1991) Taphonomy and time-averaging of shelly marine faunas. In: Taphonomy- Releasing Information from the Fossil Record (Ed. by D. E. G. Briggs & P. A. Allison). Plenum Press, New York (in press).

KIDWELL, S.M., FURSICH, F.T. & AIGNER, T. (1986)

ogy, 27,501-536.

543-560.

73,125-142.

1156-1167.

1795--1801.

934 I. Banerjee and S. M. Kidwell

Conceptual framework for the analysis and classification of shell concentrations. Palaios, 1,228-238.

LORANGER, D.M. (1951) Useful Blairmore microfossil zones in central and southern Alberta, Canada. Bull. Am. Ass. petrol. Geol., 35,2348-2361.

MCLEAN, J.R. & WALL, J.H. (1981) The Early Cretaceous Moosebar sea in Alberta. Bull. Can. petrol. Geol., 29, 334- 317.

MELLON, G.B. (1967) Stratigraphy and petrology of the Lower Cretaceous Blairmore and Mannville groups, Alberta foothills and plains. Bull. Res. Coun. Alta., 21, 270.

PORTER, J.W., PRICE, R.A. & MCCROSSAN, R.G. (1982)The Western Canada Sedimentary Basin. Phil. Trans. Roy. SOC. Lond., A305, 169-192.

ROSENTHAL, L. (1988) Wave dominated shorelines and incised channel trends : Lower Cretaceous Glauconite Formation, west-central Alberta. In: Sequences, Stratig- raphy, Sedimentology : Surface and Subsurface (Ed. by D. P.

James & D. A. Leckie), Mem. Can. SOC. petrol. Geol., 15,

SALAZAR-JIMENEZ, A,, FREY, R.W. & HOWARD, J.D. (1982) Concavity orientations of bivalve shells in estuarine and nearshore shelf sediments, Georgia. J. sedim. Petrol., 52,

TAYLOR, D.R. & WALKER, R.G. (1984) Depositional environments and paleogeography in the Albian Moosebar Formation and adjacent fluvial Gladstone and Beaver Mines formations, Alberta. Can. J . Earth Sci., 21, 698- 714.

VAN WAGONER, J.C., POSAMENTIER, H.W., MITCHUM, R.M., VAIL, P.R., SARG, J.F., LOUTIT, T.S. & HARDENBOL, J. (1988) An overview of the fundamentals of sequence stratigraphy and key definitions. Spec. publs SOC. econ. Paleont. Miner., 42, 39-45.

WANKLYN, R.P. (1 985) Stratigraphy and depositional environments of the Ostracode Member of the McMurray Formation (Lower Cretaceous; Late Aptian-Early Albian) in west- central Alberta. MSc thesis, University of Colorado, 289 pp.

207-220.

565-586.

(Manuscript received 27 December 1990; revision received 18 April 1991)

Mannvile Sedimentology-Stratigraphy Article

Documents

Transcript of Mannvile Sedimentology-Stratigraphy Article