Major Flooding Surfaces Research Articles

Defining lithic patterns within river-dominated delta deposits for geostatistical simulation

Reservoir development forecasts depend on accurate descriptions of the spatial distribution of rock properties that impact subsurface fluid-flow pathways and volume connectivity. Reservoir models constructed using geostatistical methods combine analogous facies dimension data with sparse subsurface data to predict spatial variations in rock properties. This study uses a physics-based depositional process model to define realistic facies variations within a river-dominated delta deposit formed during multiple shoreline regressions and transgressions. Geostatistical models are conditioned to varying amounts of information extracted from the depositional model to examine how well they reproduce the facies patterns. Reservoir simulation is used to examine the impact of analogous dimension data and varying conditioning constraints on reservoir performance predictions of water displacing oil. The dimensions of surface depositional features underestimate the continuity of preserved facies patterns, proportional grids following major flooding surfaces allow significantly better predictions than uniform rectangular grids, and trend constraints are more important when defined facies correlation length is significantly less than well spacing. When geostatistical model parameters are poorly chosen, reservoir simulation of the resulting weakly-structured facies patterns overpredict recovery and water breakthrough time. It is demonstrated that process-based depositional models can be used to optimize geostatistical model construction methods and input parameters to reduce uncertainty of reservoir development assessments.

Taphonomy of diploporite (Echinodermata) holdfasts from a Silurian hardground, southeastern Indiana, United States: palaeoecologic and stratigraphic significance

AbstractA microbioherm-bearing hardground within the middle Silurian (Wenlock) Massie Formation near Napoleon, southeastern Indiana, United States is encrusted by the attachment structures of numerous pelmatozoan echinoderms. Among the most common of these holdfasts are multi-plated discoidal structures representing the thecal attachments of diploporite ‘cystoids’. This large population of holdfasts permits the first detailed taphonomic and palaeoecologic study of hardground diploporite attachments, allowing for increased morphological understanding of these rarely studied structures and facilitating identification of holdfasts in deposits where they might have been overlooked or misidentified. The biostratinomic sequence commences with detachment of thecae, followed by weathering of isolated discoidal holdfasts to bring out radiating canal structures and plate sutures, eventually leading to removal of the interior floor to expose the underlying substrate. Continued exposure can result in separation of component holdfast plates, though cementation to the substrate prevents scattering of plates. Diagenetic precipitation of pyrite occurred after burial; the large size of crystals suggests late diagenesis, perhaps seeded by early diagenetic pyrite crystallites produced by decay of ligamentary tissue. Extrinsic taphonomic factors include overgrowth of holdfasts by laminar stenolaemate bryozoans and other echinoderm attachment structures. Diploporite holdfasts are not bored and are absent on microbioherms. Taphonomic data indicate the time-averaged nature of this hardground and its diploporite assemblage and permit prediction of similar occurrences at major flooding surfaces.

Crustal thickening or isostatic rebound of orogenic wedge deduced from tectonostratigraphic units in Indosinian foreland basin, Longmen Shan, China

Abstract Longmen Shan is located at the boundary between the Sichuan Basin and Tibetan Plateau, representing the steepest gradient of any edges of the plateau. Three endmember models of uplift process and mechanism have been proposed, including crustal thickening, crustal flow, and crustal isostatic rebound. Here, we use tectonostratigraphic units in the Late Triassic foreland basin to restraint uplift process and mechanism in the Longmen Shan during Indosinian orogeny. The Late Triassic foreland basin developed as a flexural foredeep on the Yangtze passive continental margin during the Indosinian orogeny. The basin fill includes the Maantang Formation, the Xiaotangzi Formation and the Xujiahe Formation, and it is divided into four tectonostratigraphic units with wedge-shaped or tabular cross-sectional geometry by unconformities and flooding surfaces in this paper. The first and third ones are wedge-shaped tectonostratigraphic units and correspond to underfilled condition with basal unconformities or major flooding surfaces, the high rate of subsidence and sediment accumulation, coarsening-upward succession and a dual sediment supply, and them may link to strong active thrust loading events or the rapidly advance rate of the orogenic wedge; the second and fourth ones are tabular tectonostratigraphic units and correspond to overfilled condition with the unconformity, the low rate of subsidence and sediment accumulation, fining-upward succession, a single sediment supply, the mass emergence of conglomerate layers, and they may be related to isostatic rebound and erosion unloading. Two endmember models of uplift process and mechanism in the Longmen Shan during Indosinian cycle have been proposed by coeval Late Triassic sedimentary sequences in the foreland basin here: (1) crustal thickening during the wedge-shaped tectonostratigraphic units, (2) crustal isostatic rebound during the tabular tectonostratigraphic units. This two endmember models proposed in this paper may be helpful to understand the mechanism of the Wenchuan Earthquake.

Sedimentology and sequence stratigraphy of the Middle–Upper Jurassic Krossfjord and Fensfjord formations, Troll Field, northern North Sea

The Middle–Upper Jurassic Krossfjord and Fensfjord formations are secondary reservoir targets in the super-giant Troll oil and gas field, Horda Platform, offshore Norway. The formations comprise sandstones ( c . 195 m thick) sourced from the Norwegian mainland to the east, that pinch out basinwards into offshore shales of the Heather Formation to the west. Sedimentological analysis of cores from the Troll Field has identified six facies associations, which represent wave- and tide-dominated deltaic, shoreline and shelf depositional environments. Resulting depositional models highlight the complex distribution of depositional environments, and reflect spatial and temporal variations in physical processes at the shoreline, rate of sediment supply and accommodation development. These models are further complicated by the absence of coastal plain facies, which implies that the Troll Field was fully subaqueous during deposition, that shoreline regression was forced by falling sea level or that coastal plain deposits were removed by transgression. Genetic sequences bounded by major flooding surfaces (‘series’) exhibit laterally uniform thicknesses, implying no major tectonic influence on sedimentation. The recognition of pronounced variability in facies character and stratigraphical architecture emphasize the need for a robust depositional model of the formations in order to drive future exploration in these, and coeval, reservoirs.

The Composition and Depositional Environments of Mesoarchean Iron Formations of the West Rand Group of the Witwatersrand Supergroup, South Africa

This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictiteassociated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted δ13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive postArchean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.

Wave-influenced deltaic sandstone bodies and offshore deposits in the Viking Formation, Hamilton Lake area, south-central Alberta, Canada

Abstract Subtle indications of deltaic influence can be overlooked in coarsening-upward units that resemble shoreface successions such as those found in the Cretaceous Viking Formation at Hamilton Lake (HL). Particular facies within HL strata contain physical and biogenic indicators of riverine discharge, and are interpreted as deltaic in origin. This study focuses on distinguishing between these deltaic deposits and strata reflecting normal-marine depositional conditions and relating facies within the stratigraphic framework. Based on detailed facies analysis, Viking deposits include: delta front, prodelta, upper offshore, lower offshore, shelf, slump and transgressive lag deposits. The principal group of facies, Facies Association 1 (FA1), is interpreted as deltaic in nature, which is indicated by evidence of: high sedimentation rates, salinity fluctuations, riverine discharge, hypopycnal and hyperpycnal flows, and moderately to subtly stressed ichnological suites. Trace fossil suites indicate suppression of suspension-feeding activities despite the presence of hummocky cross-stratified units that resemble shoreface deposition. Filter-feeding organisms are typically abundant in shoreface and wave-dominated deltaic successions. In the case of HL strata, hindrance of dwellings of inferred suspension-feeding animals in FA1 is attributed to increased water turbidity and suspended sediment associated with deltaic outflow. The presence of a restricted suite of structures of inferred suspension-feeders is uncharacteristic of river-dominated deltaic units, which are primarily reworked by deposit feeders. Accordingly, sedimentological evidence in combination with the proportion of dwellings of inferred suspension-feeders signifies moderate wave mitigation of environmental stresses induced by riverine discharge indicative of a wave-influenced deltaic system. In contrast to FA1, muddy sandstones to sandy mudstones of Facies Association 2 contain abundant and diverse suites of trace fossils interpreted to reflect deposition in the distal lower shoreface to upper offshore. Facies Association 3 is predominantly comprised of sandy mudstones with varying degrees of bioturbation indicative of lower offshore, shelfal, slump and transgressive lag deposits. The depositional history at HL includes periods of progradation alternating with transgressive flooding, subaerial exposure, stillstand progradation and wave ravinement. Four major bounding discontinuities and two major transgressive flooding surfaces separate units reflecting predominantly deltaic deposition, strictly offshore deposition, and mixed offshore and deltaic deposition. Variation in the deltaic and normal-marine signature of the deposits suggests changes in source area(s) over time, shifting shore-parallel deltaic influence, or fluctuations in riverine influx.

THE STRATIGRAPHIC DISTRIBUTION OF FOSSILS IN A TROPICAL CARBONATE SUCCESSION: ORDOVICIAN BIGHORN DOLOMITE, WYOMING, USA

Stratigraphic architecture exerts a powerful influence on the vertical distribution of fossils and should be considered before biostratigraphic and paleobiologic interpretations to avoid interpretations of patterns that are actually stratigraphic artifacts. Previous numerical models of the stratigraphic distribution of fossils were developed for siliciclastic settings, which also work well for mixed carbonate-siliciclastic settings. These models are evaluated here using the Upper Ordovician Bighorn Dolomite, which was deposited on a tropical carbonate platform. The Bighorn Dolomite contains three third-order, unconformity-bounded depositional sequences, which correspond closely to the Steamboat Point, Leigh, and Horseshoe Mountain Members and are correlative with the C2, C3, and C5 sequences of the eastern United States. The Steamboat Point and Leigh Members record greenhouse conditions and are characterized by meter-scale parasequences with weakly defined flooding surfaces and minor internal facies changes. Absent in these units are well-defined parasequence sets with strong upward-deepening or upward-shallowing trends. In contrast, the Horseshoe Mountain Member reflects a transitional climate and consists of 10-m-thick, well-developed parasequences that clearly stack into upward-deepening (transgressive) and upward-shallowing (highstand) systems tracts. Fossil associations within the Bighorn Dolomite fall into five biofacies that reflect depositional environment and age. The brachiopod biofacies is found in deep subtidal facies, with the gastropod facies and crinoid biofacies in shallow subtidal facies. The dasyclad biofacies and coral biofacies are limited to the shallow subtidal facies of the Steamboat Point and Horseshoe Mountain sequences, respectively. In the greenhouse sequences, biofacies transitions are gradational, whereas they are abrupt and correspond to major flooding surfaces in the transitional climate sequences. Similarly, first and last occurrences are not clustered in the greenhouse-climate sequences, although they are elevated near the maximum flooding zone. First and last occurrences are clustered in the transitional-climate sequence where open marine facies border the sequence boundary, at major flooding surfaces, and where open marine facies are last expressed.

High-resolution stratigraphy and depositional model of wind- and water-laid deposits in the ordovician Guaritas rift (Southernmost Brazil)

The upper portion of the Pedra Pintada Alloformation includes about 100 m of mostly eolian deposits. This paper emphasizes the vertical succession and lateral association of sedimentary facies, based on analysis of outcrop data and aerial photographs, as well as the hierarchy and origin of bounding surfaces. It aims to propose a high-resolution stratigraphic and depositional model that may be useful to exploitation of eolian reservoirs. The succession has been preserved due to basin subsidence, and is described in terms of four facies associations that constitute three dominantly eolian units. These units are sharply bounded by major flooding surfaces (super surfaces) that, in turn, are overlain by 1 to 2 m thick, dominantly water-laid facies (lacustrine, fluvial, deltaic and eolian). Both their internal organization and boundaries were controlled by changes in the base level rise rate. The basal Eolian Unit is composed of crescentic eolian dunes and damp interdune deposits ascribed to a wet eolian system. On the other hand, eolian units II and III, also characterized by crescentic eolian dunes (simple and compound) deposits, were related to dry eolian systems, since they comprise dry (eventually wet) interdune facies. Eolian Unit III is truncated by basinwide unconformity, which is then overlain by the ephemeral fluvial deposits (Varzinha Alloformation). This second type of super surface is related to climate-induced wind erosion (deflation) down to the water table level (regional Stokes surface) followed by fluvial incision linked to tectonic activity.

GRADIENT ECOLOGY OF A BIOTIC INVASION: BIOFACIES OF THE TYPE CINCINNATIAN SERIES (UPPER ORDOVICIAN), CINCINNATI, OHIO REGION, USA

Recent studies have emphasized that faunal change is typically brief and most commonly occurs at sequence boundaries and major flooding surfaces. The Upper Ordovician of the Cincinnati, Ohio, region records a major biotic invasion in the Richmondian Stage, which offers an opportunity to test these generalizations and to understand how episodes of faunal change are reflected in the structure of ecological gradients. The early Cincinnatian (C1–early C4 depositional sequences) displays two relatively stable faunal gradients, with the primary gradient reflecting onshore-offshore setting and the secondary gradient reflecting substrate consistency. During the mid-C4 sequence, dominant taxa of the shallow subtidal are extirpated, while deep subtidal taxa expand into those habitats, leading to a loss of cross-shelf faunal differentiation. This faunal breakdown is accompanied into the mid-C5 by a series of ecological epiboles, indicating an ongoing flux in ecological associations. The onshore-offshore gradient is reestablished in the C5, albeit with new associations dominated by or containing immigrant taxa. Recognition of this gradient is hindered by widespread increased abundance of bryozoans and by the delayed appearance of at least seven common genera of brachiopods and corals. The Richmondian Invasion plays out over multiple sequences and is not confined to a brief interval at the beginning of a sequence. These faunal changes do not coincide with sequence boundaries or major flooding surfaces and therefore cannot be sequence stratigraphic artifacts, nor can they represent a geologically instantaneous faunal response to sea-level change.

Patterns of early post-depositional and burial cementation in distal shallow-marine sandstones: Upper Cretaceous Kenilworth Member, Book Cliffs, Utah, USA

There is a limited understanding of the spatial linkage between cementation processes in shoreface/coastal plain successions and those operating in distal offshore successions. To address this, an integrated sedimentological, sequence stratigraphic and diagenetic study was undertaken on a distal shallow-marine succession, the Upper Cretaceous Kenilworth Member Book, Cliffs, Utah, USA. Laterally extensive carbonate cement is the most volumetrically significant diagenetic feature. It constitutes up to 55 vol.% of the distal lower-shoreface sandstone and sandy siltstone units beneath major flooding surfaces. During rising relative sea-level major flooding events resulted in low sedimentation, which extended residence time of sediments in early diagenetic zones and promoted enhanced cementation beneath these boundaries. Carbonate for early calcite cements was probably derived from sulphate reduction, marine water and oxidation of organic matter. Stable-isotopic composition of later ankerite ( δ 18O ∼ − 11.9 to − 6.0‰ VDPB, δ 13C ∼ − 4.5 to − 0.3‰ VDPB) suggests precipitation during progressive burial ( c. 1.7 to 3.4 km, 72 to 122 °C) from evolved marine fluids. The carbonate and other mineralizing solutes were derived, increasingly with burial, from decarboxylation of organic matter and clay mineral transformations in adjacent siltstones and shales. These interbedded, overlying and underlying facies provided an important source of mineralizing solutes that influenced the extent of pervasive cementation during burial. The combination of stratigraphic and lithofacies control resulted in laterally extensive cements that differ to shoreface successions , although are similar in form to those described for many offshore mudstone-dominated successions. Limited early concretionary ferroan dolomite beneath the expression of the sequence boundary and some flooding surfaces, however, suggests cementation by meteoric fluids at times of relative sea-level lowstand. Similar processes have been demonstrated in more proximal successions in the Book Cliffs.

Litho- and biofacies analyses of the Buckinghorse Formation: the Albian Western Interior Sea in northeastern British Columbia (Canada)

ABSTRACT During Albian time the Western Interior Sea formed an embayment of the northern Boreal Sea and paleogeography, depositional processes and relative sea level were controlled by a global sea level rise and active thrusting within the Cordillera. Along the Sikanni Chief River in northeastern British Columbia, the Buckinghorse Formation is exposed as an over 1000 m thick marine mudstone package that was deposited in the western foredeep of the Western Canada Sedimentary Basin. The section appears lithologically relatively uniform, however, close examination allowed recognition of seven lithofacies responding to subtle changes in shelf to upper offshore depositional environments. On regional wireline log cross-sections seven major flooding surfaces that subdivide the Buckinghorse Formation into six depositional sequences are widely correlative. These are independent from lithofacies. A new informal foraminiferal subzonation is proposed and correlated to the existing standard zonation based mainly on the intensely studied Peace River region to the south. In the northern continuous offshore setting foraminiferal assemblages responded with gradual faunal changeover. Each marine transgression brought new, mainly Boreal taxa to the basin. The Viking marker, described and faunally distinguished in the Hudson Hope area within the Hasler shale, is identified in the Sikanni Chief River area as a distinct silt to fine sandstone interval with a change from the Haplophragmoides gigas Zone to the Miliammina manitobensis Zone. On well logs this horizon is a major flooding surface, correlated with the top of the Paddy Member. End_Page 234------------------------

Stratigraphy of the Lower Paleozoic Haima Supergroup of Oman

ABSTRACT Following the discovery of significant gas/condensate reserves in Lower Paleozoic clastics in Central Oman a regional geological review using all available well data and a set of regional seismic lines was carried out by Petroleum Development Oman. The objective of this study was to support further appraisal and exploration activities. A chronostratigraphic framework of regional correlatable major flooding surfaces improved the understanding of the stratigraphic relationship between the different units and the regional distribution of the reservoir seal pairs. The interval of interest, the Cambrian to Lower Silurian Haima Supergroup, is a late syn- to post-rift siliciclastic infill of an extensive graben system. The depositional setting was initially continental but higher in the sequence of marine deltaic setting prevailed. The sequence is characterised by the occurrence of laterally extensive sand sheets typical of pre-vegetational times but difficult to interpret sedimentologically. At least six major transgressive regressive cycles can be recognised which can be regionally correlated. Marine intercalations are limited to Central and North Oman. The clastic influx into the grabens was initially of local origin but as the grabens filled up the sediments were predominantly derived from a southern source.

Sedimentological and geochemical controls on ooidal ironstone and ‘bone-bed’ formation and some comments on their sequence-stratigraphical significance

Abstract Ironstones and ‘bone-beds’ are both formed in shallow marine settings where the prevailing sediment accumulation rates are very low. A review of published work shows that both sediment types cap coarsening upward successions and are commonly developed in mudstonedominated settings. Both can be shown to contain similar sedimentary structures (e.g. rip-up clasts, disarticulated shell debris) and a diverse assemblage of ichnogenera. However, the early diagenetic assemblages within each sediment type are both different from each other and from the mudstones within which they are enclosed. In ooidal ironstones, the cement assemblage comprises glaucony, berthierine and siderite; in ‘bone-beds’ it comprises early apatite, glaucony and later calcite and pyrite; in the mudstones it is predominantly pyrite. It is argued that the physical environment in which the ironstones and ‘bone-beds’ formed was similar, whereas their diagenetic environments were rather different. In particular, the characteristic cement assemblage of ironstones was produced in suboxic pore waters which were subjected to intense Fe reduction, whereas the characteristic cement assemblage of ‘bone-beds’ was produced in suboxic pore waters where there was limited Fe reduction. As a consequence of these chemical and stratigraphical constraints it is believed that ‘bone-beds’ formed on marine flooding surfaces and ironstones formed either at sequence boundaries, major flooding surfaces or maximum flooding surfaces.

Surface to Subsurface Correlation of the Claiborne and Jackson Groups, Colorado River to Trinity River: ABSTRACT

Surface to Subsurface Correlation of the Claiborne and Jackson Groups, Colorado River to Trinity River: ABSTRACT

Paleocommunity information retrieval vs. shell accumulation mode in Paleozoic carbonates: examples from the Lebanon Limestone (Middle Ordovician), Tennessee, U.S.A.

Compared to their terrigenous counterparts, carbonate shell accumulations have until recently been relatively little studied to determine either descriptive or genetic classifications of shell bed types, the preservation potential of each type, or their relative ability to preserve community-level information. A partial classification of Paleozoic carbonate shell-rich soft sediment accumulations is proposed using sedimentation patterns in the Lebanon limestone of the Stones River Group. Paleoecological information preserved therein is then contrasted by shell bed type. The Lebanon represents typical Ordovician shallow to moderate subtidal carbonate shelf deposits in outcrops flanking the Nashville Dome and peritidal deposits in the Sequatchie Anticline of Eastern Tennessee; shell beds alternate with shell poor sediments (micrites, wackestones and diagenetically enhanced dolomites and clay-rich partings).None of the analyzed shell beds was strictly biological in origin; most are sedimentological although >10% are combined sedimentological/diagenetic. While the majority are single simple shell beds, >20% are amalgamated. All are thin (1 shell to 15 cm) stringers that pinch and swell showing poor lateral continuity (outcrop scale, tens to hundreds of meters) likely enhanced by burial dissolution. These shell beds differ greatly in fabric (packing/sorting), clast composition, taphonomic signature, and intensity of time averaging; thus community information retrieval is biased in predictable patterns. Virtually no shell beds show common shell dissolution or encrustation from long-term sediment surface exposure or hardground formation. Five major categories of accumulation are herein proposed using a DESCRIPTIVE, non-genetic terminology modified from previous works of DJD, as well as a Genetic interpretation for each. These are easily distinguished in the field and are also discriminated by Q-mode cluster analysis.Categories include, in decreasing frequency of occurrence: 1. SHELL GRAVELS; Storm/“event” beds: Sharp bases; poorly sorted coarse basal bioclasts and/or intraclasts, often with no preferred orientation; clasts fine upward to comminuted shell material and micrite. Horizontal platy brachiopods often cap the beds. High diversity and a wide range in shell alteration is represented, from whole unaltered brachiopods to minor abraded fragments, indicating extreme time averaging and poor resolution of short-term community dynamics. 2. COMMINUTED SHELLY LS; Current/ripple concentrations: Small tidal channel fill and discrete ripple trough accumulations are composed of cross-stratified bioclastic deposits with local concentrations of rip-ups. Beds are not graded; typically clasts are abraded, rounded and concordant with cross-beds. Intense time averaging and mixing of discrete communities is inferred due to continual reworking in these background deposits. 3. SHELL/CEMENT LS; Early cementation beds: Intense early diagenetic alteration is inferred due to red discoloration and rapid intergranular cementation; some beds show diagenetic micritic rinds. Beds may be brecciated and show deep burial stylolitization cutting bioclasts and cement. They may represent zones of preferred early cementation rather than a change in shell accumulation rate. Many shells from some beds show little postmortem alteration; these units may preserve much of the original community structure. 4. DENSE SHELL PAVEMENTS; Subtidal surficial pavements: Single layers of shells, commonly concave down, overlie mudstones/wackestones with no basal erosion. No obrution deposits were noted. Bioclasts are typically disarticulated and reoriented, but are not substantially abraded, broken, or dissolved. Diversity is low. Only minor temporal and lateral community mixing with small environmental fluctuation is indicated. 5. VERTICALLY IMBRICATE SHELLY LS; High energy beach zones: Platy whole and major fragments of brachiopods are deposited in low diversity, high angle imbricate beds. Less postmortem reworking and time averaging is evident compared to types 1 and 2.Thus, the most common (physically reworked) shell bed types show the most intense loss of short-term paleocommunity information. There are surprisingly few insitu community pavements or obligate long-term accumulations. This pattern differs from some described Ordovician carbonates, which may contain common community beds or hardgrounds/hiatal accumulations. This implies a relatively low rate of net sediment accumulation on a shallow, periodically wave swept shelf, and no major flooding surfaces or other indications of significant sea level change. Delineation of the sequence stratigraphic position of these carbonates is enhanced from this type of integrated community/biostratinomic analysis.

Sequence Correlations and Stacking Patterns of Lower Mississippian Strata in Montana and Idaho: Evidence for Eustatic Controls on Sequence Development and Platform Evolution

The Lower Mississippian Mission Canyon Formation and stratigraphic equivalents in southwestern Montana and east-central Idaho consistent of stacked sequences of shallowing-upward platform carbonates that grade westward into correlative slope and basinal sequences. Deeper water facies were deposited in a northern extension of the Antler foreland basin. Detailed biostratigraphic and lithostratigraphic correlations across the platform and into the Antler foredeep document regional stacking patterns of third- to fourth-order depositional sequences across this wide area. Twelve to fourteen (depending on amount of erosion/karstification on top of the Mission Canyon Formation) sequences have been identified and correlated across the platform slope. Major flooding surfaces can be identified and correlated from the Antler foredeep across the Mission Canyon platform and into stratigraphic equivalents in the Williston basin. These regional flooding surfaces, along with biostratigraphic boundaries, allow detailed correlation of individual sequences and sequence sets from the Williston basin to slope environments of the Mission Canyon platform in east-central Idaho, a palinspastic distance of over 1500 km. This study illustrates the application of sequence stratigraphy in a tectonically shortened, carbonate platform-to-basin setting where lateral correlations must be made between isolated and widely separated exposures. The correlation of sequences with very similar stacking patterns from Willistonmore » basin and into the Antler foredeep also suggest that these sequences were caused by eustatic sea-level oscillations.« less