Northern Powder River Basin Research Articles

Mercury capture by boiler modifications with sub-bituminous coals

The extent of mercury (Hg) emissions at a particular coal-fired boiler is unit-specific and determined by factors that include the type of coal fired, boiler operating conditions, and the amount of Hg removed at the boiler back-end by the pollution control devices. Mercury speciation, adsorption and removal in the boiler are affected by parameters such as the flue gas and fly ash characteristics, and back-end boiler configuration/operation. Various studies have reported that boiler operating conditions can be optimized to enhance the “naturally” occurring Hg capture in coal-fired boilers.A study was carried out to investigate the impact of boiler operating conditions on Hg emissions at a full-scale coal-fired power plant. Testing was performed at a 810MW coal-fired power plant firing a Northern Powder River Basin (PRB) coal, and equipped with a low-NOx system and a wet flue gas desulfurization (FGD) system for particulate and sulfur dioxide (SO2) control. Speciated (total and elemental, Hg0) Hg measurements and analytical procedures were performed according to the Ontario Hydro Method (OHM)/ASTM D6784-02 Method. OHM measurements were conducted at the air preheater (APH) inlet and the stack. Mercury emissions were also continuously monitored using single-point, non-isokinetic sampling with semi-continuous emissions monitors (SCEM). Readings from these analyzers were used for guidance during boiler control setting manipulations. Individual semi-continuous sampling modules were installed at four single-point locations along the convective pass: APH inlet, FGD inlet, FGD outlet and the stack.The test results obtained in this study showed the benefit of boiler modifications for Hg control and the extent to which the “natural” Hg control could be influenced. Parametric testing indicated a modest, but inconsistent improvement in the stack Hg (HgT) emissions at reduced excess oxygen. This was probably due to the fact that the fly ash unburned carbon levels were very low (less than 1.0%). Modifications to the low-NOx system over-fire registers, burner tilt and FGD liquid-to-gas (L/G) ratio resulted in stack HgT reductions in excess of 25%. The FGD L/G ratio manipulations also resulted in an improvement in oxidized mercury removal across the FGD. However, none of the test data suggested any reduction in Hg0 across the FGD. The combination of optimal boiler control settings for reduced Hg emission operation resulted in a 34.5% reduction in HgT at the stack.

English

Large scale coal-strip mining has been on-going in southeastern Montana for over three decades. In order to address the potential hydrologic impacts on aquifer systems, extensive long-term monitoring has been conducted since the inception of this mining. As a result, an excellent ground-water database has evolved. This database allows for refinement of key hydraulic parameters in the affected hydrogeologic strata in the vicinity of these mines. In turn, this facilitates prediction of the impacts of future hydraulic/hydrologic stresses imposed on coal bed aquifer systems via any of the following: 1) Mine expansion; 2) Mine development; and 3) Development of coal-bed methane (CBM). One method of refining the understanding of aquifer systems is to use ground-water modeling. To this end, Nicklin Earth & Water, Inc. (NE&W) has developed four separate ground-water models representing coal mines for predicting the impacts of surface mining on aquifer systems in and near the vicinity of coal mines in the northern Powder River Basin. The mine modeling simulations demonstrate that the applicable range of hydraulic conductivity at a mine scale is from 0.1 to 0.3 m/d. The mine simulations also demonstrate that vertical hydraulic conductivity for overlying and underlying strata bounding coal beds is very low. Vertical hydraulic conductivity values in the range of 10 -5 to 10 -6 m/d produced simulation results reasonably consistent with field observations at clustered vertical well sequences. The issue of vertical hydraulic conductivity (or very low vertical leakance) in confining units is a significant issue in estimating CBM production. The very low leakance rates lead to very little contribution of water from the confining units bounding the coals in the Northern Powder River Basin. In general, methods, such as Lohman (1972) and MODFLOW are best employed before the full on-set of methane gas flow begins. Once the on-set of gas flow becomes significant, it has been our experience that both these methods tend to significantly over-estimate actual water production rates during CBM production.

Hardgrove grindability study of Powder River Basin and Appalachian coal components in a Midwestern power station blend

Five coals blended at a Midwestern United States power station were subjected to detailed analysis of their Hardgrove grindability. The coals, representing four distinct coal beds, are a low-sulfur, high-volatile A bituminous Upper Elkhorn No. 3 coal (Pike County, Kentucky); a medium-sulfur, high-volatile A bituminous Pittsburgh coal (southwestern Pennsylvania); a low-sulfur, subbituminous coal from the northern Powder River Basin (Montana); and two low-sulfur, subbituminous Wyodak coals from two different mines located in the eastern Powder River Basin (Campbell County, Wyoming). All samples that were processed in the Hardgrove grindability tests were also analyzed for their maceral and microlithotype content. The high-vitrinite Pittsburgh coal and the relatively petrographically complex Upper Elkhorn No. 3 coal exhibited different grindability behavior. The petrographic compositions of the Pittsburgh raw feed, the 16 × 30-mesh fraction (HGI test fraction) and the -30-mesh fraction (HGI test reject) are similar, suggesting that the HGI test fraction typifies the whole feed. The eastern Kentucky test sample is not as representative of the whole feed, with the HGI test fraction having lower vitrinite amounts than the rejected -30-mesh fraction. The Powder River Basin coals are high in vitrinite content and show behavior similar to the Pittsburgh coal.

Lean Source Rock Expulsion Efficiency and Products: Constraints from the Mowry Shale, Northern Powder River Basin : ABSTRACTS

Lean Source Rock Expulsion Efficiency and Products: Constraints from the Mowry Shale, Northern Powder River Basin : ABSTRACTS

Geochemistry of Water in the Fort Union Formation of the Northern Powder River Basin, Southeastern Montana

Geochemistry of Water in the Fort Union Formation of the Northern Powder River Basin, Southeastern Montana

Geologic and Seismic Models, Muddy Sandstone, Lower Cretaceous, Bell Creek-Rocky Point Area, Powder River Basin, Montana and Wyoming: ABSTRACT

Abstract The Muddy Sandstone in the northern Powder River Basin consists of two distinct members (genetic units) that are separated by a widespread subaerial surface of erosion. The older sandstone member, the reservoir rock at the giant Bell Creek field, was deposited in shoreline and associated nearshore marine environments. It is genetically related by intertonguing to the underlying Skull Creek Shale. The younger member is a valley fill deposit of fluvial, estuarine and tidal flat environments. The deposition and distribution of these two genetic units were controlled primarily by relative sea level changes. The Bell Creek sandstone was deposited as a widespread regressive sandstone during a highstand of sea level. A following sea level lowstand caused valley cutting, erosion of all or portions of the Bell Creek sandstone, and paleosoil development causing early diagenesis, especially of kaolinite, siderite and quartz. A rising sea level resulted in valley filling, coastal onlap and a transgressive surface of erosion that is overlain by black marine shale (Nefsy or Shell Creek). Recurrent movement on basement-controlled fault blocks appears to have controlled distribution of the Muddy members, drainage incisement patterns, present structure and heat flow, and possibly petroleum migration. Seismic modeling indicates that it may be possible to recognize seismically the distribution of Muddy reservoir facies that occur in the area. Seismic modeling suggests that the amplitude response of the Muddy is influenced by Muddy facies and thickness changes. The lowest Muddy amplitude observed occurs when the Bell Creek sandstone is prevalent. An increase in the Muddy amplitude is noted when thick valley fill deposits are present. Results from seismic modeling compare well to actual seismic data. Amplitude changes noted in the Muddy reflector in the Bell Creek – Rocky Point area may help explorationists in the future to distinguish between nonprospective and reservoir quality Muddy Sandstone.

Structural Control on Paleovalley Development, Muddy Sandstone, Powder River Basin, Wyoming: ABSTRACT

A subaerial unconformity within the Lower Cretaceous Muddy Sandstone in the Powder River basin developed during a late Albian sea level lowstand and resulted in a markedly rectangular drainage pattern. Numerous right-angle bends and perpendicular confluences of Muddy paleovalleys are believed to reflect syndepositional movement on basement faults and dissolution of salts in the Goose Egg Formation. A detailed subsurface analysis of geophysical logs from closely spaced wells reveals that up to 30 ft of vertical displacement occurred along northwest- and northeast-trending faults prior to and during the development of the subaerial unconformity. An analysis of a high-resolution magnetic survey (NewMag) of the Powder River basin reveals that numerous paleovalleys parallel the boundaries, or basement shear zones, between basement blocks. Small, irregularly shaped, thin intervals of the Permian Goose Egg Formation, which resemble karst topography, also occur along these northwest- and northeast-trending basement faults beneath Muddy paleovalleys. An arcuate Muddy paleovalley located in the northern Powder River basin parallels contours of isopach and trend surface maps of the Goose Egg Formation. These relationships suggest that the location and orientation of Muddy paleovalleys were controlled by a combination of movement along northwest- and northeast-trending faults and syntectonic dissolution of salt withinmore » the Goose Egg Formation. Simultaneous dissolution of Goose Egg salts and headward erosion of Muddy paleovalleys along this conjugate fault pattern also indicate that the Powder River basin was influenced by wrench fault tectonics during the late Albian.« less

Palynomorph biozones for the fort Union and Wasatch formations (upper Paleocene‐lower Eocene), powder river basin, Wyoming and Montana, U.S.A.

Abstract Three biozones are proposed for the Tongue River Member of the Fort Union Formation and the overlying Wasatch Formation in the northern Powder River Basin, Wyoming and Montana. The biozones are, in ascending order, the Aquilapollenites spinulosus Zone, Caryapollenites Assemblage Zone (incorporating the Pistillipollenites mcgregorii Subzone and Insulapollenites rugulatus Subzone), and Platycarya platycaryoides Zone. The zones are based on the stratigraphic ranges of several palynomorphs and changes in abundance of certain plant groups. The apparent evolution within the Momipites‐Caryapollenites lineage, previously defined and employed stratigraphically in the Wind River Basin, Wyoming, can also be applied in the Powder River Basin. The change in dominance upsection from Momipites to Caryapollenites essentially coincides with a major sedimentological change from a fluvial channel‐dominated environment to a fluvial‐lacustrine environment. The Paleocene‐Eocene boundary in the Powder River Basin lies ...

Taphonomy and Paleoecology of Nonmarine Mollusca: Indicators of Alluvial Plain Lacustrine Sedimentation, Upper Part of the Tongue River Member, Fort Union Formation (Paleocene), Northern Powder River Basin, Wyoming and Montana

Paleoenvironments are interpreted from the taphonomy of freshwater mollusc assemblages in the Tongue River Member of the Fort Union Formation (Paleocene) in the northern Powder River Basin, Wyoming and Montana. The composition, species abundances, and spatial and temporal distributions of these assemblages were controlled by the environments in which the molluscs lived and the depositional processes that affected the molluscs after death and before final burial. Post-mortem transport, reworking and concentration of shells, and mixing of faunal elements from discrete habitats produced a taphonomic overprint on assemblage characteristics that directly reflects the processes of alluvial plain and floodbasin lacustrine sedimentation. The overprint can be interpreted from outcrop analysis of molluscan biofabric, which consists of: 1) orientation, fragmentation, size-sorting, abrasion, density, and dispersion of shells, 2) the nature and extent of shell-infilling, and 3) ratio of articulated to disarticulated bivalves. In this study, taphonomic characteristics were used with sedimentological properties to differentiate in-place, reworked, transported, and ecologically mixed mollusc assemblages. This study also defines the paleoecology of habitat preferences of mollusc species as a basis for recognition of the environments in which these assemblages were deposited: 1) large floodbasin lakes (onand off-shore), 2) small floodbasin lakes, and 3) crevasse deltas and splays. Integration of sedimentology and paleoecology provides an interdisciplinary approach to the interpretation of alluvial environments through time in the Tongue River Member.

Geologic Setting of a New Paleocene Mammal Locality in the Northern Powder River Basin, Montana

Geologic Setting of a New Paleocene Mammal Locality in the Northern Powder River Basin, Montana

Types of Stratigraphic Traps in Lower Cretaceous Muddy Formation, Northern Powder River Basin, Wyoming: ABSTRACT

Stratigraphic traps account for most of the oil produced from the Muddy Sandstone in the northern Powder River basin. Two categories of traps exist. The first trap type is the result of lateral and vertical facies changes. Reservoir facies include tidal channels, point bars, bayhead deltas, barrier islands, and strand-plain sandstones; trapping facies include bay-fill and estuarine sediments, mud-filled tidal channels, and flood-plain deposits. The second of the two categories of traps results from an unconformity that juxtaposes permeable and impermeable sediments of quite different ages. Structural and diagenetic factors often modify and locally enhance reservoir quality within both categories of stratigraphic traps. The various types of traps are demonstrated by studies of six field areas: (1) barrier-island sandstones, sealed updip by back-barrier shales, produce at Ute and Kitty fields; (2) tidal channels produce at Collums and Kitty fields; (3) bayhead deltas, encased in estuarine sediments, form traps at Oedekoven and Kitty fields; (4) fluvial point-bar sandstones form traps at Oedekoven, Store, and Kitty fields; (5) unconformity-related traps exist where Muddy fluvial valley-fill sediments lap out against impermeable valley walls of Skull Creek Shale on the updip side at Store, Oedekoven, and Kitty fields; and (6) the clay-rich weathered zone,more » directly beneath an intraformational unconformity, forms the seal to the reservoirs at Amos Draw field.« less

Lithologic Variations and Diagenesis of Lower Cretaceous Muddy Formation in Northern Powder River Basin, Wyoming: ABSTRACT

Lithologic Variations and Diagenesis of Lower Cretaceous Muddy Formation in Northern Powder River Basin, Wyoming: ABSTRACT

Unconformities and Facies Relationships of Muddy Sandstone, Northern Powder River Basin, Wyoming and Montana: ABSTRACT

The Muddy Sandstone, although commonly less than 100 ft thick, is one of the most complicated formations in the Powder River basin. Utilizing an integrated sedimentologic and stratigraphic analysis of outcrop, core, and well logs, the Muddy can be divided into six isochronous members. The recognition of two widespread, subaerial unconformities is critical to understanding the complex facies relationships and member-level stratigraphy of the Muddy. Over much of the Powder River basin, the Muddy contains an upward coarsening sequence at its base. At first glance, the sequence appears to be conformable with the underlying Skull Creek Shale. East-west cross sections, however, reveal westward truncation of siltstone and dahllite marker beds in the upper part of the Skull Creek. Rapid transgression of the sea planed off most of the erosional topography on the Skull Creek unconformity, reworking the dahllite concretions and concentrating them as a lag. A regressive, sand-rich strand plain, the Rozet member, was deposited during a subsequent sea level drop. As regression continued, a second unconformity, the Rozet, developed as streams eroded through it and into the underlying Skull Creek Shale, forming deep valleys. The next sea level rise resulted in the deposition of five regionally correlatable members withinmore » the previously eroded valleys and onlapping Rozet hillsides. Layers of volcanic ash frequently fell on the surface, preserved in the valleys as beds of bentonite. Ash that fell on the hillsides was dissolved by rainwater and contributed to growth of interstitial kaolinite within the upper part of the Rozet sand.« less

Stratigraphy, Sea Level Fluctuations, and Depositional History of Lower Cretaceous Muddy Sandtone, Northern Powder River Basin, Wyoming: ABSTRACT

The Muddy Sandstone may be the most complicated Cretaceous formation in the Powder River basin. Although generally less than 100 ft thick, it contains a wide variety of facies. Two unconformities, one of which is largely intraformational, are associated with the Muddy. It has long been known that Muddy deposition was preceded by a lowering of sea level in the Western Interior. Their study indicates that there were, in fact, two cycles of lowering/rising of sea level. The first lowering and the subsequent rise were abrupt; they were accompanied by only minor deposition of sandy sediments. The second lowering of sea level was slower. Sand provided by longshore drift from a southwestern source accumulated as an extensive strand-plain system. As sea level continued to lower, rivers cut downward locally through the old strand plain and into the Skull Creek Shale. The deepest Muddy valleys formed at this time. The following transgression was not continuous but comprised at least five pulses of abrupt rise separated by periods of stability. During periods of abrupt rise, valleys flooded to form estuaries; during still-stands, valleys were largely filled with estuarine, bayhead-deltaic, and fluvial sediments. Barrier islands and associated tidal inlets formed near the mouthsmore » of the estuaries. All of the sandy facies of the Muddy - fluvial, strand plain, barrier island, and bayhead delta, in decreasing order of importance - serve locally as reservoirs for oil and gas.« less

Coal Stratigraphy of Northern and Central Powder River Basin: ABSTRACT

Coal Stratigraphy of Northern and Central Powder River Basin: ABSTRACT

Distribution and Age of Clinker in Northern Powder River Basin, Montana: ABSTRACT

Clinker, rock that has been baked or fused by the burning of underlying coal beds, is abundant in the Tongue River Member of the Paleocene Fort Union Formation in the northern Powder River basin. Being more resistant than unbaked rocks above and below, the clinker commonly caps ridges and plateaus, and forms topographic benches and escarpments. This clinker is primarily red and orange baked sandstone and shale, but it includes gray sintered siltstones (porcellanite) and bodies of black, fused and welded breccia. Detrital zircons in the sandstones are annealed during baking and yield fission-track ages that show the time of cooling. An inventory of clinker areas has been completed for the Montana part of the northern Powder River basin east of the Crow Indian Reservation. The study area lies within the Powder River resource area of the U.S. Bureau of Land Management. Data were compiled from existing literature, color infrared aerial photographs, and unpublished mapping provided by R. B. Colton, W. C. Culbertson, and S. J. Luft of the U.S. Geological Survey. Clinker covers approximately 2,700 km2 (1,050 mi2) or 20% of the Tongue River exposures in the study area. Assuming a 15 to 25 m (50 to 80 ft) average thickness, the volume of clinker would be 40 to 70 km3 (10 to 17 mi3). The most extensive clinker layers locally exceed 60 m (200 ft) in thickness. They are produced by the thickest coal beds, some of which exceed 15 m (50 ft). Two major clinker layers form extensive topographic surfaces and escarpments. (1) Near Decker, Montana, the clinker produced by the Anderson-Dietz coal zone forms benches adjacent to the Tongue River. Because of the gentle southerly regional dip of the beds, this clinker zone rises to the north, where it caps large plateaus dividing the valley of the Tongue River from the valleys of Otter Creek and Rosebud Creek. These clinker plateaus stand up to 400 m (1,300 ft) above the Tongue River. (2) Near Ashland, Montana, the clinker produced by the Knobloch-Nance coal zone, which lies about 300 m (980 ft) stratigraphically below the Ande son-Dietz zone, forms broad benches bordering the Tongue River. The distribution of fission-track ages shows that coal has burned to form clinker in the region at least since the late Pliocene. A clinker boulder from the base of a gravel deposit 365 m (1,200 ft) above the level of the Yellowstone River west of Forsyth has been dated at 4.0 ± 0.7 m.y. This age establishes a maximum age for the gravel. The oldest in-place clinker sample dated thus far comes from the summit of the Little Wolf Mountains west of Colstrip and is dated at 2.8 ± 0.6 m.y. Clinker from the Anderson-Dietz plateau that rims the Tongue River Valley west and south of Ashland ranges in age from 1.4 ± 0.4 m.y. to 0.7 ± 0.3 m.y. Ages from clinker of the Knobloch-Nance coal zone range from 0.5 ± 0.3 m.y. to < 0.06 m.y. The older ages are from topographic lly higher clinker layers. Inasmuch as a coal bed cannot start burning until it is exposed by erosion, these ages indicate the Tongue River cut its present valley primarily during the Pleistocene. End_of_Article - Last_Page 1342------------

Channeling in Paleocene Coals, Northern Powder River Basin, Montana: ABSTRACT

Interpretation of 1,200 geophysical logs in the northern Powder River basin, Montana, reveals the paleodrainages influencing coal deposition during the deposition of the Tongue River member (Paleocene, Fort Union Formation). Four channels with associated crevasse splay deposits are recognized: (1) an east-west Rosebud drainage near Colstrip, (2) a north-south Wall channel near Birney, (3) a north-south Dietz drainage near Tongue River Reservoir, and (4) a north-south Anderson channel in the vicinity of Moorhead. These channels support the concept of a major northeast-flowing drainage system during deposition of the Tongue River Member. Identification of these channels serves as a guide to future coal exploration. End_of_Article - Last_Page 1340------------

Depositional Environments in an Alluvial-Lacustrine System: Molluscan Paleoecology and Lithofacies Relations in Upper Part of Tongue River Member of Fort Union Formation, Powder River Basin, Wyoming: ABSTRACT

The upper part of the Tongue River Member of the Fort Union Formation (Paleocene) in the northern Powder River basin, Wyoming, contains assemblages of excellently preserved nonmarine mollusks which occur in laterally continuous outcrops of diverse lithologic sequences and sedimentary structures. These attributes offer a unique opportunity for interdisciplinary interpretation of depositional environments based on molluscan paleoecology and lithofacies relations. Taphonomic histories of mollusk assemblages as reflected by molluscan biofabric (size, End_Page 478------------------------------ shape, orientation, and distribution of faunal elements), and the taxonomic composition, diversity, and abundance representation of species in assemblages are of particular importance in interpretations of paleoecology and depositional environments. Three facies are recognized vertically within an alluvial-lacustrine system. The interfluvial lake and lake splay facies is characterized by sequences of coarsening-upward detritus, abundant continuous limestone beds, and few beds of discontinuous coal and continuous carbonaceous shale. Limestones, contain two lacustrine mollusk assemblages: a locally reworked assemblage dominated by the bivalve Plesielliptio (two species), and the gastropods Viviparus, Lioplacodes (three species), and Clenchiella; and a quiet-water assemblage dominated by sphaeriid bivalves. This facies reflects development of numerous lakes bordered by well-drained backswamps in interfluvial flood basins. Lakes were repeated infilled by carbonate mud, splay detritus, and backswamp deposits. The interfluvial crevasse splay-crevasse channel facies is characterized by sequences of coarsening-upward detritus and few discontinuous limestone beds, separated vertically by thick, continuous coal and carbonaceous shale beds. This facies includes small crevasse channel sandstones which scour into splay sandstones. Biofabric of lacustrine mollusk assemblages, which are identical in composition (but with dwarfed species of Plesielliptio) to locally reworked lacustrine assemblages of the interfluvial lake and lake splay facies, reflects deterioration of lakes through active infilling by crevasses. The fluvial channel and interchannel facies is typified by thick channel sandstones laterally separated by sequences of coarsening-upward detritus, overbank sediments, and rare limestones. This facies includes thick, continuous coal and carbonaceous shale beds. Muddy-substrate flood basin lakes are characterized by an untransported lacustrine mollusk assemblage differing from those of the interfluvial lake and lake splay facies in both composition and relative abundance of species. Transported mollusk assemblages dominated by the gastropod Hydrobia eulimoides occur in crevasse sandstones. Relative to northerly flowing major channels, proximal and distal parts of splay deposits can be differentiated by their mollusk assemblage composition. The fluvial channel and interchannel facies re lects deposition on flood basins that were formerly occupied by lake, crevasse, and backswamp deposits. Thick, continuous coal beds reflect the spread of backswamps from interchannel depressions to abandoned major channel ridges. Cessation of fluvial channel and interchannel sedimentation was marked by widespread lacustrine deposition. The vertical sequence of facies just described also reflects the areal distribution of deposits of major channels that pass laterally into overbank and crevasse splay channel areas, which in turn merge into large and small lakes. Alluvial-lacustrine deposition in the upper part of the Tongue River Member is similar to that of anastomosed reach of the Saskatchewan River, Canada, as reported in a 1980 study by Smith and Putnam. End_of_Article - Last_Page 479------------

Fluvial Facies Associations--Guide to Tertiary Coal Development and Exploration in Powder River Basin, Wyoming and Montana: ABSTRACT

The Paleocene Tongue River Member of the Fort Union Formation supports widespread coal-mining activity and contains most of the recoverable coals in the Powder River basin. Coals are well-developed in eastern and northern Powder River basin, but the best exposures are in the northern part. Most mining is from the very thick (as much as 125 ft or 38 m) coals; however, as these coal reserves are depleted, future exploration will focus on moderately thick coals. Exploration guides for the moderately thick coals may be their facies associations. In the northern Powder River basin, 175 mi (280 km) of cross sections, constructed from closely spaced outcrop sections and drill holes, provide detailed facies associations of the coals. One facies association is dominated by thick fluvial-channel sandstones and coals as much as 40 ft (12 m) thick and 12 mi (19 km) in extent. The coals formed in poorly drained backswamps in which uniformly thick peat bogs accumulated, subparallel to meandering channels and opposite their migration and avulsion directions. A second facies association is characterized by abundant crevasse sandstones, subordinate thin channel sandstones, and lacustrine limestones, shales, and siltstones. Facies-associated coals, as much as 8.5 ft (17 m) thick and 5 mi (8 km) in extent, formed in well-drained backswamps frequently interrupted by crevasse splays that debouched into flood-plain lakes. The best development of economic coals of the fluvial-channel dominated facies is in the lower part of the ongue River Member, and that of the flood-plain-lacustrine coal-facies association is in the upper part. End_of_Article - Last_Page 926------------

Application of Nonmarine Mollusca to Paleoenvironmental Interpretation and Correlation of Paleogene Rocks: ABSTRACT

Nonmarine mollusks, the most consistently occurring element of Mesozoic and Cenozoic nonmarine macrofaunas, are abundant and locally dominant in many preserved terrestrial and freshwater paleocommunities. Maximum interpretive potential of mollusk assemblages is derived from detailed analysis of several biologic and physical factors: (1) taxonomy based on modern malacologic and paleontologic concepts that include differentiation of genetic and nongenetic morphologic variability; (2) biostratonomy (the history of the faunal assemblage from death to final burial); (3) community structure; (4) time-space variability of assemblages relative to a detailed lithostratigraphic framework; and (5) rock types, fabric, and structures of the enclosing sedimentary rock. Collectively, hese factors indicate whether the faunal assemblage was preserved in the original environment in which it lived. Failure to gather and/or interpret adequately these data has promoted the widely held misconception that mollusks are of little value in the interpretation of depositional environments, biostratigraphy, correlation, and age determination of nonmarine rocks. Two examples of the interpretive value of Paleocene and Eocene nonmarine mollusks are (1) depositional environments and regional paleoenvironmental reconstruction of part of the Green River and Wasatch Formations, southwestern Wyoming and northwestern Colorado; End_Page 828------------------------------ and (2) biostratigraphy and correlation of part of the Tongue River Member of the Fort Union Formation, northern Powder River basin, Wyoming and Montana. The Green River and Wasatch Formations are complexly intertonguing lacustrine and fluvial units that were deposited during early and middle Eocene time in Western Interior basins of Wyoming, Colorado, and Utah. The structure of nonmarine molluscan associations within these strata delineates littoral and sublittoral lacustrine, pond, fluvial, and terrestrial habitats. Littoral and sublittoral lacustrine habitats are characterized by a low-diversity association of prosobranch gastropods and unionid bivalves. Rank and relative abundance of taxa differ in these habitats. Ponds are dominated by a diverse association of aquatic pulmonate gastropods with sphaeriid bivalves. Lowland and flood-plain habitats are characterized by a locally diverse association of terrestrial pulmonate gastropods and a fluvial association dominated by unionid bivalves with prosobranch and aquatic pulmonate gastropods. Comparison with structurally similar molluscan associations from modern habitats, paleosynecology of fossil taxa, and lithostratigraphic data provide bases for paleoenvironmental interpretation. Analysis of paleogeographic and stratigraphic distribution of these Eocene molluscan associations, relative to a detailed lithostratigraphic framework, permits regional paleoenvironmental reconstruction within the Green River and Wasatch Formations. In the northern Powder River basin, the Tongue River Member of the Fort Union Formation contains diverse, commonly excellently preserved assemblages of Paleocene nonmarine mollusks. Detailed study of the distribution of gastropods and bivalves in the stratigraphic interval from below the Wall coal bed to above the Arvada coal bed clearly indicates the value of mollusks in correlation of sedimentary sequences between the Wall, Anderson, Smith, Roland, and Arvada coal beds. Clinal morphologic variation in shell form and sculpture through time within a lineage of viviparid gastropods provides an additional method for correlation within part of the stratigraphic interval. These studies clearly indicate the value of mollusks in the interpretation of depositional environments, biostratigraphy, and correlation of Paleogene nonmarine rocks. End_of_Article - Last_Page 829------------