Marginal Lacustrine Facies Research Articles

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

Controls of oil family distribution and composition in nonmarine petroleum systems: A case study from Inner Mongolia Erlian basin, Northern China

The Erlian basin is a continental rift basin located in Inner Mongolia, Northern China. It is a typical representative of Cretaceous Northeast Asian Rift System, which includes many small petroliferous basins in Mongolia Republic and Northern China. Although Lower Cretaceous source rocks are understood to be most important in the Erlian petroleum systems, the precise identification of these source rock intervals and their determination on oil families distribution and composition are poorly understood in this tectonically complicated, nonmarine basin. New bulk data have been gathered from source rock intervals, oil sands and crude oil samples in eight main oil-producing subbasins. Geochemical analyses indicate that Lower Cretaceous Aershan formation (K1ba) and Tengger 1 formation (K1bt1) are two main source intervals in the Erlian basin and their source rock facies vary from profundal lacustrine to marginal lacustrine according to biomarker and trace elements calibration, the profundal lacustrine facies is characterised by brackish water and anoxic environment, which is similar to their correlative oils (Family 1 oils). The marginal lacustrine facies is characterised by freshwater and suboxic environment, which sourced the most common Family 2 oils. Meanwhile, different maturation processes exercise the second control on oil groups and their compositions, the profundal lacustrine source rocks characterised by their sulphur-rich kerogens lead to two oil groups (group 1 and group 2 oils), whose maturity range from low to normal; while, the marginal lacustrine source rock only lead to normal-maturity oils. At last, biodegradation affected the composition of a certain oils and formed group 4 heavy oils. In addition, short migration distance in small subbasins made the contamination or fractionation less notable in the Erlian basin.

Middle to late Pleistocene climatic and depositional environmental changes recorded in the drilled core of lacustrine sediments in the Kathmandu Valley, central Nepal

Since October 2000, we have undertaken core-drilling for pure academic purposes at Kathmandu, Nepal, in order to clarify the continuous paleoclimatic records in the southern slope of the Himalaya and to reconstruct the depositional environmental changes of the ancient lake in Kathmandu (Paleo-Kathmandu Lake) during the last one million years. In this project, we also aimed to clarify the uplift history of the Himalaya and its linkage to paleoclimatic and paleo-environmental changes. We could have penetrated black muddy lacustrine sediments, called the Kalimati Formation, at western central part of the valley (RB core at Rabibhawan) in 2000 (Sakai et al 2001) and southern part of the valley (CP core at Champi and JK core at Jorkhu) in 2003 (Figures 1 and 2). We also could have penetrated the lower part of the fanglomerate, called Itaiti Formation and underlying Lukundol Formation which shows marginal lacustrine facies comprising of fluvial and swamp deposits in 2001 (Sakai 2001a). The longest core drilled at Rabibhawan is 218 m in length and ranges in age from about 700 ka to 10 ka on the base of

Cycles in Perilacustrine Facies of Late Mesozoic Rift Basins, Southeastern Mongolia

Detailed study of 365 meters of core recovered from southeastern Mongolia reveals fluvial, marginal lacustrine, and profundal lacustrine depositional environments in an Early Cretaceous rift basin. Multiple examples of cyclic sedimentation are superposed on these large-scale facies transitions. In fluvial facies, more than 20 upward-fining successions (2–10 m thick) are present. These contain cross-stratified sandstone units, with erosional bases, that fine upward to mudstone with root marks and burrows. They are interpreted as channel-fill successions in a fluvial depositional environment, whereas finergrained, less erosional intervals with abundant soft-sediment deformation and climbing ripples may represent a transition to fluvial-deltaic environments at the lake margin. Coarse fluvial-deltaic facies form the most likely hydrocarbon reservoirs, as indicated by frequency of oil staining in the core. Physical transport processes dominate well-oxygenated fluvial and marginal lacustrine facies, whereas cycles in more distal lacustrine facies mainly reflect biogeochemical processes. Profundal lacustrine strata include kerogen-rich laminated micrite, distal turbidite beds, and dolomitic breccia reminiscent of lacustrine facies in the Green River Formation of the western United States. Evidence for anoxia at the lake floor includes framboidal (biogenic) pyrite, high TOC content, mainly light amorphous organic constituents, and paucity of trace fossils relative to the rest of the core. Approximately 25 dolomite layers are interbedded with laminated micrite and calcareous siltstone units in poorly defined meter-scale cycles. These beds typically have brecciated or clotted textures, and suggest evaporative concentration of magnesium ions at the lake floor. The profundal lacustrine sequence displays a distinctive high-resistivity, low spontaneous-potential log character that is observed in well logs from several fields in the Zuunbayan subbasin. Correlative units can be traced at least 60 km to the northeast and southwest of the Zuunbayan field, indicating that this is a widespread lacustrine depositional unit. These lacustrine strata include the most likely petroleum source facies found to date, and are likely contemporaneous with other large lake systems in China and Mongolia.

Trace fossil distribution in lacustrine deltas: examples from the Triassic rift lakes of the Ischigualasto-Villa Unión basin, Argentina

Abstract This paper reports six trace fossil assemblages from lacustrine deltas of the Triassic Ischigualasto-Villa Unión rift basin, northwest Argentina. They were recognized in three correlated stratigraphic sections separated by about 100 km, and come from river- and wave-influenced deltas developed in low- and high-accommodation lacustrine basins. Trace fossil assemblages correspond to delta front (six), delta plain (two) and marginal lacustrine (one) facies associations. Each trace fossil assemblage is described, together with a detailed lithofacies characterization of the trace fossil-bearing interval. They are analysed in conjunction with previously described ichnological assemblages from partially correlative sections of the same basin. Delta front facies contains a high-diversity assemblage (22 ichnotaxa), including Cochlichnus (a ubiquitous form), Gordia, Helminthoidichnites, Helminthopsis, Didymaulichnus, Diplichnites, Stiaria, Cruziana, Bifurculapes, Protichnites, Diplopodichnus, Archaeonassa, Palaeophycus, Treptichnus, Rusophycus, Avolatichnium , ‘rhomboidal traces’, ‘fusiform structures’ and ‘millimetre burrows’. Trace fossil assemblages from delta plain facies are much less diverse (seven ichnotaxa), but display representatives of a greater variety of ethological categories, including Rhynchosauroides, Skolithos, Palaeophycus , ‘horseshoeshaped structures’, escape trace and drab-haloed root traces. Marginal lacustrine deposits of a river-dominated delta yielded a monospecific assemblage consisting of Cochlichnus anguineus . Comparison of trace fossil assemblages in wave- and river-dominated lacustrine deltas from the basin (mainly those of the delta front facies) revealed important differences in ichnodiversity that can be useful in the discrimination between these lacustrine delta types. Lacustrine delta deposits contain trace fossils that can be ascribed to three different ichnofacies: a high-diversity occurrence of the Mermia ichnofacies in subaqueous delta front sediments, a low-diversity occurrence of Mermia ichnofacies in subaqueous marginal lacustrine facies, the Skolithos ichnofacies in high-energy upper delta front/lower delta plain facies, and the Scoyenia ichnofacies in intermittently exposed upper delta plain facies. The analysed trace fossil assemblages from delta front and marginal lacustrine settings suggest environmental gradients within the Mermia ichnofacies.

Sedimentary facies changes recorded in the Plio-Pleistocene Kathmandu Basin Group in the southern part of the Kathmandu Valley, Nepal

Sedimentological study has revealed vertical and lateral sedimentary facies changes of the Plio-Pleistocene Kathmandu Basin Group in the southern part of the Kathmandu Valley, Nepal. The field observation and analyses of XRD, carbon content. C/N ratio and diatom fossils indicate that depositional environments of muddy sediments in this area can be classified into four categories: i) open lacustrine, ii) shallow lacustrine. iii) swamp-shallow lacustrine, and iv) environment strongly affected by fluvial system. Distributions of sedimentary facies in Lukundol, located in the southern margin of the Kathmandu Basin, show that fluvial facies dominant sequence in the Tarebhir Formation changed to a marginal lacustrine facies in the Lukundol Formation at 2.8 Ma, and a fluvial facies in the ltaiti Formation developed at 1 Ma following the deposition of the Lukundol Formation. In the area 5 km north of Lukundol, an open lacustrine facies in the Kalimati Formation dominated during the deposition of the Lukundol and Itaiti formations. The relation between the vertical sedimentary facies changes and palaeoclimate estimated from clay mineral crystallinity indicates: (1) the termination of fluvial facies dominant sequence at 2.8 Ma corresponds to the time where a wet climate turned to a drier climate, (2) fluvial environments developed under the wet climate and swamp-shallow lacustrine environments expanded under the dry climate during the deposition of the Lukundol Formation.

Continental, brackish and marine carbonates from the Lower Cretaceous of Kolone–Barbariga (Istria, Croatia): stratigraphy, sedimentology and geochemistry

During the Early Cretaceous, wide areas of the Dinaric–Adriatic Carbonate Platform emerged for long periods. The Hauterivian–Barremian carbonates from Kolone–Barbariga show a few typical examples of lacustrine facies with dinosaur bones and brackish/palustrine facies. The sequence of the platform is made for the most part by subtidal and intertidal limestones. The bone levels are located in a large depression few meters deep in the uppermost Hauterivian marine limestones. The filling facies of this depression are made by oncolitic rudstones and algal boundstones, which represent marginal lacustrine facies, and by laminated limestones, thin stromatolitic levels and distal fringes of rudstones which represent relatively open lacustrine facies. The fossil content is characterized by rare charophyte stems, ostracods, gastropods and plant remains, while typical marine fauna is absent. At the Hauterivian–Barremian boundary a major emersion event has been observed, then a slow transgressive phase occurred. The transgressive facies are primarily made by mudstones with ostracods, charophytes and Spirillina (brackish and probably freshwater facies), wackestones with Ophtalmidiidae and rare dasyclad algae, storm layers with gastropods and miliolids and breccia-like dinoturbated beds. Wackstones, packstones and very rich in dasyclad grainstones outcrop at the top of the section, representing the maximum of the transgression. Trace elements content, carbon and oxygen stable isotope analyses have been performed to aid the palaeoenvironmental interpretation. In this geological setting, Barium seems to discriminate between brackish and freshwater facies. The isotopic values of the marine carbonates appear to depend on early diagenetic processes, meanwhile lacustrine facies seem to show a weak signal of the depositional environment.

Environmental controls on perennial and ephemeral carbonate lakes: the central palaeo-Andean Basin of Bolivia during Late Cretaceous to early Tertiary times

In the central palaeo-Andean Basin (Potosí Basin) of Bolivia, the up to 450-m-thick El Molino Formation (Campanian-Maastrichtian to early Tertiary) includes two main carbonate lacustrine episodes each of different extent and duration. These episodes are separated by an extensive development of alluvial facies (floodplain deposits) and limestones deposited in playa lakes or isolated ponds. The El Molino Formation has been investigated in six different areas of the Potosí Basin and affords the opportunity to document climatic and tectonic controls on the development of perennial and ephemeral carbonate lacustrine systems. The first lacustrine episode originated predominantly in perennial lakes with wave-dominated low-gradient ‘ramp’-type margins. Outer marginal lacustrine facies include winnowed oolitic grainstones, ostracod packstones and microbialite bioherms. More agitated areas were characterized by nearshore oolitic bars with associated thrombolite mounds. The fossil content, and the lack of significant precipitation of evaporites, indicate that the lake waters were of low salinity (probably oligohaline) during most of this period. After the first lacustrine episode, the widespread development of terrigenous alluvial facies (floodplain deposits), with interspersed playa lakes or isolated ponds in depressions on the floodplain, may indicate a change in the net water budget. The second lacustrine episode is dominated by dark micritic limestones and, to a lesser extent, microbialite bioherms which developed under semiarid climatic conditions. This episode includes the deposits of ephemeral saline, locally alkaline, shallow lakes which were characterized by low-energy, low-gradient ‘ramp’-type margins. The stable isotopic analysis of 163 carbonate samples covering the different facies and depositional settings displays a wide range of values ( −14.2 < δ 18 O < +2.8‰ and −12.9 < δ 13 C < +2.8‰ ) that is typical of nonmarine environments. The distribution of the values indicates that, during the deposition of the El Molino Formation, the basin was hydrologically closed and experienced no strong hydrogeographical changes, except for variations in the palaeolake level related to fluctuations in the regional P/E ratio. These are recorded by two main isotopic trends related to changes between perennial and ephemeral lacustrine conditions. Lacustrine sedimentation was controlled predominantly by climatically driven hydrological changes with repeated oscillations of the water level (expansion and contraction) and subsequent fluctuations in the width of lacustrine-facies belts.

Palustrine sedimentation and associated features—grainification and pseudo-microkarst—in the Middle Miocene (intermediate unit) of the Madrid Basin, Spain

A rather thick sequence (50–75 m) of lacustrine limestones forms the uppermost part of the Intermediate Unit (Middle Miocene) in the NE part of the Madrid Basin, central Spain. A gradual change from distal alluvial to open lake facies is recorded in this area. Red mudstones passing vertically to strongly mottled limestones are typical of the transition from distal alluvial to marginal lake deposits. A wide variety of facies occur in the marginal lacustrine facies association, all of them showing pedogenic features (e.g. root traces, alveolar septal structures, pseudo-microkarst). Micrites and biomicrites are interpreted as open lacustrine deposits. Although not as strongly pedogenically modified as the marginal lacustrine facies, they still show evidence of subaerial exposure. The carbonate sequence was deposited in a low-gradient, shallow-lake system in which slight fluctuations in the water level caused extensive emergence. Pedogenic processes affected most of the lake deposits, but modification was more intense in the marginal facies. Biogenic activity and mechanical reworking at lake margins led to the formation of pedogenic-diagenetic pseudo-microkarst and peloidal and/or intraclastic limestones.

Models of miocene marginal lacustrine sedimentation in response to varied depositional regimes and source areas in the Madrid Basin (Central Spain)

A succession of lake complexes formed through Miocene times in the Madrid Basin (Central Spain). The complex formed during the Aragonian stage provides an outstanding opportunity to evaluate the role of source areas and depositional regimes in the construction of coeval marginal lacustrine deposits. Sedimentary facies of the Aragonian lake indicate a shallow lake that was fringed to the NW and S by arkosic alluvium, to the N by litharenitic alluvial fans, and to the NE and E by alluvial fans distally dominated by large floodplains. Marginal lacustrine deposits in the arkosic realm mostly consist of Mg-smectites and dolomite sequences that laterally interfinger with sepiolite beds which were deposited in small discontinuous ponds in the distal alluvial fan areas. Occasionally, micaceous-rich sandy streams entered the lake forming sheets at their entry points. In contrast, the northern margin of the lake was formed by muddy flats which were influenced by fluctuation in the water table. In this setting, illitic mudstones are commonly interbedded with biotite-rich flood sands, showing a sheet-like geometry. Meandering channels occasionally cut into the mud flats. Facies assemblages on this side of the lake were mainly derived from low-grade metamorphic areas. To the northeast, the marginal lacustrine facies consist of silty clays, carbonate paleosols and paludal to lacustrine limestones. These facies reflect an interfingering between the lake and a wide stream-floodplain system derived from a carbonate source area. The mosaic of marginal lacustrine facies in the Madrid Basin during the Aragonian is discussed in the light of lithological and geochemical factors related to variations in source areas.

Paleontology and Sedimentology of Upper Clastic Member of Wanakah Formation, Chama Basin, New Mexico: Lacustrine Paleoenvironmental Implications: ABSTRACT

Lacustrine strata of the upper part of the Jurassic Wanakah Formation were restricted to the Chama basin of north-central New Mexico by mid-Jurassic tectonic activity in the Brazos and Nacimiento uplifts and along the Gallina-Archuleta anticlinorium. Lateral and vertical facies of the upper Wanakah exposed around the southern margin of the Chama basin indicate that the deeper part of the lake was north of the outcrop belt. The upper 3-5 m of the Wanakah consists of thin-bedded rippled sandstone, interbedded mudstone, and limestone containing trace fossils and freshwater mollusks characteristic of marginal lacustrine facies. Taphonomic studies of mollusks in the Wanakah Formation have been combined with application of ecophenotypic variation documented in extant unionid bivalves to produce paleoenvironmental interpretations of these lacustrine rocks.

Evidence for a shallow early or middle Wisconsin-age lake in the Bonneville Basin, Utah

Relatively complete stratigraphic records of the Bonneville cycle and of at least one and probably two earlier lacustrine are exposed along the Bear River below Cutler Dam in northern Utah between altitudes of 1290 and 1365 m. In most exposures the unconformity between the Bonneville Alloformation and the underlying unit, herein named the Cutler Dam Alloformation, is marked by slight erosional relief and by a weakly to moderately developed buried soil, herein named the Fielding Geosol. In truncated profiles, the Fielding Geosol reaches a maximum of stage II carbonate morphology. Wood from near the base of the Cutler Dam Alloformation yielded a 14C date of >36,000 yr B.P. (Beta-9845). Alloisoleucine/isoleucine (aIle/Ile) ratios of Sphaerium shells from the Cutler Dam beds average 0.15 ± 0.01 in the total hydrolysate, which is significantly greater than the average for Sphaerium shells of Bonneville age elsewhere in the basin. Therefore, the Cutler Dam Alloformation is older than 36,000 yr B.P., but much younger than deposits of the Little Valley lake cycle (140,000 yr B.P.?) which bear shells having significantly higher aIle/Ile ratios. The Cutler Dam Alloformation along the Bear River may be broadly correlative with marine oxygen-isotope stages 4 or 3. Fine-grained, fossiliferous, marginal-lacustrine facies of the Cutler Dam Alloformation are exposed at altitudes near 1340 m, and are probably the highest exposures of sediments deposited in the early or middle Wisconsin lake in the Bonneville basin.

Sunnyside Petroleum-Impregnated Sandstone Deposit, Uinta Basin, Utah: ABSTRACT

The Sunnyside oil-impregnated sandstone deposit is a giant, exhumed oil field which occurs in the Green River Formation (Eocene), southwestern Uinta basin, Utah. The updip limit of the deposit has been eroded; thus, the precise mechanism of entrapment cannot be determined. The deposit occurs within a transgressive continental and lacustrine sequence. The alluvial Colton Formation (early Eocene) underlies the deposit and grades into marginal lacustrine strata of the overlying Green River Formation. The lower part of the marginal lacustrine sequence is predominantly deltaic, but the upper third is interbedded with open lacustrine facies. The uppermost part of the Green River Formation in the area is eroded. Petroleum-impregnated sandstone beds occur throughout the approximately 1,370 ft (418 m) of exposed marginal lacustrine facies. However, the main part of the deposit has a gross oil column of at least 860 ft (270 m) of which 640 ft (195 m) are petroleum-saturated siliciclastic rocks. The deposit is exposed only on the southwest side and at the northwest corner. Only 12 significant wells were drilled in an area of 20 sq mi (52 sq km) prior to 1979. Thus, the limits of the deposit and the contained petroleum resource are difficult to assess. The delineated part of the deposit contains about 2.2 billion bbl of petroleum in place. The downdip limit of the deposit has not been defined. End_of_Article - Last_Page 556------------

Definition of Depositional Facies of Parachute Creek Member of Green River Formation, Colorado--Evidence from Sulfur Isotopes and Whole-Rock Mineralogic Data: ABSTRACT

The Parachute Creek Member of the Green River Formation in the southern Piceance Creek basin displays four major lacustrine and marginal-lacustrine facies: deltaic-interdeltaic mudflat, carbonate mudflat, proximal open lacustrine, and distal open lacustrine. These facies are strongly gradational across the basin and their definition is not clearly evident from study of the physical characteristics of each facies. Additional evidence from whole-rock mineralogy (defined by X-ray diffraction) and sulfur isotopes, however, help to clarify each facies. The deltaic-interdeltaic mudflat facies contains channel-form cross-stratified sandstone intercalated with pyritic mudrock and marlstone. Detrital quartz, K-feldspar, Na-plagioclase, and rock fragments characterize the sandstone. The mudrock and marlstone contain both calcite and dolomite plus detrital silicate minerals; ^dgr34S values of pyrite from this facies range from 2 to 26 per mill and average 12 per mill. The carbonate mudflat facies is dominated by dolomitic algal stromatolite, oolite, pisolite, and interclastic carbonate rock, and interbedded green to gray pyritic marlstone. The marlstone contains both calcite and dolomite and considerable authigenic analcime; ^dgr34S values range from 5 to 40 per mill and average 16 per mill. End_Page 824------------------------------ Proximal open-lacustrine deposition produced well-stratified marlstone and lean oil shale, and rare algal stromatolite. Detrital silicate minerals become less abundant and are volumetrically replaced by dolomite, Fe-dolomite, excess-Ca ankerite, analcime, and rare dawsonite. The ^dgr34S values of pyrite from this facies range from 22 to 54 per mill and average about 30 per mill. The distal open-lacustrine facies is represented by richer grades of oil shale (greater than 15 gal/ton) and saline minerals (nahcolite and halite). The oil shale contains abundant Ca-Fe-Mg-carbonate phases, authigenic quartz, K-feldspar, Na-plagioclase, and dawsonite. Analcime is rare. The ^dgr34S values of pyrite and marcasite range from 18 to 66 per mill and average 35 per mill. Overall, the Parachute Creek Member shows that detrital silicate minerals and calcite are concentrated in the marginal-lacustrine facies but authigenic silicate minerals, complex Mg-Ca-Fe-carbonate minerals, and saline minerals are concentrated in the open-lacustrine facies. Sulfur-isotope data show that iron sulfide minerals become progressively enriched in 34S toward the open-lacustrine environment of deposition. End_of_Article - Last_Page 825------------

Lacustrine Facies and Stromatolites From the Middle Triassic of East Greenland

ABSTRACT The Middle Triassic Edderfugledal Member of East Greenland is interpreted as lacustrine because of: (A) nonmarine fossils such as conchostracans and small bivalves, (B) non-marine trace fossils such as Isopodichnus, Cruziana, Pelecypodichnus and Fuersichnus, (C) stromatolites with algal moulds, (D) sedimentary structures, which indicate shallow water, weak to moderately strong wave action and frequent subaerial exposure, (E) absence of tidal current generated structures and (F) great lateral continuity of single beds, which indicate a low topographic gradient in the basin. Six major facies are distinguished within this ancient lake content: (1) green mudstone (open lacustrine), (2) yellow dolostone (carbonate mudflat), (3) flat pebble conglomerate (beach la or stromatolite breccia), (4) stromatolitic limestone (nearshore lacustrine), (5) greyish wave rippled sandstone (shoreline sandflat) and (6) reddish sandstone and mudstone (alluvial mudflat). The stromatolitic columns, commonly between 5 and 15 cm high, are subcylindrical to hemispheroidal, discrete and laterally linked. They consist of mm-thin couplets of organic-rich micrite laminae with both coccoid and filamentous algal moulds and dolomicrite laminae. The member displays well developed cyclic sedimentation both in the basal open lacustrine unit (the Sporfjeld Beds) and in the overlying shallow lacustrine unit (the Pingel Dal Beds). In the Sporfjeld Beds open lacustrine facies dominate and marginal lacustrine facies are restricted to carbonate mudflats. In the Pingel Dal Beds open l custrine facies become relatively rare and shoreline sandflats and alluvial mudflats are added to the marginal lake environment. This change in sedimentation is explained mainly in terms of intermittent tectonic uplift. These tectonic events were probably accompanied by a gradual change towards more humid conditions during the Middle Triassic.

Comparative mineralogy of nearshore and offshore lacustrine lithofacies, Parachute Creek Member of the Green River Formation, Piceance Creek Basin, Colorado, and eastern Uinta Basin, Utah

A comparative study of the relative distribution of analcime, quartz, K-feldspar, Na-plagioclase, calcite, dolomite, and ankerite shows variable partitioning of these minerals between the marginal-lacustrine and open-lacustrine lithofacies of the Parachute Creek Member of the Green River Formation in the southern and western Piceance Creek Basin, Colorado, and the eastern Uinta Basin, Utah. The variations correlate with changes in the depositional environments of ancient Lake Uinta. Four individual, but gradational, environments of deposition are recognized in the Parachute Creek Member: (1) deltaic lacustrine; (2) carbonate flat; (3) proximal open lacustrine; and (4) distal open lacustrine. Rocks from these depositional facies range from sandstone and algal limestone in the first two marginal-lacustrine environments to oil shale, tuff, and saline-mineral beds in the latter two open-lacustrine environments. Detrital quartz, K-feldspar, and Na-plagioclase are most abundant in channel-form sandstone beds in the deltaic-lacustrine facies. In the open-lacustrine facies where oil shale is dominant, these silicates are still abundant but are authigenic or diagenetic in origin. Analcime is found in fine-grained rocks in all environments but is least abundant in oil shale of the distal open-lacustrine environment where saline minerals such as nahcolite and halite are present. Calcite, dolomite, and ankerite show the most complex facies distribution. Calcite is apparently most abundant in the finegrained calcareous rocks intercalated with sandstone and algal limestone in the marginal-lacustrine facies, and it is least abundant in the distal oil shale. Dolomite is abundant throughout the Parachute Creek Member but is most abundant in the distal oil shale. Ankerite is also most abundant in the distal oil shale but is rare in marginal-lacustrine rocks. The overall zonation of the carbonate suggests that the more chemically complex types are found in the more basinward environments. Thus, the pattern for the carbonate distribution is a reflection of local biogeochemical and geochemical conditions and not the result of transportation of carbonate phases from a fringing mud-flat (playa fringe) environment.

Early Tertiary sedimentation in the western Uinta Basin, Utah

During latest Cretaceous through middle Eocene time, over 3,000 m of siliciclastic and carbonate sediment accumulated in the Lake Uinta depocenter in northeastern Utah. Detailed stratigraphic analysis of this extensive lacustrine system, both along the western and southern outcrops and in the subsurface of the Uinta Basin, indicates three major facies: (1) open lacustrine, (2) marginal lacustrine, and (3) alluvial. The open-lacustrine facies consists primarily of mud-supported carbonate and claystone units with minor amounts of sandstone and siltstone. Kerogen and other organic compounds produce shades of gray and brown. The open-lacustrine rock assemblages were deposited away from terrigenous clastic influxes either near the center of the lake or in nearshore settings. Rocks originating in the latter environment contain abundant fossils (mollusks and ostracodes) and scattered desiccation features. The marginal-lacustrine facies is composed of gray-green calcareous claystone, channel-form sandstone, and grain- to mud-supported carbonate units. The dominant depositional environments are interpreted to be lake-margin carbonate flat, deltaic, and interdeltaic. Lake-margin carbonate-flat deposits consist of carbonate beds as thick as 30 m which grade lakeward from an ostracode and oolite grain-supported texture to a mixed mud- and grain-supported texture. Deltaic deposits consist of channel-form sandstone units as thick as 15 m which cut adjacent thin beds of sandstone, siltstone, and gray-green claystone. Ostracode- and oolite-bearing grainstone beds, as much as 4.5 m thick, of lake-margin carbonate-flat origin are commonly interbedded with the deltaic rocks. Interdeltaic rocks exhibit characteristics similar to the deltaic rocks but contain more carbonate beds of lake-margin carbonate-flat origin and far fewer sandstone units. The alluvial facies, representing alluvial-fan, lower deltaic-plain, and high mud-flat environments, occupied the most proximal setting within the depositional system. The lower deltaic-plain environment is typified by 15- to 30-m-thick channel-form sandstone units and associated thin-bedded sandstone, siltstone, and red, mud-cracked claystone. The alluvial-fan environment is characterized by thick conglomerate beds with crude horizontal stratification. Red claystone, minor isolated channel-form sandstone, and thin fossiliferous gray-green claystone units characterize the high mud-flat environment. Thin mud- and grain-supported carbonate units of lake-margin carbonate-flat origin are locally interbedded with rocks of the above environments. During most of early Tertiary time, the Lake Uinta system exhibited a northeast-trending core of open-lacustrine facies surrounded by successive halos of marginal-lacustrine and alluvial facies. The width of the open-lacustrine core continually fluctuated depending upon climatic and tectonic conditions prevailing at the time. Southerly derived feldspathic sands dominated the sediments supplied to the south flank of the basin, whereas sand contributed to the north flank was quartzose and originated from rocks exposed in the Sevier orogenic belt to the west and the Uinta uplift to the north. The north flank of the basin had a steeper depositional slope than the south flank, as manifested by the presence of coarser grained siliciclastic sediments and the narrower band of the marginal-lacustrine facies.