J. Frederick Sarg

Petrology of the Carbonate-evaporite Facies Transition of the Seven Rivers Formation (Guadalupian, Permian), Southeast New Mexico

ABSTRACT The Lower Seven Rivers (Guadalupian) evaporite to carbonate facies change, commonly interpreted as a coastal sabkha/carbonate lagoon transition, represents instead an interfingering of dolomite and gypsum in a shallow subaqueous hypersaline shelf setting behind the main Capitan shelf-edge complex. Three mesosaline (35-120) carbonate lithofacies occur in lateral succession over a distance of 3.5 km, adjacent to the evaporites. From the evaporite margin toward the Capitan shelf crest they are: 1) evaporite-edge grapestone grainstones which are current-laminated and occur interbedded with fine-grained quartz sandstone units suggesting a moderate-energy, shallow subaqueous depositional environment; 2) skeletal-rich pellet packstone/wackestones which are commonly current-laminated or burrowed and contain a low-diversity but abundant biota of calcispheres and encrusting forams with a few molluscs and ostracods, suggesting a shallow subaqueous environment; and 3) a shallower shelf facies of pellet wackestone/mudstone characterized by increased micrite, fewer skeletal fragments, and algal stromatolites. The evaporite facies occurs as layers 1 to 7 m thick composed dominantly of mosaic gypsum deposited in a subaqueous environment (i.e., shelf basin) during times of maximum salinity, perhaps influenced partly by the evaporite-margin facies. The mosaic structure is suggested to have been formed by early diagenetic growth of sulfate nodules below the sediment-water interface. The average S34 for this gypsum is +9.3 ± 1.0 which agrees well with the average Upper Permian value of + 10. A fourth carbonate facies of skeletal-poor pellet packstone/wackestones occurs interbedded with the evaporites. It is commonly plana laminated and contains scarce skeletal fragments similar to the skeletal-rich facies. Neither the evaporite facies nor the skeletal-poor facies within it, nor the two adjacent carbonate facies, contain sabkha features such as intertidal algal laminates, desiccation features, or subtidal to supratidal cycles capped by nodular gypsum. Post-depositional diagenesis included 1) cementation of the carbonates by CaCO3 in two forms, including eogenetic isopachous (submarine) cement in the grapestone grainstones and eogenetic/mesogenetic equant spar cement in the grainstones and packstones; 2) eogenetic and probably syndepositional dolomitization of the carbonate rocks to a microcrystalline dolomite; 3) eogenetic insediment growth of evaporite crystals and nodules; 4) post-Permian telogenetic solution brecciation and dedolomitization; and 5) hydration and recrystallization of subsurface anhydrite to gypsum.

Lithofacies, stable isotopic composition, and stratigraphic evolution of microbial and associated carbonates, Green River Formation (Eocene), Piceance Basin, Colorado

Lacustrine carbonates of the Eocene Green River Formation crop out on the western margin of the Piceance Basin and the eastern margin of the Uinta Basin, in western Colorado. This area allows tracing of vertical and horizontal facies variation over hundreds of meters. Limestone beds consist of littoral to sublittoral lithofacies: bioclastic and oolitic grainstones, oolitic wackestone, intraclastic rudstone, stromatolites, and thrombolites. Facies form upward-deepening cycles that start with sharp-based grainstones and packstones followed by stromatolites or thrombolites and capped by fine-grained stromatolites and/or oil shale deposits. The vertical succession of carbonate deposits correlates with evolutionary lake stages. The succession starts with grainstone deposits rich in ostracods and gastropods that correspond to an initial freshwater lake. Thrombolites capped by laminated stromatolites or coarse-agglutinated stromatolites correlate with a higher-salinity transitional lake. Deepening-upward cycles, as much as 5 m (16 ft) thick, of thrombolites, agglutinated stromatolites, and fine-grained stromatolites occur in the highly fluctuating lake. The upper section is dominated by laminated stromatolites that correspond to a rising lake. Stable isotope 18O and 13C values covary and range from 8 to +0.8 and 3 to +5, respectively. The 18O values indicate carbonate-precipitating water evolved from fresh to saline and became less saline in the upper Green River. Negative excursions of 13C values correspond to lake level rises, and positive excursions of 13C values occur during lake level falls. Syndepositional to burial diagenesis modified carbonate porosity. Early dissolution is followed by burial compaction and fracturing. Compaction and late calcite cements occluded primary and secondary porosity.

Depositional environments and sequence stratigraphy of carbonate mudrocks using conventional geologic observations, multiscale electrofacies visualization, and geochemical analysis: The case of the Tuwaiq Mountain and Hanifa Formations in a basinal setting, Saudi Arabia

ABSTRACT Depositional interpretation and sequence stratigraphic analysis of carbonate mudrocks requires numerical analysis and data integration to achieve quantitative, predictive stratigraphic and geochemical models. A depositional and sequence stratigraphic analysis is built for a basinal interval of the Tuwaiq Mountain and Hanifa Formations, Saudi Arabia. Conventional geologic interpretation, automated electrofacies analysis, and geochemical interpretation are integrated using quantitative means. Cluster analysis of well logs using self-organizing maps and hierarchical clustering allowed for a multiscale electrofacies analysis. This is useful for identifying major lithological surfaces, which commonly correspond to sequence stratigraphic surfaces. Geochemical data were used for depositional environment interpretation, such as sediment provenance, redox, and paleoproductivity conditions. Factor analysis is used to group element data. Redox and paleoproductivity indices were calculated using electrofacies clustering of different elemental groups. Electrofacies analysis shows good correlation with the lithofacies; that is, lithofacies can be identified from logs. Five major lithofacies have been identified in the studied interval: (1) wispy laminated skeletal wackestones to mudstones; (2) differentially cemented skeletal wackestones, packstones, and grainstones; (3) laminated peloidal mudstones to wackestones; (4) bioturbated packstones and wackestones; and (5) palmate anhydrite. The majority of the interval is interpreted to be deposited by gravity processes. Four major sequences have been identified in the Tuwaiq Mountain and Hanifa Formations made of transgressive and highstand systems tracts. The uppermost bioturbated packstones of the Hanifa Formation are interpreted to be a lowstand systems tract with subsequent restriction leading to the deposition of gypsum. In the studied interval, total organic carbon (TOC) content correlates well with suboxic to anoxic intervals that have high paleoproductivity. Complete anoxia is not a prerequisite for organic matter preservation. These high TOC intervals are mainly in transgressive systems tracts.

Sedimentology of the World Class Organic-Rich Lacustrine System, Piceance Basin, Colorado

The Piceance lake basin formed between ca 54–48 Ma years ago, during the early to middle Eocene, and it contains the largest known oil shale resources in the world today (~1.5 trillion bbls in place). Based on the detailed facies and facies association study three small-scale (decimetre to metre) depositional cycles have been separated that characterize deposition along the basin margin and in the deeper part of the basin. Stacked depositional cycles form large-scale depositional sequences (metre to 10’s of metres). Depositional sequences characterize significant changes in lake regime and are divided into periods of low, rising, and high lake. During low lake level, main accumulation of deposits occurred in the deeper part of the basin where organically lean deposits formed. Following rising and high lake levels brought thick accumulations of siliciclastic and carbonate deposits along lake margin that were later, during high lake level capped by littoral to sublittoral siltstones and mudstones, and lean oil shales. In the deeper part of the lake thick organically rich deposits formed. The overall lake evolution is characterized by lake stages that reflect long-term changes in the basin controlled by both, climate and tectonics. Lake Stage 1, Fresh to Mesosaline lake was deposited during decreasing tectonic activity and increasing climate control. Lake Stages 2 and 3, Transitional and Highly Fluctuating lake are interpreted to be dominated by an increasingly warm and arid climate and characterized by laterally discontinuous and very rich oil shale deposits. The following Stages 4 and 5, Rising and High lake record the change to a wetter climate and increasing tectonic activity, resulting in increased runoff, development of a widespread deep lake that extended across the Piceance basin, and are marked by the laterally continuous rich oil shale deposits. Stage 6, Closing lake marks high siliciclastic input, the beginning of the closing of the Piceance basin, and progressively decreasing organic richness.