A core drilled in the center of the Piceance Creek Basin, Northwest Colorado, penetrated 580 m of profundal lacustrine sediments of the Green River Formation (Eocene), from the top of the Parachute Creek Member to the lower part of the Garden Gulch Member. Seven zones of rich oil shale, separated by thinner zones of lean oil shale, were cored, the uppermost rich zone being the Mahogany zone.
The Mahogany zone is 51 m of kerogen-rich carbonates with minor silicate-rich tuffs. We distinguished rocks with %Corg > 10% (kerogenites) from those of the overlying “A-groove” and underlying “B-groove” which have %Corg between 0.5% and 10% (kerogenous carbonates). No rocks with %Corg < 0.5% occur in these intervals. The organic matter is predominantly amorphous kerogen, with only minor amounts of extractable organic matter (EOM). Minerals either were deposited as detrital grains from suspension or as volcanic ash, or were formed as chemical precipitates in the lake waters or in interstitial pore fluids.
A diversity of stratification types and thicknesses occur in these rocks, reflecting variations in depositional rates and sources of both organic matter and minerals. With increasing amount of organic matter, laminae become thinner and less continuous or are not present.
The sedimentary structures in these rocks record compressive ductile (early), compressive brittle (middle), and tensile brittle (late) deformation after deposition. The degree of ductility was controlled by the amount of organic matter in the sediments. Kerogenous carbonates, containing less than 10 %Corg, were relatively non-ductile during early compaction and dewatering. Conversely, organic-rich kerogenites were highly ductile and flowed in response to early burial. Compressive brittle structures (such as stylolites and vertical, hairline fractures) occur only in kerogenous carbonates, which were less ductile following early compaction than were kerogenites. Irregular fractures and breccias caused by dissolution of evaporites formed due to later unloading of overlying strata.
Based on petrology, stratification types and sedimentary structures, 11 lithofacies were identified in the core, of which 7 are most common. In order of increasing organic content, these lithofacies are: 1) tuff, 2) kerogenous dolomite, 3) laminated kerogenite, 4) massive kerogenite, 5) laminated kerogenite with selvages of kerogen, 6) microlaminated (laminae < 0.1 mm thick) kerogenite, and 7) auto-brecciated kerogenite. These lithofacies occur in recurrent associations in the core, reflecting periodic influxes of minerals from the littoral zone into the profundal zone of the lake.
