Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Lacustrine Carbonates and Evaporites – Facies Evolution and Diagenesis: Eocene Green River Formation, Piceance Creek Basin, Colorado
(1) Department of Geology & Geological Engineering, Colorado School of Mines, Golden, CO.
The lacustrine carbonates of the Eocene Green River Formation crop out on the western margin of the Piceance Creek Basin, in Douglas Pass. Carbonates consist of littoral to sub-littoral facies: skeletal-oolitic-quartz grainstone, oolitic packstone or grainstone, skeletal grainstone, oolitic wackestone, intraclast rudstone, stromatolites, and thrombolites. Facies commonly form thin, 1-2m thick, upward-deepening cycles that start with intraclast rudstone or oolitic grainstone followed by coarse-agglutinated stromatolites or thrombolites, and capped by fine-grained laminated stromatolites and shale.
The overall vertical succession of carbonate deposits correlate with lake evolutionary stages. The carbonate succession in the Douglas Pass area starts with skeletal-oolitic-quartz grainstone and skeletal grainstone deposits that occur in an initial Freshwater Lake Stage 1. Thrombolites capped by coarse-agglutinated stromatolites or fine-grained laminated stromatolite were deposited during the High Siliciclastic Input Stage 2. This Stage 1 to Stage 2 change also corresponds to a change in the lake waters to higher salinity. Nahcolite is precipitated in lake-center environments. Repeated deepening-upward cycles of thrombolites, coarse-agglutinated dendrolitic stromatolites, coarse-aggulutinated stromatolites, and fine-grained laminated stromatolites occur in the Rapidly Fluctuating Lake Stage 3. During this lake stage, periodic hypersalinity existed in the lake, and halite was deposited in the lake center. Thin, and the richest oil shale units, occur interbedded with the halite beds. The uppermost part of the carbonate section is dominated by laminated stromatolites that correlate with an overall deepening of the lake, and the beginning of the Rising Lake Stage 4. The lake remains restricted during this period and Nahcolite is common. Nahcolite precipitation continues into the ensuing High Lake Stage 5, and is accompanied by widespread, organic-rich oil shale deposition (Mahogany Zone). Changes in Carbon and Oxygen stable isotope values reflect changes in lake chemistry and correspond well to Lake Stages.
The Green River Formation carbonates have undergone a complex diagenetic history including micritization, dissolution, neomorphism, siderite cementation, mechanical and chemical compaction, dolomite cementation, shallow burial dolomitization, fracturing, cementation (equant, block, and poikilotopic cements), late burial dolomitization, and dedolomitization.