Geological Engin. - Mines Magazine - Colorado School of Mines

Figure I. Index Map, Piceance Creek Basin, showing locationof s+rafigraphic sections and contact between Parachute Creekand Evacuation Creek members of the Green River formation.SECTION (I - BFigure 2. Structure Contour Map, ("A"-Groove) and CrossSection, Piceance Creek Basin, Colorado.depression and lacustrine depositionprevailed. Consequently the characteristiclithologies of the GreenRiver Formation are well laminated,lacustrine mudstone, shale, marlstone,and dolomite. The Green River Formationcan be subdivided on thebases of sandstone content and presenceor absence of substantial quantitiesof kerogen into five mappablemembers (from oldest to youngest): Douglas Creek, Anvil Points,Garden Gulch, Parachute Creek, andEvacuation Creek (Donnell, 1961) (seeFigure 2).The Douglas Creek and Anvil PointsMembers are dominated by brownto buff, cross-bedded, ripple-markedsandstone, algal and ooUtic limestone,and gray shale, which is barren ofkerogen. These beds accumulated ator near the shore of the newly formedEocene lake in the high energy environmentsof the delta, shoreline, andalluvial plain. The Douglas Creek andAnvil Points Members thin abruptlybasinward by facies change to thewell laminated shales and kerogenousmarlstones of the Garden Gulch andParachute Creek Members.The Garden Gulch Member is composeddominantly of well laminatedshale which is barren of kerogen.Locally marlstones, thin sandstonebeds, and limestones occur in theMember. The principal difference betweenthe Garden Gulch Member andthe overlying Parachute Creek Memberis the relative lack of kerogen andcarbonate in the Garden Gulch. Locallyin the center of the basin, theGarden Gulch Member does containsignificant concentrations of kerogen.Figures 3 and 4 display the facies relationshipsof the Garden Gulch withthe overlying Parachute Creek andunderlying Anvil Points and DouglasCreek. The shoreward divergence ofthe bedding planes and time lineswithin the Garden Gulch Membershows that a greater thickness ofallogenic lacustrine mud was accumulatingnear the shore than in thecenter of the lake during the sametime.The Parachute Creek Member containsthe gibbsite, dawsonite, nahcoiite,halite and the thickest and richestkerogen-bearing beds in the GreenFUver Formation, The Member iscomposed almost entirely of dolomiteand marlstone, most of which yieldoil when distilled. The thickness andoil yield grade of the kerogen-richdolomite increases greatly toward thecenter of the basin from all sides, asshown on Figures 3, 4, and 5 (Stanfield,1966, p. 36). The coincidence ofthe present structural low of the basinand the thickest area of rich "oilshale" shows that chemical stratificationof the lake must have controlledthe preservation of the organicmaterial on the lake bottom. Thus,the relatively clear and fresh surfacewater supported a prolific flora, whilethe bottom conditions beyond a criticaldepth were reducing and saline, favorablefor the preservation of the organicdebris. Figures 3 and 4 showthat the basal 1000 feet of kerogenbearingdolomite accumulated in thecenter of the basin while kerogenleanmarlstones were deposited nearerthe edges of the lake. The gibbsite,dawsonite, nahcolite and halite areconcentrated in this same interval.The gibbsite and dawsonite are concentratedin a stratigraphic intervalbetween 400 and 1400 feet below theA-groove marker bed (Figures 3and 4) in the center of the basinwithin the richest "oil-shale" units.The shoreward convergence of thebedding planes and time lines withinthe Parachute Creek beds below theA-groove marker bed illustrates thatthe chemical-density stratification ofthe lake allowed a greater thicknessof kerogen-rich dolomite to accumulatein the deep central part of thedepositional basin than near the edgesduring contemporaneous deposition.This same chemical-density stratificationallowed the preservation of agreater concentration of organic debrisand was probably a factor contributingto the formation of the gibbsite,dawsonite, and nahcolite. The conditionswhich allowed the preservationof the organic debris became morewidespread with the passage of time,permitting the accumulation of thebasinwide kerogen-rich Mahogonyzone ("a Mahogony Ledge") near thetop of the Parachute Creek Member.The A-groove marker bed is a widespread,thin, relatively kerogen-lean,tuffaceous dolomite bed which can beused effectively as an electric-logmarker for correlation purposes.Hence, the stratigraphic sections (Figures3 and 4) are hung on this marker.The upper 800 feet of the ParachuteCreek Member contains dawsonite,gibbsite, and nahcolite in significantquantities, although it is much moreirregularly distributed than in thelower portion.The Evacuation Creek Member iscomposed of buff to brown, lenticularsandstones, siltstones, kerogen-leanmarlstones, and shales, which accumulatedin the waning stages of thelacustrine deposition. Thus, the Memberincluded interbedded facies resultingfrom varying depositional environments: lacustrine (laminatedmarlstones and shales), shoreline,deltaic, and alluvial (standstones, siltstones,and nonlaminated shales). Thegradual restriction of the lake causedthe contact between the ParachuteCreek and the overlying EvacuationCreek Members to be gradational anda facies relationship. Hence, kerogenbearingmarlstone deposition persistedin the much-restricted lake in thebasin center long after non-lacustrineconditions overtook the rest of thebasin.Dawsonite and Gibbsite.—The distributiotiof significant concentration18 MAY, 1970 • THE MINES MAGAZINETHE MINES MAGAZINE • MAY, 1970 19