Counting marmots…
An anclent stratlgraphic trap for petroleum exists in continental deposits at Rangely oil field, about 24 km south of Dinosaur National Monument, where the eol Ian Weber Sandstone (pennsy lvanlan-permian) intertongues with the Maroon Formation of fluvlal origin. The stratigraphic trap developed as a result of the progradation of eolian dunes toward the ancient Uncompahgre uplift. Fine silt and conglomeratic material brought into the dunes along the margins of the dunefield became impermeable layers--due to postdepositional diagenetic cementation and intrinsic textural properties. The conditions which created the stratlgraphic trap at Rangely may have developed ' in other areas along the margins of ancient Pennsylvanian uplifts in Colorado, Wyoming, and Utah. Analysis of cores from Rangely oil field indicates that porosity and permeability within the oil-productive sandstone Is affected by primary and secondary diagenetic processes. Reduction of porosity and permeability in the reservoir sandstones Is due to burrowing and contortlon of laminations, which destroyed or ~dified previously well-sorted laminations of sandstone units. Cementation is most complete in the burrowed and contorted Intervals. Evidence for eol Ian origin of the Weber Sandstone near Dinosaur National Honument includes: (a) large scale tabular-planar cross-stratified u nits with few horizontal symmetric ripples, but many low-relief, asymnetric ripples oriented up and down slipface deposits; (b) raindrop imprints on slipface deposits at Deerlodge Park, Colorado; (c) characteristics of contorted stratification, lamination style, and burrowing that e xactly match those of modern eolian deposits and some inferred ancient eolian deposits, and which can be interpreted in the l ight of known eol Ian processes; (d) interbedding of we1 I-sorted quartz sandstones and poor1 y sorted, mica-r ich siltstones and conglomerates at Rangely oil field, which Is interpreted to be interbedded eolian dune and fluvial sediments; (e) consistent southward transport directions In the Weber Sandstone are more compatible with a wind- driven depositional system than a marine depositional system, since steady ocean currents flowing south to south- east would be forced to flow upslope toward the anclent Uncompahgre and Front Range up1 ifts; (f) absence of appre- ciable clay or chert In the Weber Sandstone in the study area, in contrast to the occurrence of these minerals in marine rocks of the Weber Sandstone farther west; (g) thin, lentlcular carbonates (usually only .31-.61 m thick) restricted to extensive dlastems In the Weber Sandstone In the study area which indicate t hat the carbonates were deposited in non-marine ponds associated with interdune areas; (h) striking dl fferences between the lenticular, brecciated, unfossiliferous dolomites In the Weber Sandstone and the t hick, fossiliferous cherty limestones of approximately the same age in marine deposfts within the Weber Sandstone, and the older underlying Morgan Forma- tion. Shortly after deposition. the eollan deposlts of the Weber Sandstone became saturated with water and were then subjected to penecontemporaneous deformation that produced complex folding and breaking of laminations.
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