IMPLICATIONS OF REPETITIVE, VERTICAL STACKING OF PROGRADING SHORELINE DEPOSITS OF CRETACEOUS PICTURED CLIFFS SANDSTONE, NORTHERN SAN JUAN BASIN

The Pictured Cliffs Sandstone (Upper Cretaceous) is a progradational nearshore-marine sand body that was deposited in the San Juan basin (Colorado-New Mexico). Previous workers interpreted the unit as deltaic or beach-barrier environments. The unit serves as a hydrocarbon reservoir where unconventional stratigraphic traps are created by shale or coal beds in the northern part of the basin. In these instances, most of the hydrocarbons are found in landward pinchouts of nearshore and foreshore marine sandstone bodies. Outcrops of Pictured Cliffs Sandstone in the northern part of the San Juan basin show multiple transgressive-regressive sequences with the thickness of the sandstone layers varying both vertically and laterally. In outcrops, the Pictured Cliffs Sandstone is up to 100 m thick, extending from an intercalated basal contact with Lewis Shale (offshore marine) to an abrupt contact with the Fruitland Formation (nonmarine coal beds). Detailed outcrop analysis reveals 14 different lithofacies. The lithofacies are organized into sequences at meso-scale and macro-scale. At the meso-scale, four typical sequences can be recognized: (1) erosional scours overlain by hummocky stratified sandstone, followed by planar bedded sandstone, overlaid by ripple laminated sandstone and/or mudstones (interpreted as a tempestite sequence); (2) heterolithic, wavy bedded or ripple laminated sandstone and mudstone drapes (interpreted as tidal rhythmites); (3) interbedded mudstone and distal turbidite sequences up to 3 m thick (interpreted as prodelta deposits); and (4) massive sandstones up to 3 m thick containing intraclasts (interpreted as flood deposits). At the macro-scale, megasequences up to 10 m thick can be recognized. These consist of mudstones interbedded with individual tempestites (typically 30-40 cm thick), overlain by a sequence of amalgamated tempestites (1-8 m thick), overlain by cross-bedded sandstone. This megasequence thickens- and coarsens-upward, and is interpreted as a prograding shoreline sequence. There are approximately 10 of these parasequences observable in the northern part of the San Juan basin, indicating a subtle relationship between tectonic subsidence and eustasy.