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Shoreline Architecture and Sequence Stratigraphy of Campanian IIes Clastic Wedge, Piceance Basin Colorado: Influence of Laramide Movements in Western Interior Seaway

Abstract

Facies and sequence stratigraphic analysis of the Campanian Iles Formation of the Mesaverde Group in northwestern Colorado identify 11 shoreline tongues that form 3.3 My duration Iles Clastic Wedge extending east and southeastwards from the Sevier orogenic belt to the Western Interior Seaway. Iles Formation deposits and their coeval alluvial and coastal plain equivalents (Neslen Formation, Trail and Rusty Members of the Ericson Formation) are well exposed from Utah and southern Wyoming into northwestern Colorado. The Iles Clastic Wedge was examined in the subsurface Piceance Basin and at outcrops in Meeker and south of Rangely, NW Colorado. The correlation of 9 outcrop measured sections and log data of 110 wells reveal that the clastic wedge contains low-accommodation regressive-transgressive sequences (8–39 m thick) of Loyd Sandstone, Sego Sandstone, Corcoran Member, and Cozzette Member and their updip-equivalent Neslen Formation strata. A series of dip and strike oriented northern Piceance Basin transects makes it possible to evaluate coastline variability, and the progressive southeasterly pinchout of the 11 coastline tongues within the larger Iles Clastic Wedge. Facies associations of the sandstone succession indicate storm-wave dominated coasts that transition seaward into offshore/prodelta mudstones with thin-bedded sandstones and extend landward into tidal/fluvial channels and coal-bearing strata; facies associations also indicate interdeltaic coastal embayment with moderate tidal influence. The sequence architecture of the Iles Clastic Wedge indicates that the wedge differs from underlying and overlying wedge sequences. Since the maximum stratal thickness and the average shoreline thickness of the Iles Clastic Wedge indicate relatively thin, amalgamated and low accommodation succession than the overlying and underlying wedges, it is suggested that this low accommodation setting in northern Piceance Basin was caused by combined effects of embryonic Laramide uplifts or reduced Sevier subsidence across the Western Interior Seaway. Uplift or greatly reduced subsidence across the region would have caused an increase in coastal embayment as well as generally accelerated coastal regressions and transgressions in this 3.3 My interval.