--> Abstract: Local Tectonic Control from Parasequence Architecture: Upper Cretaceous Second Frontier Sandstone, Powder River Basin, Wyoming, by Boyan Vakarelov and Janok P. Bhattacharya; #90078 (2008)

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Local Tectonic Control from Parasequence Architecture: Upper Cretaceous Second Frontier Sandstone, Powder River Basin, Wyoming

Boyan Vakarelov1 and Janok P. Bhattacharya2
1Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia
2Geosciences Department, University of Houston, Houston, TX

Wave-dominated, shallow-marine successions are typically considered to be homogeneous ‘tank-like’ sand bodies. Internally they are shown to be built by highly elongated parasequences that are form by pulses of linear-shoreline progradation. These geometries govern various reservoirs, such as the shape and orientation of compartments and distribution barriers and baffles.

The parasequence architecture of the Cenomanian Second Frontier sandstone (Frontier Formation) in east-central Wyoming shows that the above assumptions do not necessarily apply to all wave-dominated settings. Even though this outcrop, core, and well-log-based study shows that the sandstone is clearly wave-dominated throughout and has a well-defined NNW-SSE elongated mapview outline, the orientation of internal parasequences do not conform to this trend. Parasequences are oriented obliquely and show well-defined southward (along stike) compensational migration with time. It is argued that the observed parasequences architecture is governed by syn-depositional tectonic control, which affected position of river trunk streams, governed the mode of shoreline progradation, and limited the extent of the Second Frontier sandstone to the west.

These finding are compatible with observations in Holocene wave-dominated systems that: (1) often show complex internal architecture, (2) rarely extent for more than several tens of kilometres long strike as a linear shoreline, and (3) are frequently affected by local topographic control. It is argued that ancient shoreface models, which are based on tectonically quiescent passive margin settings, may not be fully applicable to many areas that have been affected by local tectonism or other factors, and should be applied with care.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas