Vakarelov, Boyan K.¹, Janok P. Bhattacharya¹ 
(1) University of Texas at Dallas, Richardson, TX

ABSTRACT: High-Frequency Tectonic Sequences in a Cretaceous Foreland

Tectonically-related accommodation changes that mimic glacio-eustatic frequency are documented in the Cenomanian Lower Belle Fourche Member of the Frontier Formation, Wyoming. Mapped using thickness variations between numerous bentonite layers, positions of transgressive surfaces, and angular truncation of units, subtle tectonic movements of amplitudes of 20-30m and wavelengths of up to a hundred kilometers can be identified. These structures control positions of delta lobes and sand distribution within the Frontier. Three distinct uplift pulses result in truncation of units that can be mapped between the Clay Spur bentonite dated at 97.17 ± 0.69Ma and a younger unnamed bentonite dated at 95.78 ± 0.69Ma. This results in an average frequency of uplift equal to 0.46m.y. As five bentonite-bounded units are mapped between the Clay Spur and the first truncation surface, which suggest up to two more episodes of uplift, a more realistic estimate of frequency of tectonic uplift in the study area is 0.2m.y. The results of this study dispel the notion that parasequence scale deposition must be of purely glacio-eustatic origin and highlights the importance of tectonic controls in sequence stratigraphy.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.