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Topographic Irregularities on the Base Zuni Supersequence Boundary and their Initial Cretaceous Sediment Fill, Central Texas

S. C. Atchley1, L. M. Zygo2, and J. R. Wallgren1
1Baylor University, Dept. of Geology, Waco, TX
2Baylor University, Center for Applied Geographic and Spatial Research, Waco, TX

The Cretaceous-pre-Cretaceous unconformity, i.e., base Zuni supersequence boundary, crops out across much of central and west Texas. Topographic features on the unconformity surface are exhumed along the retreating margin of the nearly flat-lying Cretaceous strata that overlie the surface within Lampasas and Mills counties, Texas. Digital elevation data displayed as a topographic profile atop Pennsylvanian outcrops adjacent the Cretaceous margin suggest that paleotopography was influenced by northwest-dipping Pennsylvanian strata that underlie the unconformity surface. Differential erosion of Pennsylvanian limestone, sandstone, and shale prior to Cretaceous deposition created a trellis network of northeast-trending valleys and divides.

Local tectonic features, climate, and rising sea level influenced the nature of the Cretaceous fluvial fill that directly overlies the unconformity surface. The Sycamore Conglomerate is the earliest Cretaceous deposit, and accumulated within a semi-arid climate as a down-filling braided alluvial wedge shed off the Llano Uplift. The Travis Peak Formation onlaps and disconformably overlies the Sycamore Conglomerate, and was deposited as an aggradational gravelsand meandering river complex during sea level rise.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana