LAST TECTONIC SIGNAL FROM ARM'S WICHITA-AMARILLO BLOCK IS RECORDED BY PERMIAN POST OAK CONGLOMERATE AND PALEOTOPOGRAPHY

The Wichita-Amarillo and Arbuckle crustal blocks are fartherest east of the Ancestral Rocky Mountains (ARM). The Wichita Mountains of southwestern Oklahoma, and part of the Arbuckle Mountains of southern Oklahoma, are the surface expression of the Cambrian Southern Oklahoma Aulacogen. This rift zone was uplifted as discrete crustal blocks in the Pennsylvanian, recognized by many as beginning in the Morrowan, through sedimentological signals in units in adjacent Anadarko and related basins (~330Ma). Timing and nature of the last tectonic signal in the Wichita-Amarillo block, and thus length of tectonism, have not been much emphasized. End of tectonism for this block is recorded by the Permian (Leonardian) Post Oak Conglomerate, a facies of the Garber Sandstone-Hennessey Group (~270Ma).

The Post Oak was recognized as a distinct stratigraphic unit by Chase in 1954, with 3 distinct facies: granite clast, rhyolite clast, and limestone clast types, reflecting local sources. The characteristically rounded granitic clasts formed through spheroidal weathering in an interval of tectonic quiescence, allowing widespread low-relief landscapes and deep weathering, just preceding final uplift. Then a concentrated period of uplift, where erosion rates greatly exceeded weathering rates, dismantled the deep regolith of active spheroidal weathering, and distributed the corestones and finer debris as sedimentary aprons around the newly uplifted, less-weathered bedrock. Maximum thickness of the Post Oak provides a minimum estimate of the amount of vertical uplift: <100m. However, relief of the Permian topography, formed during this uplift and subsequently buried and preserved by the Hennessey shales, gives a better estimate: 300-400m. Donovan used different arguments, relating to lateral offsets of the limestone clast facies, to give potential strike-slips of several km.

Thus, while final ARM tectonism in this block yielded offsets 1-2 orders of magnitude less than that "forming" the blocks at the beginning of uplift, the end and last stage of tectonism, approximately 60Ma later, was still significant.