Petrography and Petrogenesis of the Alibates Dolomite and Chert (Permian), Northern Panhandle of Texas

Abstract

The Permian (Guadalupian) Alibates Dolomite crops out along the Canadian River in the northern Panhandle of Texas. The dolomite is underlain by the Permian Whitehorse Formation and is overlain by the Permian Quartermaster and Tertiary Ogallala formations. The 5-meter section of Alibates consists of three members: a lower gray dolomite and an upper gray dolomite separated by an interval of red to brown calcareous mudstone. The upper dolomite and middle red beds are locally absent. Regional paleogeographic and stratigraphic studies suggest that the Alibates Dolomite was deposited in a shallow lagoonal to supratidal carbonate mudflat environment. Lamination and brecciation in the dolomite members are similar to algal laminates in other ancient rocks and in modern sabkha algal-mat environments. The three members represent a transgressive-regressive-transgressive sequence. Extensive calcitization of both dolomite members has occurred at several places. Calcitization has formed a "boxworks" structure where it has proceeded along fractures and porous zones in the dolomite. Chertification has occurred locally in both dolomite members forming chert beads and sporadic, irregular chert masses. Massive sheets of chert have completely replaced the upper dolomite at the Alibates National Monument. Evidence for a replacement origin for the chert includes: 1) sharp dolomite/chert contacts, 2) clasts of dolomite floating in a chert matrix, 3) relic lamination in the chert, 4) chertified sedimentary breccia, and 5) length-slow chalcedony. Sources of silica were stratigraphically above the Alibates, and silica-bearing solutions percolated downward into the dolomite along fractures and porous zones. Both calcitization and chertification can be related to calichification which is a common process in this climate: calcitized and chertified zones in the Alibates may have formed by this or a similar process.