Stratigraphy and petrography of the Upper Chert and Shale Member, Caballos Formation, Brewster County, West Texas

Hovorka, Susan D. (Susan Davis)
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The Upper Chert and Shale Member of the Devonian (?) Caballos Formation is a sequence of interbedded multicolored bedded radiolarian chert and green and gray siliceous shale. Models for depositional environments in which bedded chert formed are still in the early stages of development. A model for deposition of the Upper Chert and Shale Member is presented: 1) Bedded green and black radiolarian chert was deposited, probably unconformably, on top of the Upper Novaculite Member of the Caballos Formation. The chert was deposited as an organic-silica/clay ooze which was modified by mild soft-sediment deformation and by dissolution/reprecipitation of silica to its present form. Bedded radiolarian chert was probably deposited in water more than a hundred meters deep. Callixylon logs floated to the basin from adjacent land areas; sparse carbonate beds within the member were formed from resedimented carbonate from shelf areas. 2) A basin-wide change in geochemical conditions caused deposition of two unusual beds of chert breccia and conglomerate within a 10-m-thick interval of red shale near the top of the Upper Chert and Shale Member. The breccia precursor was an unusual, soft, siliceous material (silica gel, magadiite, siliceous ooze?) formed either as a direct precipitate in a restricted basin or by intense early diagenetic alteration of organic-silica/clay ooze. The breccia beds show evidence of two episodes of deformation. Loading of unusually soft sediment after shallow burial caused lateral flow of sediment and resulted in rolled, folded, and cracked surfaces and irregular thicknesses of the beds. The second episode of disruption was brought about by extreme internal shrinkage (50 to 70%) to produce septarian-nodule-like cracks. Displacive crystallization of evaporite minerals may have further expanded voids in some places. Voids are filled with a complex sequence of cements, including chalcedony (normal bright, "gray flannel," and zebraic, all length-fast), quartzine (length-slow), "cubic" quartz, and "harlequin" quartz. Cements typically form isopachous crusts but may produce microstalactitic forms. Pelleted internal sediment similar to the chert precursor sediment interlayered with cement indicates that cementation was early. Restriction of the basin and unusual diagenetic fabrics in the breccia beds are most conveniently explained if the breccia beds formed in shallow water with intermittent subaerial exposure. The two breccia beds persist throughout the outcrop area. Red hematitic jasper and its unoxidized precursor, black pyritic chert, are found in the north-central part of the area and represent the most extreme development of the brecciated fabric. In other areas, green (colored by illite, chlorite, and siderite) breccia predominates. In the southeastern part of the area, where sedimentation was most rapid, thick conglomerate overlies or is the lateral equivalent of the breccia. The fragments of soft sediment were transported short distances by currents or gravity. 3) A return to the conditions described in "1" caused deposition of more green chert. Chert deposition was ended by the gradual encroachment of Tesnus clastics from the southeast.