Petrology of carbonate units in the Canyon Group (Missourian Series), Central Texas



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After filling the Fort Worth basin earlier in Pennsylvanian time, sediments derived from the uplifted Ouachita fold belt spilled across the Bend axis onto the eastern shelf of the Midland basin. Streams entering the shallow seas that covered the shelf constructed delta systems. In the area of this report, in southwestern Brown County, southeastern Coleman County, and north central McCulloch County, Canyon Group sediments are wholly marine deposits, in contrast to underlying Strawn Group and overlying Cisco Group, which contain coal beds and paralic sediments. Deltaic fans of terrigenous sediments, complete with channel-filling sandstones and conglomerates, were abruptly abandoned by their distributaries on at least 11 occasions during Canyon Group deposition. The abandoned surfaces were invaded by marine plants, including calcareous algae, and associated faunas, which generated and trapped carbonate mud on the shallow surface of the abandoned delta. Analyses of the microfacies, defined by distinctive biota and matrix textures, in the carbonate units permit recognition of a succession of depositional environments that is common to most of the Canyon Group limestone units. Muddy or sandy basal beds of most of the units are succeeded by Archaeolithophyllum - encrusting foraminifer biopelsparite which constitutes the majority of each unit. In most units the algal-foraminifer microfacies is gradational upward into a biopelsparite in which Fusulinids and benthic foraminifers are prominent constituents. Archaeolithophyllum is rarely present in this microfacies, although phylloid green algae have been observed. In four of the carbonate units, a rounded-fragment biosparite interval caps the microfacies succession. The sequences were deposited in shallow water that remained at approximately uniform depth as carbonate accumulation kept pace with subsidence due to compaction of underlying mud. This is indicated by the approximately uniform ratios of limestone unit thicknesses to the thicknesses of the underlying shales, it being assumed that compaction, and therefore space available for superposed carbonate accumulation, is a roughly uniform fraction of total mud thickness. Penecontemporaneous faulting defined a graben during the latter part of Canyon Group deposition. The graben, which is subparallel to depositional strike, persisted as a trough during the deposition of Ranger Limestone, Colony Creek Shale, and Home Creek Limestone #1, and was covered shortly thereafter by shale and limestone units of the upper part of Home Creek Formation. Subsequent displacement along portions of the bounding faults is due at least in part to differential compaction of the predominantly shale section within the graben. Similar features (e.g. Fort Chadbourne fault system) have been recognized in subcrops of Middle and Late Pennsylvanian age in Sutton and Schleicher Counties, Texas (Rall and Rall, 1958), but this is the first known report of such a shelf-edge fault system in outcrop