ABSTRACT

The Cap Mountain Limestone Member of the Riley Formation, Upper Cambrian of central Texas, consists of terrigenous and carbonate facies that accumulated in an overall transgressire-regressive sequence. The terrigenous strata comprise one-third to one-half of the member and are most abundant in the northern and western parts of the area.

The terrigenous strata are divisible into three facies. The low-angle cross-stratified very fine sandstone to coarse siltstone facies is an arkose. Other, less common sedimentary structures found in this facies include: terrigenous-carbonate flaser bedding, isolated vertical burrows, symmetrical and asymmetrical ripples convoluted and microfaulted laminae, desiccation cracks, and clay drapes. This facies accumulated on a tidal flat, primarily within the intertidal reaches. The trough cross-stratified fine-to-medium sandstone facies fills channels within the tidal-flat accumulations and displays a quartz-rich, fossiliferous, and intraclastic texture. Large channel-fills, 30 cm to 7 m thick, exhibit bipolar trough cross-stratification directions and represent trunk-channel sedimentation within the tidal flats. Small channel-fills, 5 to 30 cm thick, exhibit evidence of vast lateral migration and represent tributary channels draining the tidal flats. The thoroughly bioturbated very fine sandstone to coarse siltstone facies is the lateral equivalent of the low-angle cross-stratified sandstone-siltstone facies and, likewise, is an arkose. In addition to thorough bioturbation, this facies exhibits some terrigenous-carbonate wavy and lenticular bedding developed in micriterich areas. This facies represents shallow subtidal sedimentation. The freshness and abundance of feldspars in the tidal flat and adjacent areas, lack of abundant clay, and salt hopper impressions in clay drapes suggest that arid to semi-arid conditions existed in the source area and that eolian processes pr vided a high percentage of the terrigenous sediment to the area.

The carbonate strata, the lateral equivalents of the terrigenous strata, have three facies that are distinguished by their allochemical constituents. The brachiopod-trilobite-pelmatazoan biomicrite facies exhibits terrigenous-carbonate lenticular bedding, rippled and lumpy bedding, and bioturbation. This facies accumulated under shallow-shelf conditions. The sponge megasclere biomicrite facies contains an average 19 percent Chancelloria megascleres and segments of megascleres. These accumulations, which have no distinctive biohermal shape, constitute organic banks or loose skeletal buildups that developed several kilometers offshore on the shallow shelf and leeward of oolite shoals. The oosparite facies exhibits rippled beds, 1 to 2 m thick, which represent low-amplitude, long- avelength platform carpets of oolite. Ooclasts, abraded chunks of oolitic rock, are relatively common in this facies and were derived from hardgrounds.

The depositional history of the transgressire portion of Cap Mountain Limestone Member in central Texas outcrops may be subdivided into two phases. In the first phase, the strata in the lower part of the member accumulated on an extensive tidal flat cut by migrating tidal channels and in adjacent, broad, shallow subtidal areas. In the second phase, the strata in the upper part of the member accumulated mainly in shallow subtidal areas, on a shallow shelf, in sponge organic banks, and adjacent to oolite shoals.