Glen Rose Cycles and Facies, Paluxy River Valley, Somervell County, Texas

Paleoenvironmental analysis of the alternating beds of the basal Cretaceous Glen Rose Formation in Central Texas indicates a cyclical alternation of subtidal through supratidal facies. These facies were reconstructed through detailed analysis of lithology, biota (including macrofauna, microfauna, ichnofauna, and macroflora), detrital shell dispersal, and primary sedimentary structures. Facies succession indicates that each cycle represents a subtidal to supratidal depositional regime transit; cycles are regressional, with the transgressional phase being poorly developed, if at all. This conclusion is in agreement with results of studies of the Holocene transgression, which reveal a regressional succession developed by progradation of shorelines. Some other formations show similar depositional regime transit cycles; hence the Glen Rose model may be widely applicable. Facies variations within and between cycles indicate (1) that there is no typical or ideal cycle, (2) that the cycles record successive sedimentation units within a marginal marine hypersaline lagoon or bay system, and (3) that depositional regime transit cycles have limited correlation value. Facies in the seven cycles in the lower Glen Rose, Paluxy Valley, include subtidal to supratidal flat deposits, marsh, flood plain, shoal patch reef, bay, marsh island, and shifting sand lobe deposits; the only typical aspect of the cycles is their general subtidal to supratidal succession. Mixed terrestrial, marginal marine, and marine faunas and floras within these facies tracts indicate that the lower Glen Rose in this area is a lagoonal or bay depositional system. Paucity of terrigenous elastics, abundance of evaporites, presence of serpulid-patch reefs,and relatively low diversity of faunas with an abundance of thick- shelled ostracods indicate that at least the earlier phases of the lagoonal system were hypersaline. Areal stratigraphic correlation of these depositional regime transit cycles is unwise because the cycles tend to proliferate downdip, and because units that could be used as key beds commonly are local in extent. These beds reflect similar depositional conditions which recur in homotaxial phases of different cycles; indeed potential key beds may change facies within a few miles, even when bed thickness remains nearly constant. Correlation by matching cyclesin sections less than a mile apart, however, is feasible. Several particulars are useful in recognizing position in the cycle. Burrow fillings change upwards from lime-mud to sparry calcite or dolomite--a reflection of diagenetic regime. Burrows with unlithified or partially indurated calcite mud and shells occur in subtidal sediments. Diagenetically modified burrows first appear in or above the zone of sorted and abraded shell, inferred to be the wave or beach zone. Diagenetic burrow modification is intrastratal; this plus the dense fabric with unaltered shells indicate early, soft sediment diagenesis. Paleoenvironmental evidence plus studies of the Recent support the idea that these diagenetic modifications are associated with exposure and solutional modification, dolomite by hypersalinity, sparry calcite by fresh water. Subtidal deposits contain articulated and unsorted shells, many in living position; nearshore wave zone or intertidal shells occur in beds of well-sorted and abraded detrital plates. Supratidal deposits abound in rollers or may contain abraded detrital plates in texturally inverted sediments. Supratidal marshes contain abundant remains of the plant Frenelopsis. Its tiny cells, thick epiderm, protected stomata, and poorly developed roots indicate it is a physiological xerophyte similar to Salicornia of present-day salt marshes.