Two Depositional Models for Pliocene Coastal Plain Fluvial Systems, Goliad Formation, South Texas Gulf Coastal Plain: ABSTRACT

The Goliad Formation consists of four depositional systems--the Realitos and Mathis bed-load fluvial systems in the southwest and the Cuero and Eagle Lake mixed-load fluvial systems in the northeast. Detailed facies analysis indicates that Goliad bed-load and mixed-load fluvial systems represent two contrasting depositional environments. Five facies are recognized in the Realitos and Mathis bed-load fluvial systems: (1) primary channel-fill facies, (2) chaotic flood channel-fill facies, (3) complex splay facies, (4) flood plain facies, and (5) playa facies. Both channel-fill and splay deposits consist primarily of gravel, sandy gravel, and coarse to medium gravelly sand. Primary channel-fill deposits are characterized by large-scale accretionary foresets and trough cross-beds, localized scour-and-fill structures, and massive beds. Fining upward trends are crudely developed to absent, and no diagnostic vertical sequence of sedimentary textures and structures has been recognized. Chaotic flood channel-fill deposits are characterized by erratic interbedding of coarse and fine units, and of erosional and accretionary feat res. Individual beds have chaotic or poorly ordered textural profiles, and an absence of well-developed internal structures. Extensive flocks of stacked scour-and-fill structures are common. Complex splay deposits share characteristics of both crevasse and sheet splays. A model for Realitos-Mathis depositional environments shows arid-climate braided stream complexes with extremely coarse sediment load, highly variable discharge, and marked channel instability. Broad, shallow, straight to slightly sinuous primary channels were flanked by wide flood channels. These acted as part of the flood plain under most conditions, but as channels during high intensity flood flow. Flood channels passed laterally into broad, low-relief flood plains. Small playas occupied topographic lows near large channel axes. Three facies are recognized in the Cuero and Eagle Lake mixed-load fluvial systems: (1) channel-fill facies, (2) crevasse splay facies, and (3) flood plain facies. Channel-fill deposits consist of coarse to medium sand and gravelly sand, with a variety of large- and intermediate-scale sedimentary structures. Small-scale structures are common in finer grained beds. Fining-upward sequences are moderately well developed and commonly stacked. Crevasse splay deposits consist of medium to fine sand and silt, with abundant small-scale sedimentary structures. A model for Cuero-Eagle Lake depositional environments shows coarse-grained meander belts in a semi-arid climate. Slightly to moderately sinuous meandering streams were flanked by low, poorly developed natural levees. Crevasse splays were common, but tended to be broad and ill-defined. Extensive, low-relief flood plains occupied interaxial areas. While a number of models for coarse-grained meander belts exist in the literature, there are few models for arid-climate, gravel-rich braided stream complexes. These systems are likely to be characterized by extreme hydrodynamic complexity, which will be reflected in the resultant sedimentary package. The model proposed for the Realitos and Mathis fluvial systems may aid in recognition of analogous ancient depositional systems. In addition, since facies characteristics exercise broad controls on Goliad uranium mineralization, the proposed depositional models aid in defining target zones for Goliad uranium exploration. End_of_Article - Last_Page 484------------