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The Norias delta system was the major depositional component of the Frio Formation, South Texas. The Norias was an expansive, predominantly sandy, progradational complex fed by the updip Gueydan fluvial system. This progradational complex effectively filled the Rio Grande embayment of South Texas by the end of Frio deposition. Initiation of Norias deltaic progradation was probably associated with the early rise in sea level following a lowstand. As sea level rose following the lowstand, topographic expression of the preexisting Vicksburg progradational wedge caused a northward deflection of the Gueydan fluvial system within the Rio Grande embayment. Subsequently, the Norias delta complex was established as a stable progradational system in a position lateral to the north lanking margin of the main Vicksburg deltaic platform. The Norias delta system was flanked laterally with time-equivalent depositional systems, including a northerly strand-plain/barrier-bar system and to the south there are indications of a possible deltaic complex in northern Mexico. Entrenchment of the Vicksburg shelf during sea level lowstand probably led to development of a submarine canyon adjacent to the south flank of the Norias delta system. Incorporated within the complex body of the progradational sequence a well-defined evolution of Norias delta styles can be documented through the time of Frio deposition. Early Frio high-constructive, fluvially dominated, lobate geometries were characterized by meandering distributary systems as well as thick delta-front and delta-margin san s.
Middle Norias deltaic deposits continued to be generally high-constructive lobate in style, but evidence for significant reworking of deltaic sediments can be inferred based on the strand-plain/barrier system developed to the north. Late Norias deltas were high-destructive wave dominated in nature. Essentially the Norias depositional complex had become a broad alluvial plain feeding a strand-plain/barrier-bar system with little physiographic expression of a delta lobe.
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