Interactions of Biological and Geological Processes in the Beach and Nearshore Environments, Northern Padre Island, Texas

Hill, Gary; Hunter, Ralph

Padre Island, a barrier island off the southern Texas coast, experiences a low tidal range, is affected by waves of moderate height. and the northern part of the island is composed largely of fine sand. The seaward part of the island and the adjacent nearshore can be differentiated geomorphically into a beach (backshore and foreshore) and a bar-trough system.Macrobenthos zonation is related to geomorphic features and can be defined in terms of species distribution and density, diversity-equitability. and niche classification. Dominant species characterize each landform: Ocypode quadrata (ghost crab) is concentrated in the beach. and Callianassa is/agrande (ghost shrimp) and Melfita quinquiesperforala (sand dollar) are mainly in the nearshore bar-trough system. Diversity is lowest in the backshore and highest in the foreshore and the bar-trough system. The backshore has a homogeneous trophic nucleus(scavengers) and the foreshore and bar-trough system have a mixed trophic nucleus of suspension and deposit feeders. Characteristic depositional structures are common to each landform: (I) beach. gently seaward-dipping planar laminations produced by wave swash; (2) bar-trough system. small-scale lenticular crossbedding usually without any well-defined preferred orientation of dip direction and produced by deposition on sand ripples. planar lamination produced when bar crests are planar because of high W~lVes. and medium-scale cross-bedding produced by migrating megaripples and dipping in the direction of the longshore current. The depositional structures commonly are destroyed by organisms. Characteristic biogenic sedimentary structures are found in each geomorphic element. Variation in the areal density. size. morphology. and orientation of O. quadrata burrows can be used to definesubenvironments of the beach. Deep oblique to vertical burrows of C. islagrande and shallow horizontal intrastratal trails of M. quinquiesperjorata characterize the bar-trough system.

Interactions of Biological and Geological Processes in the Beach and Nearshore Environments, Northern Padre Island, Texas

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