Physiography and surface sediment facies of a recent tidal delta, Harbor Island, Central Texas coast
Creators: name: Hoover, Richard Alan, 1939-
Publisher: name: The University of Texas at Austin
Year: 1968
Subjects: subject: Harbor Island
subject: Harbor Island tidal delta
subject: Texas
subject: Aransas Pass
subject: Harbor Island sedimentation
subject: Tidal delta sediment
subject: Tidal deltas
subject: Deltas
subject: Inlets
subject: Texas barrier islands
subject: Physiography
subject: Sedimentation and deposition--Texas--Aransas Pass
subject: Deltas--Texas--Aransas Pass
subject: Inlets--Texas--Aransas Pass
subject: Barrier islands--Texas--Aransas Pass

description: Lobate tidal deltas are associated with passes between the barrier islands of the Texas coast. Although tide range is low (1.5 feet), wide spacing of inlets and variable lagoon and estuary levels cause strong currents in inlets. Deposition occurs at the lagoonal terminus of the inlets due to waning currents. Lagoon and estuary levels are affected more by runoff from rains, seasonally strong onshore and offshore winds, and hurricane storm-surge tides than by diurnal astronomical tides. Nine physiographic and vegetational units were mapped on Harbor Island at a scale of 1:24000. Subaerial parts of the tidal delta (vegetated mound, shell beach and storm apron, and supratidal flat) are found near the inlet (Lydia Ann Channel) and at the southern edge of Aransas Bay. Salt marsh, tidal creeks, and tidal channels are found near the inlet entrance (Aransas Pass). Shallow submerged areas (unvegetated subtidal flat, grassflat, and oyster banks) make up the remainder of the area. Although maximum relief is only 12 feet, elevation differences are clearly indicated by plant zonation both in submerged and subaerial environments. Current baffling and sediment entrapment by marine grasses (Diplanthera wrightii , Ruppia maritima, and Thalassia testudinum) and by marsh plants are important depositional processes on grassflats, at the inlet edge, and in the marsh. Progradation of marsh over unvegetated subtidal flat occurs in two stages: (1) colonization of small areas by Spartina alterniflora, and (2) coalescence of marsh islands by lateral accretion, leading to development of broad areas of marsh transected by tidal creeks. Subaerial environments are sites of sediment accumulation only during high lagoonal tides (resulting in deposition of mud), northers (deposition of shell on beaches facing Aransas Bay), and hurricanes (deposition of shell on storm apron and sand on supratidal flat). Molluscan shell assemblages are composed mostly of pelecypods. Comparisons of relative abundances in individual samples with frequencies of occurrence in sets of samples from each environment provide a basis for interpretation of shell distributions. Small environmental differences cannot be detected with the use of preserved shell remains alone, because of selective preservation, lateral transport, and vertical mixing (bioturbation) of community components. Seven surficial facies are described from shallow cores: bay margin grassflat, oyster bank, subtidal flat, marsh, supratidal flat, vegetated mound, and storm ridge. Primary sedimentary structures are rarely all facies except supratidal flat and storm ridge are characterized by burrows and mottles produced by organic activity. Depositional history of Harbor Island is interpreted as representing accumulation of sediment on a broad, submerged, sandy platform. Tidal currents formed longitudinal bars separated by channels. Vegetation stabilized these bars and caused them to accrete both vertically and laterally. Continued accretion and deposition of sand by hurricanes led to development of the present subaerial physiography, a complex of marsh, supratidal flat, vegetated mound, and storm ridge