ABSTRACT

Sedimentary and hydrographic patterns of western Gulf of Mexico barrier and barrier lagoons have been influenced by dominant southwestward longshore drift. This influence has been both direct and indirect as the drift deflected rivers to the southwest from their mouths.

The entrenched, drowned, filled and undrained late Pleistocene river valleys pass diagonally under the Holocene lagoons and barrier chains at depths around 100 ft or more. These drowned river valleys invariably follow deeply entrenched fault systems of Mesozoic origin.

Of the 40 or more historic storm-washovers per 100 mi of barrier chain, only seven natural passes have remained well established during the past 150 years. Five of the seven barrier breaks cut diagonally over broad submerged or subsurface valleys of present-day rivers. Four are located at downdrift bay ends.

Cyclic migrations and abnormally high relative sea level rise rates, indicated by tide gauges located at barrier inlets, suggest vertical instability. A recorded example of such instability is subsidence of 1.2 ft during the past 47 years of the east end of the Galveston seawall, overlying the subsurface valley of the Trinity River. Tide gauge records for the same period at Pensacola, Florida, presumed tectonically stable, reveal slight eustatic changes, but not the 1.2 ft higher levels found at Galveston. The Galveston seawall subsidence may not antedate the onset of subsurface fluid extraction in the Houston-Galveston area.

The counterclockwise spiraling of hurricanes crossing the barrier chains produces surges through barrier gaps and topographic lows. Inlet migrations over unstable valley-fill have been halted at the south against a large fan, characteristically formed at inlets. These fans help segment lagoons into a series of bay basins. The inlet-fan morphology forms a funnel mouth for the ebb tide jet flowing out of the inlet, dominating flow patterns of bays.