Abstract

The Pleistocene Port Durnford Formation comprises fossiliferous, upward coarsening sandy clays of lagoonal origin overlain by backbarrier peat or bioturbated sand with Ophiomorpha , followed by barrier overwash sands and eolian deposits. The basal lagoonal deposits contain oysters and other molluscs, cirripeds, fossil wood, and a variety of vertebrate remains including crocodile, turtle, fish, antelope, rhinoceros, buffalo, elephant and hippopotamus. Primary structures such as lenticular, wavy and flaser bedding are largely destroyed by bioturbation. Overwash deposits consist of landward dipping plane-beds, with planar and trough cross-bed intrasets. Lagoonal infilling and landward barrier migration are thought to have accompanied the rise of the sea to its last interglacial (Sangamon) level of approximately +8 m. Eolian sands, in large-scale sets of high-angle cross-beds, were probably deposited as coastal dunes during the subsequent Wisconsin regression. Syndepositional deformation took place in two main phases, the first immediately following infilling of the lagoon, and the second after a cover of washover sands had been deposited. The resulting coherent structures, chiefly upright and locally overturned folds, and subordinate overthrusts and normal faults, resemble, on a reduced scale, those produced by tectonism. Deformation was concentrated in the upper few meters of lagoonal strata and extended to the prevailing sediment-water interface, resulting in small growth faults, open-cast folds, and other contemporaneously buried structures. Deformation is thought to have involved gravity gliding and clay diapirism, probably operating in concert. Landward gravity gliding would have been facilitated by the steep NW to WNW paleoslope on dissected basement. The presence of two dominant fold wavelengths, giving rise to anticlmoria with second-order folds, is explained by the diapiric rise of two pressurized, buoyant, clay-rich layers beneath denser barrier and eolian sands.

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