An early mid-Miocene, strike-parallel shelfal trough and possible karstification in the Northern Carnarvon Basin, northwest Australia

Abstract

As part of a study to investigate relative sea level history from the Tertiary North West Shelf of Australia, we describe a seismically imaged, stratigraphic architectural element not previously associated with prograding carbonate clinoforms, a strike-oriented trough on an early mid-Miocene sequence boundary. This trough, located on a shallowly dipping carbonate ramp, is recognized on both 2-D and 3-D seismic data by truncation, reflector interruption and amplitude variations. It is composed of three individual segments, each up to 500 m wide and 60 m deep, with variable cross-sections. Their combined length, orthogonal to the progradation direction, is ∼8 km. The trough occurs where there is a slight basinward increase in dip, and a facies change on the ramp from sand-sized calcarenite to clay-sized calcilutite. It has both a cross-cutting and sub-parallel relationship with underlying Paleogene faults reactivated within a Mesozoic basin-forming trend. The trough, does not conform to known drainage patterns, or contour current incisions mapped in similar environments elsewhere. We propose that this seismically mapped trough is karst topography that developed as a result of preferential dissolution focused by heterogeneities within the exposed carbonate ramp. Stable isotope analyses conducted on bulk carbonate samples ∼2 km updip, suggest a mixed, marine to slightly meteoric origin for associated porewaters. Furthermore, the sequence boundary on which the trough is developed is contemporaneous with recognized subaerial exposure surfaces to the northeast and southwest. The presence of such a karst feature indicates a minimum fall in relative sea level at this location of 80–160 m during the early mid-Miocene.