Abstract

The morphology of three basin-wide erosion surfaces has been defined using 7869 resistivity well logs, over an area of about 50 000 km2. The lowest of these, designated E6, is topographically smooth, and has no incised steps or low areas. It cuts into marine mudstones, and indicates either an uninterrupted erosive transgression following a major lowering of relative sea level, or wave scouring of the substrate following a very minor lowering of sea level. Erosion surface E6.5 also cuts into marine mudstones, but its overall morphology is less well understood owing to subsequent dissection by surface E7. This surface (E7) is characterized by three strike-parallel linear low areas that die out northwestward. These linear lows are up to 40 km wide, 36 m deep, and have irregular to symmetrical cross-sectional profiles. They cut into, and are filled with, marine mudstones, and their origin is enigmatic. The differences in the morphologies of the surfaces suggest the interplay of several controlling parameters: (i) steady transgression versus transgression interrupted by stillstands; (ii) absolute depth of erosion during transgression, which in turn controls the preservation or erosion of incised river valleys; (iii) wave scouring of substrate in response to minor lowerings of sea level; and (iv) availability of coarse sediment and river gradients steep enough for its transport to the shoreline. The average time span between erosion surfaces is 300 000 – 350 000 years. This timing is hard to explain tectonically, or in terms of global eustacy, suggesting the possibility of glacio-eustatic control of fluctuations in relative sea level.

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