A bio- and litho- stratigraphic study of the Ecca-Beaufort contact in the southeastern Karoo basin (Albany District, Eastern Cape Province)
Date
2008-10-27T10:44:30Z
Authors
Mason, Richard
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Abstract
The rocks of the Carboniferous to Jurassic aged main Karoo Supergroup of
South Africa preserve an internationally significant and stratigraphically
continuous retro-arc foreland depositional sequence. This succession
documents environmental change from glacial-marine, through fluvio-deltaic to
continental fluvial and aeolian, culminating in rift associated continental flood
basalt extrusions. The fluvio-deltaic transition from marine to continental
deposition has been the subject of much recent research and corresponds
with the position of the Ecca-Beaufort contact. Over the entire basin this
transition comprises three separate lithofacies associations deposited in the
prodelta, deltafront and delta plain environments. Anomolously the
southeastern contact is currently mapped as reflecting fluvial deposits of the
Koonap Formation lying unconformably on prodelta deposits of the Fort Brown
Formation.
Detailed study across the Ecca-Beaufort contact in this part of the basin now
reveals the same lithological transition as is present in the rest of the basin.
Contrary to previuos work, this has shown that the Waterford Formation is
indeed present in this part of the basin. This in effect means that the
Grahamstown map sheet (1:250 000, Map 3326) requires modification to
include this Formation and the new contact placement of the Ecca-Beaufort
contact occurs some 70 to 120m above the presently mapped contact.
Fossils collected during this and previous studies show that
biostratigraphically Tapinocephalus Assemblage Zone fauna occurs in the
rocks of the Koonap Formation in this part of the basin, indicating that the
Ecca-Beaufort contact in the southern part of the basin youngs towards the
east. This supports the prograding shoreline deposition model that has been previously proposed for the Ecca-Beaufort contact.
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Keywords
Ecca, Beaufort, biostratigraphy, lithostratigraphy, Karoo