Trace fossils in shallow-marine deposits of the Lower Devonian Gamka Formation (Bokkeveld Group), southern South Africa
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Zimmermann, Udo۱; Mلngano, Gabriela۲; Buatios, Luis۲; Beukes, NJ۳|
|Holding Date||08 October 2008|
The Gamka Formation is the lowermost quart-arenitic unit of the Devonian Bokkeveld Group (Cape Supergroup). The rocks are exposed around the modern margin of southern South Africa and are interpreted as shallow marine to terrestrial, formed after a massive rift event during the early Palaeozoic. The Lower Devonian Gamka Formation is the first quartz-rich succession in the Bokkeveld Group. The contacts to the underlying (Gydo Formation) and overlying shales (Voorstehook Formation) are conformable. Asterozoan trace fossils assigned to the ichnospecies Asteriacites lumbricalis occur in this lower part of the Bokkeveld Group and are for the first time described. Five-rayed, star shaped resting trace fossils (c. 1-1.5 cm wide) are preserved as positive hyporeliefs and negative epireliefs in two layers of moderately sorted, relatively quartz-rich, wave-rippled sandstone. In addition, Brooksella-like radial structures preserved as negative epireliefs and having five unbranched leaf-like, petaloid lobes radiating from a vertical shaft are present in the same beds. These radial feeding structures penetrate c. 1-1.5 cm into the sediment, cutting through different levels, including mica-rich layers. The trace fossil-bearing layers represent a transition facies from muddy shale to muddy sandstone (Gydo Formation) to well-sorted clean quartz-arenite (top of the Gamka Formation). Our finds confirm a shallow-marine environment for the northernmost part of the Bokkeveld basin, c. 300 km to the north of Cape Town. This implies that age-equivalent deposits further south would represent deeper-water deposits.