Silurian Onlap in Time and Space
Pieter Spaak, Peter Burgess, Cees Van Oosterhout, and Andy Bell
Exploration New Ventures, SHELL E&P, Rijswijk, Netherlands
In the Middle East and North Africa (MENA area), source rocks associated with the post-glacial ‘base’ Silurian flood, are responsible for numerous hydrocarbon accumulations, including the Arabian ‘super-giants’ (North Dome/South Pars and Ghawar) and major Algerian gas fields (Hassi R’Mel and Hassi Messaoud). This contribution takes a closer look at the Silurian flood and onlap in time and space. Over 200 biostratigraphically constrained data points from this central Gondwana area, document distribution and ‘duration’ of onlap, which typically lasted 5-9Ma. Some areas only became flooded after 24Ma in the Late Silurian (Ludlow).
The melting of an Ordovician polar ice cap will have caused a considerable sea level rise. However, analogous to more recent deglaciation events, such a ‘glacial’ sea level rise will probably have been relatively fast and consequently, the extended duration of onlap more likely represents ongoing tectonic subsidence of the Silurian Gondwana margin and cratonic interior. In addition, the onlap ‘distribution’ reflects flooding of a structurally and glacially modified and varied landscape, and this topographic variability was instrumental in the formation of the ‘source beds’.
Commonly, the latest Ordovician glaciation as observed in the MENA area is considered to have been a discrete and (very) short-lived event. Integration of MENA and South America data does not only illustrate the spatial relations of Silurian depocentres, but also highlights ongoing glacial conditions in the early Silurian, with the south pole now shifted towards South America. Moreover, major shelf-margin incisions are observed in Arabia with Caradocian infill. If incisions are also considered as evidence for increased ice build-up, a picture emerges of glacial conditions straddling the Ordovician-Silurian boundary lasting perhaps for 10-15Ma.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery