--> Abstract: A Tale of Two Glaciations, by Pieter Spaak and Malcolm I. Ross; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

A Tale of Two Glaciations

Pieter Spaak2; Malcolm I. Ross1

(1) Basin Scanning and Evaluation, Shell International Exploration and Production, Houston, TX.

(2) New Play Development, Shell International Exploration and Production, Rijswijk, Netherlands.

Glacial deposits form important reservoir successions in the Palaeozoic of Gondwana. Focusing on Arabia in a ‘mid-latitude’ position during the entire Palaeozoic, two major glacial events can be recognized. Glacial and post-glacial sediments of the Al Khlata and Unayzah ‘formations’ are the reservoirs of numerous Permo-Carboniferous fields in southern and central Arabia. Latest Ordovician glacial deposits and erosional remnants form the target of wells and prospects in south and north Arabia. Following the Ordovician glaciation, the large scale, second order ‘base’ Silurian flood is linked with the most significant (Qusaiba) source rock of the region, which can be traced over vast areas in North Africa.

The extent of the Late Ordovician and Permo-Carboniferous ‘ice-sheet’ is tremendous, covering in both cases very large parts of Gondwana. In that context, it is remarkable that during the Devonian, ice-cover over Gondwana is limited, notwithstanding a ‘very polar’ position of the super-continent in that period.

In this contribution, the observations outlined above are illustrated in the form of reconstructed palaeogeographic maps. Reviewing these reconstructions, we suggest that the appearance of the relatively ice-free Devonian, sandwiched between two glaciations, was caused by the disruption of the normal oceanic-driven Hadley cell circulation. During the Devonian the vast Gondwana landmass was centered over the South Pole and dominated the entire southern hemisphere. This configuration dramatically reduced the continental frozen precipitation required to create and support large ice sheets. During the bracketing Upper Carboniferous and Upper Ordovician, the South Pole was close enough to the margin of the super-continent to allow a more ‘normal’ Hadley cell circulation, with associated precipitation at high latitudes. We will present a palaeoclimate simulations that support this conclusion.