The Jurassic Megasequence in Egypt
Karen Romine1, David M. Allard2, Salah Karamat2, Tom Loutit3
(1) SRK Consulting, Deakin West, Australia (2) Apache Egypt Companies, Houston, TX (3) SRK Consulting, Deakin West, ACT, Australia
Deposition of the Jurassic Megasequence commenced during rifting in the Early Jurassic and terminated in latest Jurassic with uplift, block rotation and erosion. Relative movement between the European and African plates produced a NW-directed extensional stress regime in northeastern Africa, resulting in NE-trending normal faults and basins in Egypt. In the Western Desert, most of the producing fields lie along these northeasterly Jurassic structural trends.
Basement structure and composition fundamentally controlled the development and compartmentalization of Jurassic sub-basin depocentres. Jurassic extensional subsidence was confined to the north and east of a NW-trending Precambrian basement shear zone that extends from northeastern Libya to southeastern Egypt, including the Nile Valley region. Jurassic basin development was further compartmentalized by a NNW-trending accommodation zone which coincides with the upper Nile Valley. NE-trending Jurassic half-graben depocentres (and the Jurassic shoreline) stepped far to the NW across this zone, resulting in rapid and dramatic changes in facies from marine carbonates in the east to non-marine clastics in the west.
Lateral variation in facies in the Western Desert indicates a broad ramp-like shelf that dipped to the NE. Minor base level changes produced large lateral shifts in facies and environments, and resulted in the vertical juxtaposition of marine carbonates and coals. Marine deposition reached its greatest landward extent during the Late Jurassic maximum transgression (Masajid Formation), and has been interpreted as far south as the Komombo-1 well. Although the stratigraphic evidence was largely removed during uplift and erosion at the end of the Jurassic, much of the Nile Valley Region and Eastern Desert could have experienced this marine flooding event.