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Title: The structure and stratigraphy of the South Hadhramaut region, Yemen
Author: Watchorn, Frederick George
ISNI:       0000 0001 3563 8168
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1997
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The Cretaceous-Recent geological history of the south Hadhramaut region of Yemen in explored in this thesis, focusing on the geometry, timing and sedimentation history of Gulf of Aden rifting. A wide array of data and methods were used during this work: detailed field mapping of the structures and measurement of the stratigraphy; Landsat and SPOT image interpretation; sedimentary facies analysis; balanced cross-section construction; subsidence analysis and geomorphological analysis were all carried out. The pre-rift sediments, spanning the Cretaceous-Eocene, are described and analysed from measured sections. During deposition of the Tawilah Gp. (Cretaceous), the south Hadhramaut region was a broad clastic shelf which opened oceanwards to the east. This shelf was punctuated by numerous transgressions depositing shallow marine limestones throughout the Cretaceous. A large but localised erosional unconformity separates the Hadhramaut Gp. from the Tawilah Gp. This unconformity is associated with fault-block uplifts during the latest Maastrichtian. The early Tertiary evolution of the region, where southern Yemen was under a broad and homogenous carbonate shelf, is only briefly described. Absence of late Eocene strata and minor (1 00-150m) erosion of middle Eocene beds suggest some pre-rifting uplift and related development of a proto-Gulf of Aden marine embayment. Rift sedimentation began in the early Oligocene and continued to the early Miocene. Three sequences related to rifting are recognised. Sequence 1 is a patchily developed and highly variable alluvial plain and saline playa facies assemblage of probable Oligocene age. Sequence 2 comprises fine-grained fluvial sands grading into marginal marine coastal sabkha-type deposits with some local reefal beds. It has a cumulative thickness of between one and two kilometres and displays evidence of upward gradation to isolated depocentres. Sequence 3 facies reflect the current physiography of the margin with alluvial sediments interfingering with coastal clastic and carbonate facies. The morphology of the rift margin developed early in the rift phase and has changed little since then. The extensional form of the margin is well preserved and superbly exposed in 3-D. The basin margin fault systems form a north dipping (i.e. away from the oceanic axis), en-echelon, left stepping fault array, characteristic of an oblique rift. No systematic reversals in fault polarity are seen across strike. Fault linkage at all scales is 'soft' and no strike slip transfer faults were found. Faults are dominantly planar and show both convex and concave shapes in plan view. Total extension at the margin, calculated from regional cross-sections and subsidence analysis, is uniformly low and shows significant variation both along and across strike. Along strike variations in structural styles are related to proximity to basement highs, with these quasi-rectangular blocks being regions of complicated, dense fault arrays often characterised by oblique-slip faulting, pop-up structures and short fault length. A number of reactivated Cretaceous faults are documented. Intra-high regions are dominated by classic "softdomino" style fault patterns. Structural pattern varies systematically from S to N: coastal exposures are deformed by terraced, large displacement, domino-type fault blocks; rift shoulder arrays are deformed by interlocking, conjugate fault sets, and; plateau exposures are characterised by long (composite) and steep linear faults. The rift shoulder is preferentially located along a Mesozoic basement high.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available