Abstract
A fairly thick sequence of sandstones with very subordinate shales, designated as Pre-Upper Cretaceous (PUC) by the operating company (Winthershall), are met at B1, B2, B3, and B4-96 in the subsurface of Maradah Trough, Sirt Basin. The Amal Platform defines its eastern limits. The operating company divided the PUC sequence into PUC ‘B’ (older) and PUC ‘A’ (younger) units. The PUC ‘B’ is 995+ feet thick and comprises mainly sandstones with very rare and thin levels of shales. The PUC ‘A’ unconformably overlies the PUC ‘B’ and consists of sandstones and shales. It is divisible into Units I-IV on the basis of gross lithology and log characteristics. The thickness of PUC ‘A’ ranges from 510 feet in the east to 765 feet in the west. It thickens to 1450 feet further west towards Jakhira.
PUC ‘B’ and PUC ‘A’ sequences comprise mostly quartz-arenites of fairly similar petrographic characters. It, however, is possible to resolve one from the other on the basis of gross lithologic and palynologic criteria.
The palynofossil assemblage, though scarce, suggests PUC ‘B’ to be ?Precambrian-Cambrian. The assemblage recovered from PUC ‘A’ is relatively very rich and referable to Middle Triassic (Anisian and ?Ladinian). These and other data suggest an unconformity between PUC ‘B’ and the overlying PUC ‘A’. Lithologic characters, petrographic attributes, and palynofossil contents suggest that PUC ‘B’ was probably laid in a tectonically active graben under mainly non-marine oxic environments during ?Pre-Cambrian. The detritus appears to be partly recycled. Subsequently, the depositional area underwent uplift and erosion till the close of Early Triassic. The PUC ‘A’ appears to have been laid in a graben under fluctuating oxic-anoxic and non-marine to at times probable transitional environments during Middle Triassic (Anisian and ?Ladinian). Tectonically the graben was fairly subdued during most of the Anisian and promoted erosion of mainly non-marine shales with subordinate sandstones. The later period was marked by relatively accelerated negative tectonism and interludes of marine influence. Westerly increments in thickness and overall increase in sandstone contents of PUC ‘A’ indicate that the Marada Trough was asymmetric and the faults defining its western fringes were tectonically more active during the Middle Triassic.
Relatively colder climates appear to have prevailed during ?Precambrian-Cambrian when the PUC ‘B’ sequence was developed. The palaeoclimates during the evolution of the lower part of PUC ‘A’ sequence, dated as Anisian, were mainly hygrophytic. These, however, switched over to mixed hygrophytic-xerophytic types during Ladinian when the upper part of the PUC ‘A’ sequence was laid.