Controls on facies evolution in low accommodation, continental-scale fluvio-paralic systems (Messak Fm, SW Libya)

Abstract

The evolution and controls on lithofacies distribution in ancient fluvio-paralic successions have been the focus of many studies. However, the evolution of ancient continental-scale fluvio-paralic depositional systems is not well documented due to a lack of suitable outcropping or subcropping examples. Features inherent to continental-scale fluvial systems, such as low overall gradients and large distances separating source from sink are likely to have a strong influence on vertical and lateral lithofacies evolution.During the Early Cretaceous a vast fluvio-paralic system drained the interior of the north Gondwana continent, terminating at approximately the present day coastline of North African/Saharan platform, and extending for >1500km across the margin. In the Murzuq Basin, ~600km from the shelf edge, extensive outcrops of Lower Cretaceous (named the Messak Fm) comprise ~300m of fluvio-paralic sediments. Correlation with more distal equivalent outcrops on the northern margin of the Ghadames Basin presents an excellent opportunity to assess facies development of a large-scale continental system, and clearly illustrates the effects of limited accommodation availability.For the first time, we identify evidence of marine influence in the Murzuq Basin during the early Cretaceous and interpret this as the record of transgressions associated with the regionally well-documented early Aptian sea level rise. The distribution of fluvial and paralic facies is interpreted to be controlled by a combination of limited accommodation availability, repeated relative sea level rises, and autocyclic fluvial processes. Relative sea level rises were able to rapidly inundate large parts of the Gondwanan margin due to the low gradient of the platform, but transgressive facies distribution and preservation were restricted due to the balance between sediment supply and limited long-term accommodation availability. Thus, while relative sea level exerts a strong influence in such a setting, marine-influenced deposits are likely to be relatively rare. The Messak Fm provides a clear example of how sediments preserved by a large-scale low-gradient fluvial system do not necessarily reflect the relative balance of depositional controls.