Depositional environments and sequence stratigraphy of paralic glacial, paraglacial and postglacial Upper Ordovician siliciclastic deposits in the Murzuq Basin, SW Libya

摘要

The application of sequence stratigraphy methods to glaciogenic deposits is more problematic than in normal paralic deposits because changes in the relative sea level are strongly influenced by an interplay between glacial advance and retreat, and concomitant isostatic loading and rebound of the continental shelf. The study of outcrop and subsurface sections of the Upper Ordovician glaciogenic Melaz Shuqran and the Mamuniyat formations, Murzuq Basin SW Libya, yielded important information on understanding changes in the relative sea level related to glacier movements. Three depositional sequences were recognized. Depositional sequence one (DS-1) corresponds to the entire Melaz Shuqran Formation that was bounded below by subglacial erosion surface (i.e. sequence boundary, SB) formed during glacier advance into shallow water areas. This surface correlates with a transgressive surface (TS) in deep water areas and records an initial rise in the relative sea level owing to glacier advance and loading of the continental shelf. Transgression occurred due to the slower rate of eustatic sea-level fall than the isostatic loading. The transgressive systems tract (TST) comprises shoreface sandstones and offshore mudstones/diamictites with ice-rafted debris. Further rise in relative sea level was associated with glacial retreat, which resulted in sediment starvation in deep water areas and the formation of a condensed section. These deposits represent glacial depositional systems. The highstand systems tract (HST) comprises prograding deltaic deposits, which was formed when the rate of sediment supply exceeds the rate of relative sea level rise. The HST deposits represent paraglacial depositional systems. Depositional sequence two (DS-2) corresponds to the lower and middle part of the Mamuniyat Formation, and is bounded below by deep erosional surface (SB) that cuts into DS-1 as an incised valley, which was formed during glacial advance across the continental shelf. Incision occurred due to the faster rate of eustatic sea-level fall than the rate of isostatic loading. Glacier retreat and the concomitant rise in relative sea level resulted in the deposition of braided fluvial sediments in a lowstand systems tract (LST) and tide-dominated estuarine sediments in a TST. These deposits represent glacial depositional systems. The HST sediments represent prograding foreshore to shoreface deposits, which were formed when the rate of sediment supply exceeded the rate of relative sea-level rise. These deposits represent paraglacial depositional systems. Depositional sequence three (DS-3) corresponds to the upper part of the Mamuniyat Formation and is bounded below by an erosional surface (SB) that was formed as a result of isostatic rebound, associated with relative sea-level fall. DS-3 LST comprises Gilbert-type, deltaic conglomeratic sandstones grading upwards into sandstones. Subsequent relative sea-level rise during the late Ordovician resulted in deposition of shoreface TST sandstone. Further rise in the relative sea level resulted in sediment starvation in deep water areas and the formation of thin hardground layer (i.e. condensed section) that marks the upper boundary of late Ordovician deposits. This study provides predictive model for the spatial and temporal distribution of ancient glaciogenic depositional facies that may have important implications in hydrocarbon explorations.