Facies analysis of Late Pleistocene mixed carbonate-siliciclastic transgressive-regressive sequence along the coast near Mogadishu (Somalia)

Abstract

The raised sedimentary complex outcropping along the coastal belt just south of the Mogadishu airport is a roughly 10-m-thick carbonate–siliciclastic, coral-bearing depositional sequence. This complex is defined by eight primary depositional facies that form a transgressive–regressive sedimentary cycle, capped by a regressive surface that gently dips seaward.

Facies analysis reveals a transgressive system tract consisting of a stratal set of siliciclastic sand deposited above a wave-ravinement surface, as indicated by an eroded quartz sand substratum. The transgressive succession continues with loose siliciclastic sand that contains a rich mollusk fauna intermixed with coarse skeletal debris of stony corals and both encrusted and articulated coralline algae. The macrofossil assemblage indicates diverse shoreface environments, where water depth, bottom morphology, and distance from the shoreline are critical in characterizing each facies.

Comparisons with the modern biological zonation of the shallow shelf along the southern Somali coast have helped clarify the depositional characteristics of the Pleistocene facies succession. The distribution of fossil coral communities—often with colonies still in growth position—suggests a series of shallow-water facies related to back reef–shore environments that migrated landward over time. In contrast, the highstand system tract differs from the underlying facies by exhibiting a drastic decrease in the siliciclastic granular fraction and corals in growth position, which is offset by an increase in articulated and encrusted coralline algae that comprise most of the sediment source. Locally, skeletal rubble rich in large coralline algal nodules is typical of the regressive phase. The deposition of these facies results in a smoothing of seafloor roughness due to the burial of coral buildups by skeletal sand that shows less facies diversification.

The marine succession—affected in its upper part by the calichification process—culminates in a marine terrace capped by aeolian sands deposited during various depositional–erosional events. Some of these deposits correlate with raised beachrock that outcrops discontinuously just above present sea level and corresponds to a minor Holocene sea level highstand.

To assess the time interval required for the deposition of this transgressive–regressive cycle, which is related to a Late Pleistocene sea level positive peak, sedimentation rates and U–Th dating of two samples of massive Porites lutea from the transgressive system tract have been considered. The ages of approximately 114 kyr indicate a flooding event on the southern Somali coast during the transition between stages 5e and 5d of the Last Interglacial, while the overall duration of the sedimentary cycle is likely much less than 10 kyr. Furthermore, by matching the investigated cycle with positive peaks on significant global and local sea level curves, additional insights have been made regarding relative sea level fluctuations following the development of the Pleistocene sedimentary wedge.