--> Sedimentologic and Oceanographic Characterization of the “Inland Sea” Lagoon and Channel System, Southeast Qatar

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Sedimentologic and Oceanographic Characterization of the “Inland Sea” Lagoon and Channel System, Southeast Qatar

Abstract

Southeastern Qatar shows unique examples of arid shoreline sedimentation patterns, dominated by the interplay of supratidal sand dune fields and inland sabkhas, intertidal microbial mats, and shallow subtidal saline lagoons of various sizes and salinities. The largest lagoon is the so-called shallow Inland Sea, measuring 14 × 9 km with an average depth of less than 2 meters. Waters of this mesohaline lagoon are connected to the Arabian Gulf through a deeper lagoon and a channel system that follows Pleistocene drainage patterns. A spectacular, approximately 3km wide flood-tidal delta marks the entrance of the channel to the Inland Sea. Oldest sediments exposed along the coastal plain are relicts of a Pleistocene and Holocene sea-level highstands. Pleistocene bivalve-rich cemented carbonates occur at an elevation of approximately 9m above present day sea level. Uranium-thorium dating shows an age of approximately 120,000 years. Unconsolidated Holocene, bivalve-rich sediments occur at an elevation of approximately 3m above present day sea level and show un-calibrated radiocarbon age dates of approximately 5,500 years before present. Southward-migrating Quartz sand dunes supply significant sediment to the modern Inland Sea depositional system. Whereas quartz sands derived from the dunes are the dominant sediment type throughout much of the existing subtidal depositional system, carbonate sedimentation is varied and largely controlled by oceanography. A diverse assemblage of open-marine biota, including corals, form sediments in the channel toward the open Arabian Gulf. Landward the lagoons become more restricted. In the deeper outer lagoon, molluscs and ooids have been mapped. In the shallow Inland Sea proper, relatively wide (up to 400 meters) intertidal environments host microbial mats of variable morphologies. Along tidal channels (tidal creeks) the microbes tend to form aragonite-cemented, cub-shaped structures. In the shallowest reaches of the Inland Sea remnants of former dunes form barriers, separating mesohaline from hypersaline lagoons. Hypersaline lagoons are marked by stromatolites that alternate with gypsum and halite. Sedimentation patterns in the channels and lagoons of the Inland Sea region reflect the interplay between sea level, climate, and oceanography. Understanding these factors and their impact on geobody development in shallow arid marine settings is useful in modeling both carbonate and siliciclastic analog reservoirs.