Paper No. 1
Presentation Time: 1:30 PM
FROM SOFT SEDIMENT TO ROCKY SHORE: TRACE FOSSILS RECORD THE DEVELOPMENT OF A CHANNELED ROCKGROUND IN CARBONATES OF THE MIDDLE JURASSIC IN SOUTHERN ISRAEL
Trace fossils in ancient shallow marine sediments can reveal many details about the paleoenvironmental changes and early diagenesis of lithifying carbonate sequences. The top of the Zohar Formation (Callovian, Middle Jurassic) in Makhtesh Qatan, southern Israel, provides an excellent example of trace fossil utility. This sequence contains two dolomites (each one to three meters thick) separated vertically by four to five meters of dolomitic sands and silts. The top surfaces of these dolomites are cut by two perpendicular sets of channels up to two meters deep and two meters wide. The trace fossils in these dolomites show three ichnocoenoses. The first is a softground assemblage dominated by Planolites. Cross-cutting this ichnofabric are large Thalassinoides suevicus burrows which were excavated by decapod crustaceans in firm sediments. The third ichnocoenosis consists of the bivalve boring Gastrochaenolites excavated into the rocky sides of the channels. Our hypothesis is that the original carbonate sediments were deposited below wavebase and quickly bioturbated by the worm-like creators of Planolites. The sediment later began to firm up, which was probably due to a combination of dewatering and early cementation. Sea level also dropped and the thickened sediments were then burrowed by decapod crustaceans (probably callianassid shrimp) producing the large galleries of T. suevicus. Early diagenetic processes finally cemented the entire carbonate unit and it was exposed in the intertidal zone, where tidal currents and karstic processes expanded joints into channels and bivalves excavated Gastrochaenolites in the channel walls. This shallowing-up-to-exposure sequence and its early diagenetic history illuminated by this succession of trace fossils has significant implications for paleoenvironmental reconstructions of this part of Israel during the Middle Jurassic.