Journal of Conference Abstracts

Volcanic Influence on Carbonate Depositional Patterns, Upper Triassic of the Coastal Plain, Israel

Dorit Korngreen (Korngreen@hotmail.com) & Chaim Benjamini (chaim@bgumail.bgu.ac.il)

Dept. Geological and Environmental Sciences, Ben Gurion University of the Negev, P.O.B. 653 Beer Sheva 84105, Israel

The Upper Triassic (Carnian-Norian) of the Asher-Atlit 1 borehole, dominated by volcanism, was compared with the same interval lacking volcanics at the Gaash 2 borehole, situated 47 km to the south. Samples from cuttings were studied by light and cathodoluminescence (CL) microscopy. The CL reveals textures, particles, and fauna obscured by diagenetic process and invisible under light microscopy, and enables classification into microfacies. The microfacies are characteristic of a variety of original carbonate facies. CL cement signatures were also used to divide the sections into sequential compositional concentric zones (SCCZ's), each with at least 2 distinguishable generations of cement. CL cement signatures enable recognition of subaerial exposure events despite obscurity of paleopedological features, as zonation during each lowstand is ordered according to the particular redox history of the vadose and phreatic zones following exposure. One side benefit of SCCZ signatures for each subunit is to eliminate confusion due to caving.

The interval in the Asher-Atlit 1 borehole to the north is represented mainly by 1200 m of limestone, intercalated by volcanic beds which comprise 30% of the section. The carbonates can be referred to lagoonal back-reef, central reef, fore-reef ramp or slope environments. Biolithites or ramp-like ooid shoals comprise the central reef. Fore-reef and offshore facies are rare. The same interval in the Gaash 2 borehole to the south is only 300 m thick, and is represented by micritic limestone, dolomite and minor shales. The carbonates can be referred to ooid shoals, rare biolithites, and back reef facies. There are some differences between the depositional patterns of the two sections. The biolithite in Asher-Atlit 1 consists of typical buildup organisms such as bryozoa, sponges and microtubes, the ooid facies is secondary, and consists mainly of superficial ooids and cortoids. At Gaash 2 the diverse reef biota is missing, the reefs are less developed, more micritic, and multilayered, concentric ooids are common.

Differences in earliest diagenetic history between these two sections are more pronounced. At Asher-Atlit 1, at least six shallowing-upwards cycles achieve subaerial exposure cycles which can be traced using cement stratigraphy. The SCCZ's cross carbonate units, but not volcanic beds. At Gaash 2, the same time interval is represented by shallowing-upward cycles with facies shifts not topped by subaerial diagenesis, and SCCZ's did not develop.

The differences between these sections can be attributed to the intercalated volcanism:

1. Volcanism-related subsidence led to preservation of a much thicker section, and the concomitant rapid burial led to better preservation of the record.

2. Despite the greater total subsidence, periodic volcanism-related uplift kept the substrate within the photic zone, exposed to more energetic or nutrient-rich water, some of the latter contributed by the volcanism itself. The net result was higher carbonate productivity.

3. Volcanogenic uplift exposed the newly deposited section to subaerial diagenetic processes.

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