COATED-GRAIN DOMINATED FACIES OF JURASSIC UPPER TOARCIAN-EARLY AALENIAN CARBONATES, SOUTHERN ADRIATIC CARBONATE PLATFORM, CROATIA

The southern coast of Croatia makes up a part of the Adriatic platform, a thick shallow-water carbonate supersequence that evolved from the Early Jurassic through the Late Cretaceous. This study focusses on the southern part of the platform, and is based on a 110.5-m-thick upper Toarcian-basal Aalenian continuous roadcut section ~14 km ENE from Dubrovnik, Croatia. Based on outcrop data coupled with sedimentary-petrographic analysis of 17 thin-sections and 210 oncoid-size measurements, the following facies were identified: lime mudstone and peloid-skeletal mudstone (deep water below storm-weather wave-base), skeletal-peloid wacke-packstone (distal middle ramp), oncoid/skeletal packstone to grainstone matrix (proximal middle ramp), large oncoid floatstone (broad and shallow subtidal channels; proximal setting downslope from the bank) and ooid-skeletal-oncoid grainstone (shoal; bank). The upper Toarcian facies are characteristically stacked into 0.25 to 4.10 m thick (commonly ~1.5 m) asymmetric shallowing-upward cycles (parasequences), with a thin basal mudstone-wackestone and capped by a thick grainy facies (oncoid packstone-floatstone, or less commonly ooid-skeletal-oncoid grainstone). A facies shift in the Early Aalenian is indicated by thicker parasequences, 0.6 to 13.2 m, typically about 5.1 meters many of which are predominated by fenestral oolite facies. The overall facies stacking pattern suggests an upward-shallowing from the late Toarcian into the early Aalenian, which is associated with an increase in oncoid size as well as ooids becoming the major limestone constituents. Sea level changes likely were moderately small in the late Toarcian and increased into the Aalenian, indicating some cooling over the interval that followed the Toarcian oceanic anoxic event.