Jurassic Upper Toarcian-Early Aalenian Oncoid and Ooid-Rich Facies of the Adriatic Carbonate Platform, Southern Croatia

Abstract

The Jurassic-Cretaceous supersequence of the Adriatic carbonate platform is superbly exposed along the islands and mainland in southern Croatia. This study focusses on the Lower Jurassic Toarcian to Middle Jurassic Aalenian tight, coated-grain dominated facies near Dubrovnik, and is based on a 110.5-m-thick continuous roadcut section. The outcrop data coupled with sedimentary-petrographic analysis of 17 thin-sections and 210 oncoid-size measurements, enabled the identification of the following facies, from deepest to shallowest: lime mudstone and peloid-skeletal mudstone (deep water below storm-weather wave-base), skeletal-peloid wacke-packstone (distal middle ramp), oncoid grainstone to floatstone with skeletal wackestone-packstone 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 facies distribution suggests a gently sloping ramp: from ooid shoals with deeper tidal channels downramp, into wackestone- and mudstone-dominated deeper marine setting below storm weather wave base. 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 (commonly ∼5.1 m), 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 followed by ooid grainstone becoming the dominant facies. The lithofacies index and relative sea level plot suggest that the sea level changes were moderately small in the late Toarcian and increased into the Aalenian, indicating some cooling over the interval that followed the oceanic anoxic event-associated warming in the early Toarcian.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016