Facies and Non-Cyclicity of Upper Jurassic Oxfordian Muddy Carbonates, Southern Adriatic Platform, Croatia

This study focuses on a 343-m-thick section of Upper Jurassic (Upper Calovian-Lower Kimmeridgean; the bulk of the section is Oxfordian) shallow-marine limestone and lesser dolostone from Jasenice near Dubrovnik, Croatia. The section was logged bed-by-bed noting color, sedimentary structures, grain size, shape, sorting, constituents, and diagenetic overprints.

The lower units in the section are characterized by small packages of cyclic subtidal to intertidal facies consisting of dolomite layers (some weakly laminated), peloidal skeletal wacke-packstone, packstone and grainstone, and capped by lime mudstone or burrowed laminite. Caps have fenestrae and rare scalloped contacts, indicative of subaerial exposure. The exposure contains some thrust faulting and associated breccia. Upsection, the facies become muddier and microbial lumps and burrows are more common. The transition into the Oxfordian is distinguished by thick packages of brown-colored lime mudstone, which alternates with coarse, thick to massive dolostone. The lime mudstones contain microbial lumps, shell fragments, and abundant nubeculariid foraminifera. Thin beds of lime wacke-mudstone and intermittent skeletal wackestone break up these larger beds. Both the lime mudstone and wacke-mudstone become barren for a span of almost 30 meters up-section. The presence of these thick mudstones suggests that the platform deposition slightly lagged accommodation due to sea level rise coupled with subsidence. The accommodation rate began to decrease or production rate began to increase in the later Oxfordian units indicated by an increase in microbial lumps, benthic foraminifera, and fragments of calcareous algae. The section extends into the basal Kimmeridgian, which is regionally identified by the appearance and abundance of calcareous alga Clypiena jurassica.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California