Upper Callovian to Oxfordian Muddy Supersequence of Southern Adriatic Platform, Croatia
Govoni, Bonnie; Husinec, Antun; Read, J. Fred
A 340-m-thick section of Upper Jurassic intraoceanic Adriatic platform carbonates was logged and sampled for stable isotopes near Dubrovnik, Croatia. An upper Callovian sequence is characterized by m-scale packages of cyclic subtidal to intertidal facies consisting of dolomite, peloidal-skeletal wackestone, packstone and grainstone, capped by lime mudstone or burrowed laminite; abundant stromatolitic cycle caps indicate semi-arid conditions. Up-section, grain-supported facies disappear, facies become muddier and microbial lumps and burrow-mottling becomes more common. The Oxfordian is a supersequence with five third order sequences (AdOx-1 to 5) each about ~1 million years duration. The Oxfordian sequences contain abundant lime mudstone (with microbial lumps, shell fragments, and abundant nubeculariid foraminifera), containing skeletal wacke-mudstone interbeds in lower parts, and barren lime mudstone in upper parts of sequences. The accommodation history plot of the supersequence supports a long-term sea-level rise and fall during the Oxfordian, the rising limb being characterized by progressively thicker subtidal parasequences, and the falling limb by progressively thinner subtidal parasequences with an up-section increase in microbial lumps, benthic foraminifera, and calcareous algal fragments. The overlying basal Kimmeridgian sequence (AdKm-1) contains low-energy subtidal facies similar to the topmost Oxfordian sequence AdOx-5.
The succession supports overall late Callovian cooling followed by mid-Oxfordian maximum flooding. The small number of Oxfordian parasequences (57 total, ~100 k.y. duration) are suggestive of short-term eccentricity forcing while the ~1 m.y. sequence durations suggest long term obliquity forcing. Moderate subsidence rates (~5cm/k.y.) coupled with low sedimentation rates likely prevented the Oxfordian platform interior from shallowing to intertidal depths, as evidenced by lack of laminites and subaerial exposure breccias. Also, many parasequences are amalgamated and represent incomplete shallowing. The bulk carbonate matrix (δ 13C -2.32 to +4.04; δ 18O from -6.11 to +1.81‰) shows two significant isotope excursions in the Oxfordian, which are evaluated in terms of global carbon cycling and eustatic changes.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013