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High Frequency Sea Level Change Recorded within Non-Cyclic Lower Aptian and Cyclic Upper Aptian Platform-Interior Facies of the Adriatic Platform, Southern Croatia

Harman, Charles 1; Sweeney, Rafferty 1; Husinec, Antun 1; Read, Fred 2
1 Geology, St. Lawrence University, Canton, NY.
2 Geosciences, Virginia Tech, Blacksburg, VA.

This study was on Aptian platform-interior carbonates exposed on Peljesac Penninsula and the islands of Korcula, Hvar, and Mljet, southern Croatia. The Lower Aptian succession has non-cyclic, thick-bedded to massive subtidal facies of lime mudstone and skeletal-intraclastic lime mudstone and wackestone with rare benthic foraminifera, calcareous algae, microbial encrusters (Bacinella), bivalve fragments, as well as subordinate pelagic crinoids (Saccocoma) and planktic foraminifers (Hedbergella). The shallowest facies are skeletal-peloid wacke-packstones and grainstones with rudists in about 0-3 meters water depth. The succession becomes more cyclic toward the Early-Late Aptian boundary. The Upper Aptian is more cyclic, with parasequences consisting of basal skeletal mudstone-wackstone overlain by peloid-intraclast-skeletal packstone and grainstone, barren mudstone, fenestral lime mudstone or microbial laminite. Subaerial breccia horizons and/or residual clay horizons form common parasequence caps. There are up to seven subaerial horizons in the lower, and six in the upper part of Upper Aptian. Estimated water depths of facies probably were no more than 5 meters based on the range of stratigraphic distances of facies below base of laminate cap, breccias, or top of barren lime mudstone, but this could be increased if there was unfilled accommodation.

The facies stacking patterns suggest an important environmental change during Aptian, with Early Aptian transgression, coeval with drowning of numerous Tethyan carbonate platforms, and pronounced Late Aptian regression marking a significant biological crisis in the peri-Adriatic region. The published data show that accumulation rates for the southern part of the Adriatic platform were 4 cm/k.y. on average, reaching a minimum value of less than 1 cm/k.y. in the Aptian. These low accommodation and accumulation rates likely caused amalgamated subtidal facies in the Early Aptian, and periodic emergence and well-developed paleosols in the Late Aptian. Thickness variations between the sections is likely due to syndepositional tectonics over fault blocks, with superimposed sea level fluctuations strongly influencing formation of the sequences and parasequences.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009