Recurrent Cooling Events in Aptian Greenhouse: Stable-Isotope Trends and Sequence-Stratigraphic Evidence from Southern Tethyan Adriatic Platform Carbonates

Husinec, Antun; Read, J. Fred

To evaluate evidence for Aptian glacio-eustasy, and effects of OAEs on the platform stratigraphy, we examined the facies stacking and sequencesof the Aptian Croatian Adriatic Platform of Southern Tethys using carbon-isotope chemostratigraphy to constrain correlations with well-dated pelagic sections.

The Aptian supersequence consists of four 3rd order depositional sequences, commonly bounded by brecciated sequence boundary zones. Sequence Ad1 (latest Barremian-Early Aptian) is dominated by shallow subtidal, commonly amalgamated carbonate parasequences and late highstand, sequence boundary zone breccias. Sequence Ad2 (Early Aptian to earliest Late Aptian) has a significant flooding event associated with OAE 1a, marked by deposition of relatively shallow water, but dysaerobic laminated carbonates. Sequences Ad2, Ad3 and Ad4 are dominated by peritidal carbonate parasequences updip, especially in the early TST and late HST, and subtidal parasequences elsewhere. Chemostratigraphy suggests that sequence Ad3 is early Late Aptian, and is separated by a major hiatus from sequence Ad4 (latest Aptian) which spans OAE1b. Sequence Ad4 is dominated by shallow subtidal parasequences, with peritidal facies and breccias occurring in the late highstand, sequence boundary zone.

The breccias capping sequences, together with the well documented evidence of sea level changes of tens of meters, suggests that 3rd order sea levels likely were driven by glacio-eustasy.The strong eccentricity dominated cyclicity observed in coeval pelagic sections are manifested on the platform by 4th order parasequences (100 to 400 k.y. duration). The low subsidence rate (~1 cm/k.y.) of the platform allowed the full effects of sea level fall to be recorded as multiple breccia horizons, although this might have been emphasized locally by tectonic warping of the platform.

Tectonically induced slowing of platform subsidence in the Aptian, coupled with intermittent cooling events and glacio- eustasy, generated a platform characterized by breccia-capped 3rd-order sequences composed of relatively few parasequences and sets. OAE1a resulted in short term incipient drowning, but OAE1b had little effect.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013