TIMING AND CAUSES OF THE DROWNING AND BACK STEP OF THE APULIA CARBONATE PLATFORM (MONTE SACRO SEQUENCE, GARGANO PROMONTORY, SOUTHERN ITALY)

We evaluate the timing and mechanisms that caused drowning and backstep of the Monte Sacro Sequence in the Apulia carbonate platform exposed in Gargano Italy.

Previous studies interpreted backstep of the Monte Sacro Sequence to represent a drowning unconformity, with pre-drowning platform strata of Berriasian age, and post drowning, onlapping strata of Valanginian age. These studies interpreted drowning to have been caused by eustatic sea-level rise and found no evidence for “poisoning” of platform carbonate production during the Weissert carbon isotope excursion (CIE), which was considered a potential factor in drowning by other authors.

Stratigraphic sections at Lame di Milo and Coppa di Montelci at the toe of slope show an upward change from thick carbonate breccia beds interpreted to represent the flank of the pre-drowned platform, to thinning breccia and skeletal packstone beds intertongued with pelagic lime mudstone, and ultimately to a continuous pelagic lime mudstone succession. Skeletal packstone composition shows evidence of continued photozoan carbonate production on the platform margin during the retrogradation and backstep. Intraclasts lack characteristics, such as mineralization, that would indicate material reworked from a long “dead” platform.

Potential models for the backstep of the Monte Sacro Sequence include: a drowning unconformity and onlap consistent with previous studies; intertonguing at the toe of the slope during initial retrogradation followed by backstep; backstep and redeposition of reworked margin sediments on the toe of the slope; “rapid” but continuous retrogradation without a distinct backstep, and; along-strike variability in drowning with shedding of debris from an active margin factory towards the onlapping toe of slope succession of a drowned segment of the margin. Testing of these models will require additional field, petrographic and biostratigraphic study.

𝛿¹³C values from pelagic lime mudstone, and the matrix of breccia and packstone in the drowning succession range from 0.0 to -3.0 ‰ (VPDB). Comparison with regional isotope data compilations indicates a late Berriasian onset of retrogradation and completion of backstepping by the early Valanginian. Thus, drowning and backstep of the Monte Sacro sequence entirely preceded the Weissert CIE.