Model for development of small carbonate platforms in the Umbria-Marche Apennines supported by strontium isotope stratigraphy

Abstract

ABSTRACT The objective of this research was to determine the age relationships between Early Jurassic sedimentary units in the Umbria-Marche Apennines using strontium isotope stratigraphy. These age relations are critical for evaluating the mechanism by which the small platforms in the region formed. I present an alternative model for carbonate platform differentiation and evolution based on the strontium isotope data; this model relies primarily on growth of individual small platforms through differential sediment accumulation rather than the generally accepted model of extensive late-stage faulting. Strontium isotope stratigraphy provides a high-resolution correlation tool for the region. Because 87Sr/86Sr values steadily decrease in the Hettangian–Pliensbachian section of the Early Jurassic, they illuminate relationships in different carbonate facies very well. This is particularly valuable where physical and/or biostratigraphic correlation is difficult, such as in the Umbria-Marche carbonate platforms. Strontium isotopic ratios from measured basinal stratigraphic sections along the Burano and Bosso Rivers indicate that at least a 100 m section of the Corniola Formation was deposited while adjacent platforms continued to grow. The small Umbria-Marche platforms drowned synchronously in the late Sinemurian, based on 87Sr/86Sr values from several platforms at the end of Calcare Massiccio Formation deposition. Measured 87Sr/86Sr values for all platforms are the same within error. These stratigraphic relationships indicate that differentiation into shallow- and deep-water facies occurred in the latest Triassic/earliest Jurassic, early in the development of the margin. Current models explain the differentiation of platforms and basins by late-stage extensive normal faulting, requiring hundreds of meters of stratigraphic throw. Large-displacement faults are not consistent with the age relations determined by strontium isotope stratigraphy in the region. Minor faulting in the Late Triassic coupled with growth by differential sediment accumulation of small, shallow-water carbonate platforms continuing for several million years would produce the exposed platform/basin relationships. Inferred accumulation rates for the platform and basin sequences suggest that the observed relief of the platform escarpments was generated in less than 7 m.y., a time period consistent with the documented stratigraphy in the region.