Hummocky cross-stratifications and trace fossils in the Middle Toarcian of some sequences of Umbria-Marche Apennines

Abstract

Some Toarcian sequences of the Umbria-Marche Apennines show variable thicknesses of marly deposits rich inorganic material (MS, Marne del M. Serrone Formation, Late Domerian — Middle Toarcian) and well-oxygenated, red and nodular marly limestones (RAUM, Rosso Ammonitico Umbro-Marchigiano, Middle/Late Toarcian). In the MS, gravity flow deposits and turbidites are locally abundant. The MS/RAUM transition, instead, exhibit sharpbased hummocky cross-stratified (HCS) deposits, that, in the Middle Toarcian, range in thickness from several centimeters to 60 cm, and show wavelengths mostly from 70 to 140 cm. A complete sequence of HCS deposits consists of three divisions: I) a lower, massive or slightly graded bivalve and echinoderm lag, (0.5 to 40 cm thick) that is generally erosive, coarse-grained and discontinuous and probably represents proximal materials, deposited during the high-energy phase of a storm event (basal lag); II) a middle, well-sorted and matrix-poor calcisiltitic division (10 to 60 cm thick), that exhibits a well preserved hummocky cross-stratification; III) an upper calcilutitic deposit (from 0.5 to 20 cm), with pervasive bioturbation in the upper part of the bed. Division III (often absent) probably reflects the waning-energy phase of depositional event. A vertical transition from turbidites at the Domerian/Toarcian transition to sharp-based HCS deposits in the upper part of the sequences (Middle Toarcian), seems to indicate a regressive trend in the investigated Umbria-Marche sections. In fact, according to literature, HCS beds were deposited generally about or above a storm wave base. At this depth the storm waves suppressed the Bouma divisions and formed HCS under a combined (oscillatory/unidirectional) flow regime. Highly packed concentrations of concave bivalve shells, showing shelter-porosity (winnowed beds) and abraded tests of benthic foraminifers seem to indicate a dominant near-bottom oscillatory flow regime below the fairweather wave base level. A shallowing-upward trend in the investigated sections from the Early Toarcian to the Middle Toarcian, as testified also by microfaunal content, seems to be due to tectonics and/or sea-level fall and ranges from an upper bathyalouter shelf in the Early Toarcian to a middle shelf in the Late Toarcian. At this last time trace fossil assemblages increase in abundance, diversity and penetration depth at the top of HCS beds and in normal basinal sediments, in relation to increased oxygen availability and to favourable substrate conditions.