Hummocky cross-stratified deposits and turbidites in some sequences of the Umbria-Marche area (central Italy) during the Toarcian

Abstract

Some sections of the Umbria-Marche basin (central Italy) were characterized during the Toarcian by alternately thicker and thinner organic-rich marls and well-oxygenated, red and nodular marly limestones. These sequences exhibit sharp-based hummocky cross-stratified (HCS) deposits and turbidites, ranging in thickness from several centimetres to 200 cm. Palaeocurrent measurements of HCS beds and turbidites indicate the same SE-NW direction. HCS deposits, that show wavelengths mostly from 70 to 140 cm, are grainstones and matrix-poor packstones. A complete sequence consists of three divisions. (a) A lower, coarse-grained bivalve and echinoderm lag, from 0.5 to 40 cm thick. The lag is generally erosive and discontinuous and probably represents a proximal material deposited during a high-energy phase of a storm event. (b) A middle, well-sorted calcarenite/calcisiltite division (from 20 to 100 cm thick), that exhibits well preserved hummocky cross-stratification. Bioclastic calcareous sands transported by offshore currents from storm surges were organized into HCS beds under an oscillatory/unidirectional flow regime near the bottom. (c) An upper, oscillatory ripple-bedded calcisiltite division from 0.5 to 25 cm thickness. Ripples were probably formed during a dominant oscillatory regime (wave action?) after the sand sedimentation had stopped. Locally a pervasive bioturbation is common in the upper part of this bed. Turbidites (10–200 cm thick) are packstones and rudstones with abundant muddy matrix and generally show the classical Bouma BC sequence. The lower and middle interval of the studied sequences, from the late Domerian to the early Toarcian, is dominated by fining-upward turbidites. On the contrary, the upper part of the sequence, middle/late Toarcian in age, is dominated by sharp-based HCS beds. The vertical transition from turbidites to sharp-based HCS deposits is indicative of a general regressive trend. The causes of the sea-bottom depth changes can be related respectively to a sea-level rise documented for the early Toarcian and to the sea-level fall of the middle Toarcian. Lateral and vertical distribution, frequency, grain size, structures and composition of detritic beds and microfaunal assemblages confirm the differentiated topography of the Umbria-Marche basin during Toarcian time.