AAPG ACE 2018

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Insights Into Internal Structure of a Megabed From Long Distance (130×30 km) Correlation: The Contessa Megaturbidite of the Miocene Marnoso-Arenacea Formation (Serravallian; Northern Apennines, Italy)

Abstract

Megaturbidites are laterally extensive event beds deposited by sediment-laden density flows of exceptionally large magnitude, capable of transferring sediment volumes in order of several Km3. Though the scale of some megaturbidites make them an attractive target for hydrocarbon exploration, their shape and facies partitioning are typically sub-seismic, making outcrop analogues important to provide insights into subsurface examples.Here we report on the 'Contessa' Megaturbidite (CM) of the Marnoso Arenacea Fm. (MAF) that represents the most extensive correlation of an individual megaturbidite yet achieved in any ancient sequence. CM was emplaced in the earliest Serravallian as a result of a collapse of the southernmost bound of the Apennines foredeep. A total of 45 sections with spacing in the range 2–8 km were loggedover an area extending for c. 130 km along the foredeep axis(i.e. along flow) and c. 30 km perpendicular to it. To help pinpointing deposit shape and quantify substrate erosion, logging included a few MAF turbidites below and above CM.

CM is sandstone-mudstone couplet of basin basin-wide extent having thicknesses in the range 12-15m and associated to a minimum sediment volume of c. 35 km3 (i.e more than one order of magnitude larger than typical MAF turbidites). Above an invariably fluted base indicating palaeoflow from the south–southeast, CM consists of the following sedimentary facies: i) a massive to crudely laminated medium-grained sandstone showing a coarse-tail grading; ii) a fine-grained bundle in which a few to several dm-thick lamina sets with either parallel, or cross, or convoluted laminations occur in turn repeatedly; iii) a relatively thick fine-very fine grained sandstones with a range of sigmoidal to hummocky laminations, and iv) a up to 8m thick massive mudstone cap, which partly compensates the thickness variations of the sandstone.

While facies i-ii record deposition from a relatively high-density waxing flow subject to a range of reflection/deflection phenomena by the confining topography, facies iii and iv suggest establishment of oscillatory flow conditions and ponding, respectively. Despite the thickness changes of the CM sandstone are subtle, field data suggest there is a complex pattern to facies proportions. Understanding such pattern is important in a reservoir perspective, because each of these facies is likely accompanied with different petrophysical properties.