IDENTIFYING FORMATIVE PROCESS AND PROVENANCE OF MUDROCKS IN A DYNAMIC DEEP-MARINE SYSTEM

Although some properties of mudrocks can be measured in the field, bioturbation intensity, grain size, and mineral composition are indiscernible without laboratory techniques. In deep-marine systems these properties are required to identify subaqueous flow depositional processes. Four mudrock facies are identified from field observations of strata of the deep-marine Miocene Marnoso-arenacea Formation (Italy) surrounding the growth of the Coniale Anticline. Graded siliciclastic mudstones (Facies I) and ungraded calcareous mudstones (Facies II) dominate pre-kinematic strata while ungraded massive marls (Facies III) and ungraded mixed composition mudstones (Facies IV) dominate syn-kinematic strata.

This research uses petrographic criteria to test hypotheses of process and provenance for the four mudrock types. Hand sample and petrographic microscope examinations were used to quantify the diversity of microscopic body fossils, ichnofossils, and carbonaceous material. Grain size and composition were also qualitatively assessed. Siliciclastic mudrocks (I) have the fewest and most disarticulated microfossils while calcareous mudrocks (II and III) have the highest abundance and diversity of microfossils and ichnofossils. Facies I is more carbonaceous than II or III. Facies III occurs only on the anticline crest and contain an elevated concentration of benthic foraminifera. Mixed-composition mudrocks (IV) in the basin have petrographic properties intermediate between I and III.

These petrographic observations support field-based interpretations of hydrodynamic depositional process and provenance. Mudrock facies I and II record long-distance transport in turbidity currents. Facies I is sourced from the Alps while Facies II is sourced from carbonate systems in the Apennines. Facies III and IV result from intrabasinal structural growth. Facies III record in-situ carbonate deposition in the photic zone on the paleo-high. Fine grained debris flows remobilize facies III from the paleo-high and entrain facies I or II deposits en route producing thick facies IV deposits in the basin floor. This research confirms that these locally-derived muddy debris flows are a previously unrecognized, yet important process for generating up to 20% of syn-sedimentary growth strata in deep-marine systems.