--> Laminated Black Shales: Process Sedimentology at Lamina Scale: Examples From the Eagle Ford Formation, TX, USA

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Laminated Black Shales: Process Sedimentology at Lamina Scale: Examples From the Eagle Ford Formation, TX, USA

Abstract

Organic-rich depositional sequences, which represent prolific source rocks for both conventional and active unconventional plays, are often characterized by rhythmically interbedded limestones and marlstones, e.g., Utica Fm (Ohio, USA), Vaca Muerta Fm (Argentina), Natih Fm (Oman). New astronomical analyses of limestone-marlstone couplets from continuous sections of the Eagle Ford Formation (Texas, USA) document that the limestone-marlstone couplets reflect climatic forcing driven by solar insolation resulting from integrated Milankovitch periodicities; in particular, obliquity (41 ka) and precession (18–21 ka) forcing on summer insulation and therefore affecting seasonality, in turn determining the lithologic alternations. In fact, the rock preserves primary environmental signals, supporting greater water-mass ventilation and current activity during the deposition of limestones. To understand the laminae in the marlstones, we performed numerous analyses at lamina scale on a continuous thin section from an entire precession cycle (35 cm thick) characterized by high organic content. These analyses include sedimentologic descriptions and micropalaeontologic assemblage reconstructions at mm scale, X-ray fluorescence with 200 μm resolution and Total Organic Carbon measurements at cm scale.