MUDSTONE COMPOSITE PARTICLES IN BLACK SHALE SUCESSIONS. EXAMPLES FROM THE VACA MUERTA AND AGRIO FORMATIONS, NEUQUÉN BASIN, ARGENTINA

COMERIO M.; OTHARAN, G.; SCHIEBER, J.; YAWAR, Z.

La Plata

Congreso; XVIII RAS-IX CLS; 2023

The Upper Jurassic–Lower Cretaceous stratigraphic record of the Neuquén Basin, west-central Argentina, contains 10’s–100’s m thick black shale successions included in the Vaca Muerta and Agrio formations. The organic richness of Vaca Muerta is considerably high compared to Agrio, nevertheless both units are characterized by carbonate–siliciclastic mud sedimentation controlled by suspension settling processes interrupted by episodic events (e.g., storm-related processes, wave/current enhanced hyperpycnal flows and turbidity currents) and frequently subject to reworking by quasi-steady bottom currents. Textural analyses of mudstone microfabrics show that, rather than a pile of discrete clay-sized particles, organic carbon-rich mudstones are typically composed of silt- to sand-sized aggregate grains (herein referred to as mud composite particles, MCPs). In the present study detailed petrographic examination by SEM of ion-milled surfaces was used to describe and interpret MCPs preserved in Vaca Muerta and Agrio formations. Organic carbon-rich intervals of both units show fecal pellets, and mud rip-up clasts. Fecal pellets were recognized as isolated aggregates as well as packed particles resulting in lenticular fabric. They are mainly composed of fragmented coccolith plates indicating not only that phytoplankton blooms occurred and acted as an important source of carbonate mud, but also an organization as grain aggregates that plausibly could have been carried to distal depositional sites by bedload processes. In addition, mud rip-up clasts were inferred from SEM images that showed flattened aggregates composed of clay minerals (mainly illite), silt-sized siliciclastic grains, organic particles, and scarce carbonate microfossils. They reflect seafloor disturbance–erosion and subsequent deposition as a result of bottom-traction transport. Notably, organic-rich levels of Vaca Muerta show silt-sized grains composed in some cases of authigenic kaolinite which probably represent the alteration of highly reactive volcanic glass, and also sand-sized lathwork lithic fragments, derived from the reworking of volcanic products. The abundance of such particles within the Vaca Muerta Formation suggest that volcanic activity plausibly enhanced fertilization in the water column, acting as an important productivity driver. Exhaustive analysis of MCPs is key to understand mudstone facies related to distal settings in terms of transport processes, bathymetric changes, oxygen availability and organic carbon preservation. Also, MCPs are relevant for the development of shale resources, since they undoubtedly impact on petrophysical and geomechanical properties, and also on the water content. As stated by Sorby (1908): “There is no necessary connection between the size of an object and the value of a fact”.