Paleoenvironmental interpretation of the Lower Ordovician Oepikodus evae Zone in the Peña Sombría section, Precordillera, Argentina

DELLA COSTA, G.M.; CAÑAS, FERNANDO L.; ALBANESI, GUILLERMO L.

Novosibirsk

Simposio; 13th ISOS; 2019

International Commission on Stratigraphy + 6

Oepikodus evae (Lindström)is considered one of the most important index fossils of the Lower Ordovicianbecause of its short stratigraphic range and wide geographical distribution. Thisconodont zone has a wide distribution in the Argentine Precordillera, where Itranges through the middle part of the SanJuan Formation, allowing an accurateregional correlation. The SanJuan Formation contacts diachronously withoverlying stratigraphic units through the Dapingian-Darriwilian stages, with O.evaerecords in the upper strata of the San Juan Formation, close to thecontact withthe Gualcamayo Formation, in the northern Precordilleran localities. Facies associationswere identified through the San Juan Formation at Cerro La Sillaand Niquivilsections represent environments from tidal and agitated shoalsettings todeeper, sub-storm wave base, basinal settings, distributed along anopen-shelfto the ramp. At the Peña Sombría section, northern Precordillera, the upperstrata of the San Juan Formation that correspond to the Oepikodus evae Zonepresents numerous species that suggests alow energy environment below the storm wave base.Microfacies analysisof these strata determine thin to medium, nodular bedded, dark carbonatemud-dominated lithologies for the upper 10 meters of the San Juan Formation.All samples are burrowed skeletal/bioclastic wackestone, mostly composed oftrilobites and pelmatozoan fragments. Articulate brachiopods andsmallgastropods are less common, and whole linguliform brachiopods, spongespiculesand ostracods are only occasionally present. Bioclasts on winnowed surfacescanbe phosphatized and stained with oxides. Mineralized bioclasts andoxidizedglauconite are reworked within the wackestones by bioturbation. Theserocks areinterpreted to have been deposited in a deeper, outer-ramp setting,bellow thestorm wave base, as is evidenced by the dominance of carbonate muddeposition. Thisfacies association correlate with lithoclastic-skeletal faciesof the La Silla and Niquivil sections, a middle ramp or open platform faciesassociation deposited in very shallow water, well within the photic zone andrecurrently reworked by frequent storms. Microfacies analysis showed ahighstand depositional pattern for this part of the San Juan Formation in thecerro La Silla section, supporting previous interpretation of the top strata ofthe San Juan Formation in the Peña Sombría section.