Insights Into the Distribution of Shallow-Marine to Estuarine Early Miocene Oysters From Southwestern Patagonia: Sedimentologic and Stable Isotope Constraints

Abstract

The lower Miocene Estancia 25 de Mayo Formation consists of shallowmarine to estuarine deposits that bear numerous oyster shell beds distributed throughout the succession. Facies analysis reveals that oysters grew in the nearshore paleoenvironments of both the lower Quién Sabe and the upper Bandurrias members. Two oyster species were identified: Crassostrea(?) hatcheri, distributed in the lower two thirds of the column, and Crassostrea orbignyi, distributed in the upper third of the column within the transition from marine to the fluvial deposits of the overlying Santa Cruz Formation. Petrographic, cathodoluminiscence, and carbon and oxygen stable isotope analyses of individual growth increments were performed on carbonate from all the oyster beds, together with consecutive time-series analyses for one specimen of each species. The isotopic composition of the shell seems to be associated with the microstructure of the growth increments. Selective diagenetic alteration affected chalky growth increments; whereas translucent (foliated and prismatic) growth increments are well preserved and can be used to infer paleoenvironmental conditions. Isotopic data indicate that C.(?) hatcheri lived in normal marine waters with a range of paleotemperatures from 10.6 to 20.5 6C. Monospecific beds of this oyster are related to opportunistic and quick colonization of the sea bottom. More negative d 13 C and d 18 O values in C. orbignyi shells likely reflect decreased paleosalinities, and the resultant environmental stress controlled the development of these low-diversity oyster accumulations. The separate stratigraphic distribution of each species was therefore determined by the paleoenvironmental conditions.

Fossil localities of the Santa Cruz Formation (Early Miocene, Patagonia, Argentina) prospected by Carlos Ameghino in 1887 revisited and the location of the Notohippidian

“…According to Cuitiño and Scasso (2010) and Cuitiño et al (2013) the uppermost marine beds of the Estancia 25 de Mayo Formation (the Bandurrias Member) are covered transitionally by the Santa Cruz Formation. At the Río Bote locality, we measured 125 m of gray to greenish siltstone, sandstone and tuff beds, with minor proportion of conglomerates in lenticular beds.…”

A multidisciplinary study of the Lower Cretaceous marine to continental transition in the northern Austral-Magallanes basin and its geodynamic significance

“…The Eocene Posadas Basalt follows after a long hiatus (Riggi, 1957), covering in angular unconformity the Albian Río Tarde tuffs. The sequence continues with Neogene sinorogenic deposits, represented by the Miocene El Chacay and Santa Cruz formations (Hatcher, 1897(Hatcher, , 1900Chiesa and Camacho, 1995;Cuitiño et al, 2013Cuitiño et al, , 2016Parras et al, 2012), covered by the Belgrano Basalt (Riggi, 1957). Sinorogenic deposits of Miocene ages indicate important generation of relief (Cuitiño et al, 2015).…”

The temperate marine Peruvian Province: How history accounts for its unusual biota

“…The Argentinian fauna likewise supports only small oysters (Shilts et al., 2007 ). Vast beds of large oysters are known from the Early Miocene and Late Miocene of Argentina (Cuitiño et al., 2013 ; Parras & Casadío, 2006 ), comprising not only species of “Crassostrea” , but also Cubitostrea alvarezi . “Crassostrea” hatcheri in the Monte León Formation (Early Miocene) reached a length of at least 250 mm and a valve thickness of 70 mm (Parras & Casadío, 2006 ).…”