Paleoecology and paleobiogeography of Darriwilian conodonts from the Las Chacritas Formation, Central Precordillera of San Juan, Argentina

SERRA FERNANDA; ALBANESI GUILLERMO L.

PUBLICACION ESPECIAL - ASOCIACION PALEONTOLOGICA ARGENTINA

ASOCIACIÓN PALEONTOLÓGICA ARGENTINA

Año: 2013 p. 103 - 108

0328-347X

The mostly calcareous Las Chacritas Formation crops out in the northern section of the La Trampa Range, southwest of Jáchal in the Central Precordillera of the San Juan Province.Peralta et al. (1999) defined the Las Chacritas Formation as a 55 m thick carbonate sequence that consists of two members. The lower member, 38 m thick, is composed of tabular, thin to medium bedded, fossiliferous dark mudstones, nodular wackestone to packstones with a bed of K-bentonite at the contact with the underlying San Juan Formation. The upper member, 17 m thick, includes thin-bedded wackestone, bioclastic grainstone, mudstone and spiculitic mudstones deposited in a continental shelf setting. At the study section, across the river and close to the stall, the upper strata of the Las Chacritas Formation are covered, although the uppermost strata are exposed northward over the hillside, where this formation is overlain by the uppermost Ordovician to lower Silurian La Chilca Formation. The whole exposure of the formation is approximately 50 m thick in the surveyed section. Previous studies on the conodont faunas from the Las Chacritas Formation have centered on biostratigraphy. As an original contribution, using a large data base of conodonts, we examine in this study aspects of the conodont paleoecology. The purpose of this paper is to describe and analyze the dynamics of the conodont communities through the upper San Juan and Las Chacritas formations in order to interpret the depositional environments and the way changes in the paleoecological conditions affected the spatial distribution of the conodont faunas through the studied time span. In addition, we investigated how conodont associations contribute to a better understanding of the position of the Cuyania Terrane during the middleDarriwilian. Conodonts are abundant and of high diversity through the contact interval between the San Juan and the Las Chacritas formations, and the same is the case in the upper part of the Las Chacritas Formation. In the middle part of the Las Chacritas Formation a decline in the relative abundance of most taxa is observed, and the presence of some prominent forms, such as Paroistodus horridus and Ansella jemtlandica, indicate a change in the environmental conditions. The major part of the carbonate sequence is dominated by two species, Periodon macrodentatus and Paroistodus horridus, allowing the recognition of this interval as representing the Periodon-Paroistodus Biofacies, which represents deep, open-sea biotopes. An open sea environment is suggested for faunas that are dominated by Periodon. e most frequent genera include Ansella, Protopanderodus, Drepanoistodus, Fahraeusodus, and Parapaltodus, which occur throughout the entire section. These taxa are interpreted as epipelagics, and are representative of deep subtidal settings. The response of the conodont fauna to the shifting environmental conditions has been studied thoroughly. The investigated sequence has been subdivided into three conodont sub-biofacies based on the taxonomic composition and abundance: a) The Rossodus-Yangtzeplacognathus Sub-biofacies; b) e Ansella-Fahraeusodus Sub-biofacies; and c) The Parapaltodus-Histiodella Sub-biofacies. In this contribution, among the 34 conodont genera identified, 47% are cosmopolitan, 23% are representative of Laurentia, 13% are Baltoscandian, and 17% are endemic in the Precordillera. The species assemblages encountered are similar to those of the Cold Domain. Nevertheless, some taxa, such as Rossodus barnesi, Semiacontiodus potrerillensis, Polonodus galerus, and two new genera are only recorded in the Precordillera, which provides it with a local signature. We support the idea that the Middle Ordovician Precordillera is a separate paleobiogeographic unit, defined as the Precordillera Province, corresponding to the Temperate Domain. It di ers from the Shallow-Sea Realm of the San Juan Formation, and the Las Chacritas Formation is referable to the Open-Sea Realm. The lithofacies types, together with the conodont distribution and abundance patterns, are indications of the occurrence of eustatic sea-level fluctuations that changed the living conditions of the fauna. The top parts of the San Juan Formation and of the Las Chacritas Formation represented favorable environments based on the highest conodont diversity. Occasional shallowing events are documented based on the presence of C. longibasis in the middle and upper strata from Las Chacritas Formation. These environmental changes caused reorganization in the conodont fauna composition, causing temporary migration events during the deepening of the Precordillera basin. We conclude that during the Middle Ordovician, the Precordillera conodont fauna shows similarity to that of the Cold Domain, but it has also a high level of endemism. This demonstrates an efficient dispersal of the conodont faunas in the Iapetus Ocean, as well as it allows us to recognize the Precordillera Province, which represents the Temperate Domain, separate from the Shallow-Sea Realm in the San Juan Formation or from the Open-Sea Realm for the Las Chacritas Formation.