Los Espejos Formation: sedimentology, biostratigraphy and biofacies analysis.

CECILIA M. RODRIGUEZ; ANA I. MESTRE; LIDIA M. SANCHEZ; SUSANA E. HEREDIA

Mendoza

Congreso; International Sedimentological Congress 2010; 2010

IAS

The aim of this work is to present the results from analyzing three profiles from Los Espejos Formation, in the Central Precordillera of San Juan province, Argentina. The three analyzed sections were: Cerro del Fuerte, Loma de Los Piojos, and Ancha creek (in Talacasto). Biostratigraphic, conodont biofacies and sedimentological studies were developed. Specifically, at Loma de Los Piojos section we carried out a detailed sedimentological analysis. From the present study, we concluded that deposits from Los Espejos Formation correspond to a storm-dominated clastic shelf. Deposits are composed of clayey silt materials with sandstone banks which become increasingly thick toward the top. At the top, tabular sand layers are intercalated with coquina banks of lenticular geometry. Identified facies were: proximal shoreface facies (f C) which show lenticular banks of middle fine to coarse sandtones with coquinoids at the top; distal shoreface facies (f A) with lenticular banks of fine to very fine sandstones with interbedded coquinoids; transition to offshore facies (f D) represented by tabular clayey silt banks interbedded with fine to very fine sandstones with hummocky, wavy and flasser structures and parallel lamination; and also by facies of very fine tabular sandstones with monotaxonomic coquinoids at the base (f B) interbedded with f D; finally, offshore facies (f E) with tabular massive fine sandstone banks interbedded with clayey silt banks. At Lomas de Los Piojos section, facies change vertically from offshore deposits or offshore transition deposits to shallower environments of distal or proximal shoreface deposits. We observed four shallowing events in the sedimentary column and interpreted them as facies successions. A regressive pattern is observed all along the outcrops.   Coquinoid layers in the Ancha creek section include several conodont species from the Kockelella variabilis variabilis Zone, indicating a Gorstian age for the middle and upper section of the profile. The distribution of conodont faunas in each section implies populations adjusted to different bathymetric conditions and preferring muddy or sandy substrates, thus suggesting different conodont biofacies: Panderodus-Pseudoneotodus biofacies, Oulodus/Coryssognatus biofacies and Ozarkodina biofacies. The discontinuity that separates Los Espejos Formation from the overlying Talacasto Formation would comprise a hiatus including the Upper Ludlovian and the Pridolian. This hiatus contrasts with what has been proposed thus far by different authors. The basal boundary of Los Espejos Formation could be associated with an important hiatus recorded in other world regions, referred to as ravinement surface that has been correlated with a generalized regression at the epicratonic and basin areas, culminating in a low sea level system tract (LST). Thus, as Los Espejos Formation presents a Gorstian age in the middle and upper sections of the profile, the lower limit of this formation could be related to the eustatic rise after the generalized regression of said limit, and therefore the age range of Los Espejos Formation could be not older than Ludlovian.