Temporal trends in shell beds accumulation. Preliminary results from the Upper Cambrian-Lower Ordovician siliciclastic nearshore-offshore environments, Cordillera Oriental, Argentina.

MARENGO, L. Y WAISFELD, B.G

Mendoza

Simposio; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

Accumulation patterns of fossil concentrations have changed through time due to changes in the nature and environmental distribution of the organisms. We present preliminary results of a large scale study encompassing the Upper Cambrian – Ordovician shell beds from the Cordillera Oriental, NW Argentina, a paleogeographic and environmental setting where the early history of shell beds has not been investigated before. Spatio-temporal distribution of fossil concentrations present in the Santa Rosita Formation exposed at several localities (Arenal, Humacha, Moya, Coquena, Chalala creeks, and Angosto del Moreno and Iruya areas) is analyzed. The siliciclastic Santa Rosita Formation records a wide array of depositional environments, including wave-dominated shallow marine deposits associated with deltaic systems, and transitions into tide-dominated estuarine complexes. Shell beds mirror the complex depositional history of the unit, including a diverse mosaic of genetic and taxonomic types. Bioclastic deposits were investigated through three time intervals: Latest Cambrian (P. (Neoparabolina) frequens argentina Zone), early Tremadoc (Jujuyaspis keideli and Kainella meridionalis Zones), late Tremadoc (“Bienvillia tetragonalis Zone”, and Notopelitis orthometopa Zone), and across four broad environmental categories: nearshore, shoreface, shoreface-offshore transition, and offshore.  Accumulation patterns of fossil concentrations have changed through time due to changes in the nature and environmental distribution of the organisms. We present preliminary results of a large scale study encompassing the Upper Cambrian – Ordovician shell beds from the Cordillera Oriental, NW Argentina, a paleogeographic and environmental setting where the early history of shell beds has not been investigated before. Spatio-temporal distribution of fossil concentrations present in the Santa Rosita Formation exposed at several localities (Arenal, Humacha, Moya, Coquena, Chalala creeks, and Angosto del Moreno and Iruya areas) is analyzed. The siliciclastic Santa Rosita Formation records a wide array of depositional environments, including wave-dominated shallow marine deposits associated with deltaic systems, and transitions into tide-dominated estuarine complexes. Shell beds mirror the complex depositional history of the unit, including a diverse mosaic of genetic and taxonomic types. Bioclastic deposits were investigated through three time intervals: Latest Cambrian (P. (Neoparabolina) frequens argentina Zone), early Tremadoc (Jujuyaspis keideli and Kainella meridionalis Zones), late Tremadoc (“Bienvillia tetragonalis Zone”, and Notopelitis orthometopa Zone), and across four broad environmental categories: nearshore, shoreface, shoreface-offshore transition, and offshore.  Accumulation patterns of fossil concentrations have changed through time due to changes in the nature and environmental distribution of the organisms. We present preliminary results of a large scale study encompassing the Upper Cambrian – Ordovician shell beds from the Cordillera Oriental, NW Argentina, a paleogeographic and environmental setting where the early history of shell beds has not been investigated before. Spatio-temporal distribution of fossil concentrations present in the Santa Rosita Formation exposed at several localities (Arenal, Humacha, Moya, Coquena, Chalala creeks, and Angosto del Moreno and Iruya areas) is analyzed. The siliciclastic Santa Rosita Formation records a wide array of depositional environments, including wave-dominated shallow marine deposits associated with deltaic systems, and transitions into tide-dominated estuarine complexes. Shell beds mirror the complex depositional history of the unit, including a diverse mosaic of genetic and taxonomic types. Bioclastic deposits were investigated through three time intervals: Latest Cambrian (P. (Neoparabolina) frequens argentina Zone), early Tremadoc (Jujuyaspis keideli and Kainella meridionalis Zones), late Tremadoc (“Bienvillia tetragonalis Zone”, and Notopelitis orthometopa Zone), and across four broad environmental categories: nearshore, shoreface, shoreface-offshore transition, and offshore.  Trilobite shell beds are the most conspicuous taxonomic type, occurring as thin pavements in offshore environments, and as beds, lenses, and pavements in distal shoreface and shoreface-offshore transition. This mode of occurrence remains relatively stable through the time interval investigated. Linguliformean shell beds are documented in nearshore settings as beds up to 5 cm thick in deltaic plain environment, as the internally complex filling of erosive bedforms (up to 2m thick) within estuarine sand bars, or as pavements and thin beds of comminuted debris in shoreface and transition environments. Echinoderms (eocrinoids) occur as separate ossicles in politaxic shell beds or as few mm “equinoderm beds”. The latter are otherwise scattered in siliciclastic setting, and their occurrence could be related to times of terrigenous sediment starvation associated with drowning of sisliciclastic systems. Rhynchonelliformean are subordinate elements in upper Cambrain and lower Tremadoc trilobite dominated bioclastic deposits. Rhynchonelliforman shell beds (20 cm thick) are exceptionally documented in lower shoreface and offshore transition settings, interbedded with trilobite shell beds in uppermost early Tremadoc. Monospecific rhynchonelliformean shell beds become abundant, thicker (up to 40 cm), more complex internally, and constitute significant sedimentary bodies in upper Tremadoc proximal shoreface settings. Finally, a wide variety of politaxic shell beds, exhibiting different combinations and relative abundances of higher taxa, mostly occur in distal shoreface environment, either in the early or late Tremadoc. At a broad scale, shell beds fit in the Archaic biostratinomic style of accumulation (cf. Kidwell and Brenchley, 1994). Trilobite and trilobite-dominated shell beds, linguliformean and linguliformean-dominated ones, could be included in the Cambrian style (cf. Li and Droser, 1999), whereas rhynchonelliformean and rhynchonelliformean-dominated shell beds might constitute early representatives of the Paleozoic biostratinomic style of these authors. During the Late Cambrian and most of the early Tremadoc shell beds of the Cambrian style dominate all the environmental categories, meanwhile the latest early Tremadoc exhibits an intermediate condition, represented by the intercalation of both styles in distal shoreface settings. Finally, in the late Tremadoc Paleozoic style shell beds become a significant stratigraphic feature in proximal shoreface environment.  Our preliminary results suggest that the onset of the Paleozoic style in the Santa Rosita Formation might have been the response to the combination of several factors: a facies-related pattern, as representative occurrences are restricted to the proximal shoreface environment; a taphonomic artifact mediated by the more resistant microarchitecture of rhynchonelliformean shells in relation to that of trilobites or linguliformean; and an evolutionary / ecological process linked to the regional radiation of plectorthoidean brachiopods in the southwest of Gondwana (cf. Benedetto, in press) and the early occupation of high energy habitats by the group. Kidwell, S.M. and P.J. Brenchley, 1994. Patterns in bioclastic accumulation through the Phanerozoic: Changes in input or in destruction?. Geology 22: 1139-1143. Li, X and Droser, M., 1999. Lower and middle Ordovician shell beds from the Basin and Range Province of the Western United Stated (California, Nevada, and Utah). Palaios 14: 215-233. Benedetto, J.L. in press. New Upper Cambrian and Tremadoc rhynchonelliformean brachiopods from Northwestern Argentina: Evolutionary lineages, heterochrony and early diversification of plectorthoideans. Journal of Paleontology.