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ICHNOLOGY OF DEVONIAN SHALLOW-MARINE DEPOSITS IN PRECORDILLERA, ARGENTINA: DELINEATING CHANGES IN ICHNODIVERSITY AND BIOTURBATION INTENSITY, AND DECIPHERING LIMITING FACTORS

Date

2024-07-17

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

0000-0002-3432-5829

Type

Thesis

Degree Level

Masters

Abstract

The Lower Devonian Talacasto Formation of western Argentina is an exceptional wave-dominated shallow-marine unit due to the good quality and accessibility of its outcrops. This unit has relevance to our general understanding of shallow-marine settings and, at a regional scale, of the ecological factors that prevailed in Gondwana during the Early Devonian. The Talacasto Formation is characterized by black, parallel-laminated mudstone in the lowermost interval, passing upwards into siltstone and very fine- to medium-grained sandstone in the uppermost interval. Seven sedimentary facies were recognized in the Loma de Los Piojos (LLP), Rio Las Casitas (RLC), Quebrada de Talacasto (QT), and Quebrada Poblete Sur sections of the San Juan Province, ranging from the shelf to the upper shoreface. Three trace-fossil assemblages are recognized: (1) Palaeophycus Assemblage, including Palaeophycus heberti, P. tubularis and Helminthopsis, (2) Phycosiphon-Zoophycos Assemblage with Zoophycos isp., Phycosiphon incertum, Nereites missouriensis and Chondrites isp., and (3) Rosselia Assemblage containing Rosselia socialis, Skolithos isp., Arenicolites isp., Palaeophycus tubularis and escape trace fossils. Two ichnofacies are identified: (1) Distal Cruziana Ichnofacies and (2) Skolithos Ichnofacies. The Talacasto Formation is an example that differs from the traditional ichnofacies model of wave-dominated shallow-marine environments since the absence of the archetypal Cruziana Ichnofacies. The absence of the archetypal Cruziana Ichnofacies is anomalous because: (1) the deposits that typify this ichnofacies are well preserved and host the distal Cruziana Ichnofacies instead, and (2) the macrofaunal content (mainly brachiopods and trilobites) suggests normal-marine salinity and oxic bottom waters, which favor the development of the archetypal expression of this ichnofacies. The interplay of ecologic (e.g., type of substrate, sedimentation rate, energy conditions, oxygen levels, food availability) and taphonomic factors (e.g., duration of colonization window, tiering) is crucial to understand this anomaly. Softgrounds are more prone to record biogenic structures. Low oxygen is the crucial controlling factor for the Palaeophycus assemblage. Low-energy conditions, low sedimentation rate, and high food supply favored extended colonization rates permitting intense degree of bioturbation (typical of the Phycosiphon-Zoophycos Assemblage) and obliteration of shallow-tier trace fossils by mid- to deep-tier structures. Whereas higher influence of storms were associated to higher energy-conditions, coarser-grained sand and food suspended in the water column favored the development of the Rosselia Assemblage.

Description

Keywords

Paleoecology, Taphonomy

Citation

Degree

Master of Science (M.Sc.)

Department

Geological Sciences

Program

Geology

Part Of

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DOI

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