DEPOSITIONAL ENVIRONMENTS OF PALEOCENE PLANT LOCALITIES WITHIN ESTUARINE FACIES OF THE SALAMANCA FORMATION, CHUBUT PROVINCE, ARGENTINA

Open Access
Author:
Comer, Emily Elizabeth
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 08, 2011
Committee Members:
  • Rudy Slingerland, Thesis Advisor
Keywords:
  • sedimentology
  • depositional environments
  • paleobotany
Abstract:
The most well preserved and diverse early Paleocene macrofloras from Gondwana occur within sediments from Chubut Province, Patagonia, Argentina. Two prolific sites of fossil preservation, Palacio de los Loros and Parque Provincial Ormachea, sit near the top of the southwestern outcrops of the Danian (65.5-61.7 Ma) Salamanca Formation. Resting just above an unconformity spanning the K-T boundary, the Salamanca Fm. provides evidence of plant recovery after the extinction event and the depositional environments in which these floras existed. This thesis provides a sedimentological description and paleoenvironmental interpretation of the Salamanca and Banco Negro Inferior Fms. in their southwestern outcrops and places macroflora fossil localities in stratigraphic context. The Salamanca Fm. in the area 25-40 km south of Sarmiento, Argentina consists of ten facies and four facies associations representing a transgression onto continental Cretaceous facies followed by regression and infilling of marine, tidal facies. Lower sections of the Salamanca Fm. contain abundant glauconite and fossils characteristic of an open marine shelf environment. These facies transition upward to bi-directional trough cross-bedded sands interspersed with flaser-bedded sandy silts. Cross beds indicate meso-tidal currents with paleocurrent directions that cluster in the NW-SW quadrants. The uppermost unit of the Salamanca Fm. is composed of sands and silts that gradationally transition to dark muds of the Banco Negro Inferior (BNI) Fm. These silts and muds are interpreted as a series of stacked gleysols and vertisols consisting of montmorillonite, illite, glauconitic, and halloysite clay phases deposited through progradation of a widespread inland swamp. High aluminum content is the cause of dark coloration within the BNI Fm. Plant localities at PL and OR occur at two stratigraphic levels within the upper Salamanca Fm. and represent accumulation in five types of sedimentary deposits- silty beds of accretion sets, transported silt lenses, mud drapes, tidal flats, and muddy deposits along low-energy tidal channels. These sedimentary deposits signify two stages of estuary infilling and the progradation of a widespread, coastal swamp represented by the Banco Negro Inferior Fm. Two-dimensional hydrodynamic modeling of tidal currents and ranges in the proto-San Jorge embayment produce tidal amplitudes of 1-3.5 m, consistent with the hypothesis that the Salamanca Fm. was deposited in an open estuary that amplified Paleocene microtidal Atlantic tides to meso-tidal range.