221-1 Redox evolution of the Rocas Verdes Basin: Testing for a Cretaceous Southern Ocean gateway and a proto-Drake Passage

Session: Phanerozoic Earth System Shifts in the Marine Sedimentary Record (Posters)

Poster Booth No.: 137

Presenting Author:

Griffin Clevenger

Authors:

Clevenger, Griffin¹, Malkowski, Matthew², Sperling, Erik A.³, Rey, Fernando M.⁴, Launderville, Emily May⁵, Crowley, James L.⁶, Pardo-Pérez, Judith⁷

(1) University of Texas at Austin, Austin, TX, USA, (2) University of Texas at Austin, Austin, TX, USA, (3) Stanford University, Stanford, CA, USA, (4) University of Texas at Austin, Austin, TX, USA, (5) University of Texas at Austin, Austin, TX, USA, (6) Boise State University, Boise, ID, USA, (7) Universidad de Magallanes, Punta Arenas, Chile,

Abstract:

Cretaceous paleogeographic reconstructions of the high-latitude Southern Hemisphere suggest that the Rocas Verdes Basin (RVB) may have acted as a proto–Drake Passage, permitting water exchange between the Pacific and the previously restricted, nascent South Atlantic Ocean. The RVB opened by back-arc extension during the latest Jurassic–Early Cretaceous in association with the breakup of the supercontinent Gondwana. Records of extension, compression, and eventual basin inversion are preserved in the hemipelagic mudstone succession of the Zapata Formation at the Tyndall Glacier locality in Torres del Paine National Park, Chile. This study aims to constrain the ventilation of the RVB and compare chemostratigraphic trends with existing South Atlantic Ocean drilling records to resolve the timing of South Atlantic oxygenation. Abundant intercalated ash layers within the Zapata Formation provide the basis for a U-Pb-constrained chronostratigraphic framework, complemented by detailed sedimentological interpretations. A 1200-meter stratigraphic section was measured to interpret the RVB's evolving depositional environment. New chemical abrasion – thermal ionization mass spectrometry (CA-TIMS) dates and Bayesian stratigraphic age modelling constrain deposition of the section to be between 150 and 111 Ma and resolve the timing of key facies transitions.  Mudstone samples were collected at 3 to 6 meter intervals and analyzed for their geochemical compositions to evaluate the redox evolution of bottom waters in the basin. Redox-sensitive trace metal and iron speciation data, combined with CA-TIMS U-Pb zircon geochronology, indicate that the early phase of the RVB development was characterized by a transition from euxinic to ferruginous bottom water conditions, with oxygenation occurring at ~143 Ma. The timing of apparent oxygenation at the Tyndall Glacier locality coincides with a transition from carbonaceous black shale to tan calcareous bioturbated mudstone. This facies change is coeval with the transition to an equivalent oxygenated facies at the Lago Argentino locality of the RVB, approximately 130 km to the north along-strike, suggesting relatively rapid and synchronous oxygenation of the basin.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10163

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