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Title: Catastrophic sedimentation at the K-Pg Boundary in the continental slope of Baja California, Mexico
Author: Santa Catharina, Amanda
ISNI:       0000 0004 9358 5510
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2020
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Controversies remain regarding the trigger, or triggers, of the Cretaceous/Paleogene Mass Extinction. The Chicxulub impact and the Deccan volcanism are the main candidates, but discussions about the timing of these events and the magnitude of their effects on the biota are ongoing. Profound alterations in the biosphere occurred at this interval, and locations in the northeastern Atlantic margin and the Gulf of Mexico show evidence of tsunamis and mass wasting deposits directly associated with the Chicxulub Impact. Close to El Rosario, Baja California, Mexico, a previously undescribed stratigraphic succession occurs that herein is interpreted to have been deposited during the Cretaceous-Paleogene event. This succession is distinct from the normal submarine slope deposits in this region (hemipelagic mudstones and slope channel deposits such as coarse- to fine-grained turbidites), and consists of six units: (1) 6 m thick muddy debrites, rich in terrestrial material including fossilized tree trunks up to 3 m long and with evidence of exposure to forest fires; (2) a pebbly mudstone up to 5 m thick, with very few, reworked foraminifera and soft-sediment deformation structures; (3) a metre-thick pebbly mudstone rich in gastropods, bivalves and fragments of corals; (4) a second interval of pebbly mudstone with soft-sediment deformation structures, that reaches up to 6 m; (5) a coarse grained lapilli crystal vitric tuff, up to 20 m thick, dacitic in composition, with lapilli (ranging from 1 to 30 cm in size) in discrete sets, within a channel-like geometry that cuts into the underlying debrites; and (6) pebbly mudstone up to 18 m thick, barren from foraminifera and palynomorphs. This succession rests with an abrupt basal contact on Upper Maastrichtian hemipelagic mudstones, and is also capped by hemipelagic mudstones. These overlying hemipelagites have Danian fauna and flora, which appear after the markedly barren unit 6. Units 1, 2, 3 and 4 likely represent material transported onto the slope by the seismic activity and/or tsunami caused by the Chicxulub impact, which destabilised the coastal region. The debrites are composed mainly of terrestrial and shallow water material, and most of the hyaline benthic foraminifera from this interval show complete decalcification. Higher amounts of sulphides were identified with SEM-EDS mapping in the debritic units, compared to the Upper Maastrichtian hemipelagic mudstones, suggesting that the decalcification of the foraminifera could have been caused by the acidification of the superficial ocean water related to the K-Pg event. The charred fossil trees, mainly Pinaceae, Cupressaceae and some angiosperms, typical of coastal environments, are interpreted to have suffered from forest fires triggered by the impact, and the absence of abrasion or bioturbation (such as Teredolites) of the wood suggests rapid burial and very little time in the air/water or water/sediment interfaces. Their presence within deepwater deposits can be explained by being carried by debris flow whose yield strength was high enough to overcome any buoyancy, and stop them from floating back up in the water column until saturation occurred. The channelized tuff unit has been dated 66.12 +/- 0.65 Ma (SHRIMP U-Pb in zircons) and is indistinguishable from the proposed ages for the K-Pg boundary. The closest volcanic activity of this age reported in the surrounding areas is in Sierra Madre Occidental, which an optimistic palaeogeographic reconstruction puts at around 300 km. Granitic rocks with K-Ar cooling ages of from 91 to 30 Ma, cut by basaltic to dacitic dykes, c. 100 km to the E provide a closer potential source. This proximal source for the tuff is more likely, given the lack of abrasion of the fragile pyroclastic elements. The age of the tuff indicates its correlated eruption was possibly triggered by the impact.
Supervisor: Kneller, Benjamin C. ; Macdonald, David I. M. Sponsor: Shell Brasil ; ANP (Science Without Borders Programme)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Sedimentation and deposition ; Cretaceous-Paleogene Extinction