Identification of OAE1b in a mid-Cretaceous carbonate platform (Aptian-Albian), southeastern Arizona: a chemostratigraphic and sedimentological approach




Helfrich-Dennis, Morgan M.

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Mesozoic climate change has been extensively studied in the Tethys. The purpose of this research is to bridge this part of the world to its counterpart in the proto-Atlantic, specifically for the time period straddling the Aptian-Albian boundary. This comparison is performed by correlating a sedimentary succession sampled in southern Arizona (Mule Mountains) to previous studies conducted in Europe. I first compared the evolution of the carbon stable isotopic composition (δ13C) to construct a robust stratigraphic framework, in which climate changes and associated perturbations of the carbon cycles are expected to be identified. I hypothesize that during the Aptian-Albian time period, significant climatic events correlate between the proto-Atlantic and the Tethys, and that the Oceanic Anoxic Event 1b, which corresponds to a period of marine anoxia straddling the Aptian-Albian boundary, impacted the shallow-marine carbonate factory now preserved in southern Arizona. To test this hypothesis, 103 m of continuous outcrop were sampled in the Mule Mountains. The stratigraphic section is described using Dunham's classification for carbonate rocks applied and encompasses the Aptian-Albian boundary (identified at 5 m). Significant macro- and microscopic fossil assemblages are identified, both for their biostratigraphic and paleoenvironmental meaning. In particular, the nature of allochems and the depositional texture is used as a proxy for past sea level. The chronology of diagenetic events that affected the successions is assessed using cathodoluminescence microscopy. δ 13C and δ18O are also be used for correlation purpose as well as proxies for paleoclimate. The main output of this study is that the chemostratigraphic correlation of the Bisbee record with the Vocontian basin of southeastern France, which is biostratigraphically constrained, indicates that this part of the Mural Fm was deposited during the Late Aptian - Early Albian, more precisely during Hypacanthoplites jacobi and Leymeriella tardefurcata ammonite zones, and thus include the OAE1b. The latter is identified by its unique carbon isotope signature. I can thus conclude that this event affected locations on both sides of the proto-Atlantic.


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