Paleogene Carbonate Dissolution Events and Their Link With Wilcox Formation Deep Water Sands

Abstract

A series of carbon isotope excursions (CIEs) mark variations in the global carbon cycle and significant changes in climate through the early Paleogene. The Paleocene Eocene Thermal Maximum (PETM) ca. 56 Ma is the most pronounced and well documented of these events, having been described in many sections across the globe. The large CIE across the PETM is marked by a clay rich layer in many deep sea sections, resulting from widespread carbonate dissolution on the seafloor, which is in turn related to shoaling of the carbonate compensation depth and lysocline. Several studies have suggested that other CIEs have a similar response, but this information is sparse. There is also debate as to how carbonate saturation in the ocean varied over the long term. Importantly, we observe that though the Eocene climate shows an overall cooling trend, yet it is punctuated by several short term hyperthermals which have been studied sparsely. Here we document changes in carbonate preservation in Eocene sediments at ODP Site 1209A (Shatsky Rise, North Pacific Ocean). This site presently lies in a middle bathyal (2387m) water depth but has undergone negligible subsidence since the Cretaceous. We generate a record of carbonate content, and couple this with magnetic susceptibility (MS) and gamma ray attenuation (GRA) records to locate possible intervals of reduced carbonate accumulation. We then examine the planktonic foraminiferal assemblages for their fragmentation which helps us to better constrain the dynamics of lysoclinal depth at this location of North Pacific Ocean in the Eocene. Throughout the Eocene there are long term variations in carbonate preservation, as well as some pronounced events of carbonate dissolution. Such documentation allows us to better constrain models for early Paleogene carbon cycling. This work will then be used to find a relationship between the hyperthermals and formation of the Wilcox Formation. How deep marine sediment deposition is perturbed in a greenhouse world of Eocene and leads to formation of high reservoir quality sands (as in the Wilcox) is the main theme of the current work.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017