Paleo-Reconstruction of the Coban Formation at Los Chorros, Central Guatemala

The Cretaceous Period (66-145 Ma) is recognized by episodes of extreme climates, associated with global greenhouse conditions that favored widespread production of shallow marine carbonate platforms. Despite their sensitivity to the quality and chemistry of seawater as well as oceanographic conditions, carbonate platforms can thrive for extended periods of time, making them a valuable record of paleoceanographic and paleoclimatic changes.

The Cobàn Formation is a thick, shallow-marine succession deposited from the Aptian to Santonian over Guatemala. In outcrop, it is divided into the lower dolomitic and upper limestone units. Three platform drownings punctuate the development of this platform from the Aptian to Cenomanian, and are represented by more basinal, anoxic deposits attributed to a relative sea level rise. Because of the lack of a robust biostratigraphic framework, the dating of its sediments and subordinated drowning facies is not well constrained.

This study focuses on a 100 m section belonging to the upper limestone unit of the Cobàn Formation. Exposure is due to a major landslide that occurred on 4 January, 2009, detaching more than 5 million cubic meters of Cretaceous limestone. The aim of this study is to constrain the age of the section through biostratigraphy evolution and chemostratigraphy.

Based on the occurrence of the miliolid Nummoloculina hemi, the section is constrained to Albian-Cenomanian. Additionally, chemostratigraphic correlation further constrained the age to middle Albian-lower Cenomanian. Petrographic analysis suggests that sediments at the measured section were deposited in a restricted platform environment with an early flooding event witnessed by an organic mudstone facies at the base of the section. Bulk rock geochemical data suggest that this mudstone facies was deposited during times of bottom water dysoxia during the early stages of a sea level rise.