Sedimentology of the Toledo Formation, Belize Basin, Central America

Abstract

The Toledo formation represents a profound shift from carbonate-dominated to clastic-dominated deposition within the Belize basin following the Late Cretaceous collision of the Maya and Chortis blocks (of the North American and Caribbean plates). The present study characterizes the lithological and petrographic makeup, depositional environments and provenance of the Toledo formation. The sedimentological characteristics can be described according to submarine-fan facies model put forward by Mutti and Ricci-Lucchi (their Facies A-G). Facies characteristics and associations indicate outer fan/depositional lobe, mid-fan, submarine canyon, and shelf depositional environments encompassing channel, depositional lobe, and sheet architectural elements. Petrographic analysis of the shelf-derived carbonate samples from the Toledo formation show a dominance of shallow-water benthics. These units are interpreted as slope deposits based on models described by Schlager. Sandstones are the dominant lithology and are identified as immature lithic arenites based on petrographic analysis. Plots of detrital modes of the sandstones on ternary diagrams identify a recycled orogen provenance related to the collision of the North American and Caribbean tectonic plates. The Toledo formation has demonstrated economic potential for hydrocarbon and groundwater resources. Petrographic results suggest that the formation is best suited as a seal for the Cretaceous hydrocarbon resources, and for groundwater resources. Preliminary geochemical analysis revealed that the total organic-carbon (TOC) content of the Toledo formation rocks range from 0.2 to 1.86 wt% with an average of TOC value of 0.85 wt% indicating fair to poor source-rock potential. These contain mostly type IV with minor type III kerogen indicating terrigenous organic matter derived from woody plants, with little to no hydrocarbon-generating potential. Based on Tmax (430 to 439 °C) and PI values, the studied samples are immature to early mature in terms of hydrocarbon generation.