Nature, age, and origin of the informal Lower Cenozoic Red Bank group, northern Belize

Abstract

The informal Lower Cenozoic Red Bank group of Belize crops out in several areas of the northern part of the country and is comprised mainly of clay with relatively minor amounts of carbonate and rare evaporite beds. The Red Bank group is of geologic importance because of its relationship to petroleum exploration, specifically the presence of this clay unit causes processing problems in seismic exploration for potential oil accumulations in underlying strata. Previous literature about the geologic nature and origin of the Red Bank group is limited due to earlier proprietary restrictions on publications. It was previously assumed that this stratigraphic unit was a relatively thin, widespread clay deposit across most of northern Belize, but recent research indicates otherwise. Thus, the objectives of this study were to better understand the dimensions of the Red Bank group and its nature and origin. These objectives were achieved using petrographic and cartographic methods. Results indicate that the Red Bank group is much thicker than previously known, is of a different age than assumed before, and has a significantly different depositional environment than was previously suggested. In conclusion, we now know that the Red Bank group is a more complex stratigraphic unit, which has a depositional history linked to global events such as sea-level high-stand. Subsurface correlation of well log shows that the Red Bank group ranges in thickness from zero to 1800 ft (549 m), as determined from well logs taken in and around the Spanish Lookout and Never Delay oil fields. Further, an analysis of well logs in the study area of northern Belize, showed that the Red Bank has an unconformable relationship with underlying formations, particularly the informal Upper Cretaceous Barton Creek formation. Correlations of the Red Bank also indicate that the nature of that basal stratigraphic contact is karstic and the Red Bank clay also fills caverns within the Barton Creek. Further, surficial studies of outcrop areas indicate that the Red Bank group itself occurs mainly in a north-west trending, fault-bounded karstic plain (a low-lying area during deposition), and occurs also within thinner, clay-filled Red Bank channel deposits atop the karstic limestone. Based on stratigraphic analysis of Red Bank nannofossils from outcrops and drill cuttings, the present study identified the chronostratigraphic position of the group as late Paleocene to early Eocene, not Miocene as earlier reports suggested. Late Paleocene to early Eocene period was a time interval of several notable global events, including the PETM (Paleocene-Eocene Thermal Maximum) global climatic event and significant high and low sea-level stands. Potential evidence of the PETM can be seen in the Red Bank group as evaporite-rich beds within the clay. Whereas, low sea-level stand likely was related to karst development in the underlying limestone; high sea-level stand likely was related to transgressive sedimentation of the Red Bank clays, which is interpreted here to have occurred within an estuarine setting, based on the Red Bank’s clay mineralogy and microfossil content. Information gleaned from the present study of the Red Bank group will help in future petroleum exploration activity in northern Belize because the dimensions and characteristics of the Red Bank group can be better accommodated in seismic processing and modeling of that area. Future work in this area would be to more thoroughly map the Red Bank group in outcrop and correlate it more widely in the subsurface.