SEDIMENTARY PETROLOGY AND STRONTIUM ISOTOPE STRATIGRAPHY WITHIN THE COROZAL BASIN, NORTHERN BELIZE

The informal Hillbank and Yalbac formations are found only in the subsurface of the Corozal basin, northern Belize. The Hillbank consists of 2 alternating sequences of genetically related facies: a shallow-water to intertidal marine sequence consisting of dolostones and evaporates and coastal plain sequence dominantly consisting of coarse to fine immature sandstones and siltstones. In the Hillbank, in stratigraphic order, are: a lower dolomitic facies package (Lower Hillbank dolostone or LHBD); a lower clastic facies package (Lower Hillbank sandstone or LHBS); a middle dolomitic facies package (Middle Hillbank dolostone or MHBD); and an upper clastic facies package (Upper Hillbank sandstone or UHBS). These packages are interpreted respectively as shallow marine facies (LHBD), coastal plain facies (LHBS), sabkha and shallow marine facies (MHBD), and coastal plain facies (UHBS). The Yalbac consists of 3 packages of genetically related facies, informal members Y1-Y3. The lower genetic package (Y3) consists of a basal transitional mixed siliclastic and dolomitic facies that grades upward into a dolomitic facies, which is interpreted as representing shallow marine, sabkha, and hypersaline environments. The middle genetic package (Y2) consists of thick-bedded dolomitic facies with rare anhydrite-rich layers, which are interpreted as being shallow restricted lagoon and tidal flat environments transitioning to sabkhas. The upper genetic package (Y1) consists of dolomitic and anhydritic facies, which are interpreted as shallow subtidal and sabkha environments as in Y2. Sr-isotope stratigraphy reveals age dates within the Hillbank and Yalbac. Sr-ratios indicating Late Triassic to Early Jurassic age dates for the Hillbank and the lower few meters of Y3. Then, there is a significant break (i.e., 190 - 133 m.y.) in the succession of interpreted ages from Sr ratios. This break specifically occurs between the lower few meters of the Y3 and the overlying main mass of the Y3. The discontinuity in Sr-ratios indicates a substantial and previously unknown stratigraphic break, a finding with profound implications for the history of the Corozal basin. The section above the stratigraphic break to the lower beds of the Barton Creek formation shows no similar breaks from Early to Late Cretaceous (i.e., 133 - 70 m.y.).