Stratigraphy and Sedimentary Petrology of the Hillbank and Yalbac Formations, Corozal Basin, Belize

Abstract

The informal Hillbank and Yalbac formations are predominantly carbonate units that are found only in the subsurface of the Corozal basin in northern Belize. The 75 to 700 m-thick Hillbank and the 200 to 3000 m-thick Yalbac are economically important stratigraphic units. The Hillbank consists of two alternating sequences of genetically related facies: a carbonate sequence dominantly consisting of dolostone and a fluvial sequence dominantly consisting of coarse, clean sandstone. In stratigraphic order, there 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 as shallow marine facies (LHBD), fluvial and alluvial plain facies (LHBS), sabkha and shallow marine facies (MHBD), and fluvial and alluvial plain facies (UHBS). The Yalbac formation was deposited conformably above the Hillbank in a continuation of marine sedimentation. The Yalbac consists of three packages of genetically related facies, members Y1-Y3. The lower genetic package (Y3) consists of a basal transitional mixed siliclastic and dolomitic facies that grades 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 that are interpreted as being shallow restricted lagoon and tidal flat environments transitioning to evaporites on sabkhas. The upper genetic package (Y1) was not studied petrographically in the present research due to a lack of available samples, but historic accounts report a dolomitic and anhydritic facies, consistent with Y2, which is interpreted as shallow subtidal and sabkha environments. Sr-isotope stratigraphy reveals an interpreted set of age dates for selected intervals within the Hillbank and Yalbac. Sr-values from these Corozal basin units indicate Upper Triassic to Lower Jurassic for the Hillbank and the lower beds of the Y3. There is a significant break in the Sr values between the lower beds and upper part of the Y3 member, specifically spanning 133-190 m.y. This indicates a substantial and previously unknown stratigraphic break within this member. The span from the base of the Barton Creek formation to the upper part of the Y3 member of the Yalbac displays sequential Sr-ages determinations suggesting that this rock section spans Lower to Upper Cretaceous. Comparison with the global sea-level cycle chart indicates that some Mesozoic sea-level drops may be related to this substantial unconformity and other breaks in the Hillbank-Yalbac section.