STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE HILLBANK AND YALBAC FORMATIONS, COROZAL BASIN, NORTHERN BELIZE

The informal Lower Cretaceous Hillbank and Yalbac formations are predominantly carbonate units that are found only in the subsurface of the Corozal basin. The 75 to 100-m thick Hillbank and the 300 to 2700-m thick Yalbac are conformable stratigraphic units, which are situated together between the Jurassic-Cretaceous unconformity and the ‘mid-Cretaceous’ unconformity. The Hillbank consists of three packages of genetically related facies (referred to in unpublished reports as HB1-HB3). In stratigraphic order, they are a lower dolomitic and anhydritic facies package (HB3), a clastic facies package (HB2), and a dolomitic facies package (HB1). These packages are interpreted as sabkha and shallow marine facies (H3), fluvial and alluvial plain facies (HB2), and shallow marine facies (HB1). The Yalbac consists of three packages of genetically related facies, which have been referred to in unpublished literature as members Y1-Y3. The lower genetic package (Y3) consists of dolomitic and anhydritic facies, which are interpreted as representing sabkha, supratidal, intertidal, and shallow subtidal environments. The middle genetic package (Y2) consists of thick-bedded dolomitic facies that are interpreted as being shallow subtidal and reefal to near-reefal deposits. The upper genetic package (Y1) consists of dolomitic and anhydritic facies, which are interpreted as representing sabkha, supratidal, intertidal, and shallow subtidal environments. Deposition of the Hillbank commences with a minor transgression and shallow marine sedimentation, spans a regressive event during which terrestrial facies prograde over marine, and ends with a new transgressive phase of shallow marine sedimentation. Yalbac deposition consists of very shallow carbonate sedimentation (Y3), followed by a deepening phase of carbonate sedimentation and reefal development (Y2), and a terminal shallowing phase (Y1).