Published Work

The Margaret Creek formation (Upper to Middle Triassic) is mainly a clastic red-bed unit, which is the lowermost among several Mesozoic formations comprising the Corozal basin of northern Belize. The Margaret Creek formation at the de facto ‘type locality’ near St. Margaret’s Village is a fluvial sedimentary deposit, which consists of numerous fining-upward packages of clastic sediments that represent arkosic braided stream channel sands and intercalated more finely grained, silty and sandy clays that are coeval fluvial overbank deposits. Detrital zircons from the Margaret Creek formation were extracted from samples taken from outcropping layers of fluvial channel sandstones in St. Margaret’s Village. Then, their age ranges were determined in an effort to better understand the provenance of these clastic sediments. Ages of detrital zircons from the Margaret Creek have a total range from approximately 300 to approximately 1600 Ma. There are three main peaks, one at about 414 Ma, another at about 1012 Ma, and the third at about 1483 Ma The younger peak probably relates to weathering of local Maya Mountain plutonic and volcanic rocks, but the older peaks suggest older, probably North American sources, which may include Grenville basement rocks. The apparent, older detrital zircon contributions apparently relate to provenance from zircon-bearing Grenville rocks during a time before the Yucatán block separated from the southern margin of North America.

Sedimentary Facies Analysis and Strontium-Isotope Stratigraphy of the Hillbank and Yalbac Formations, Corozal Basin, Belize
Karena K. Gill, David T. King, Jr., Haibo Zou, Fay Smith

September, 2018

The informal Hillbank and Yalbac formations are subsurface-only units of the Corozal basin of northern Belize. In stratigraphic order within Hillbank, are the following informal members—lower Hillbank dolostone (LHBD); lower Hillbank sandstone (LHBS); middle Hillbank dolostone (MHBD); and upper Hillbank sandstone (UHBS). The overlying Yalbac formation consists of three informal members, Y1–Y3. There are nine microfacies in the Hillbank-Yalbac stratigraphic section, including (1) shelf lagoon, (2) shallow subtidal, (3) supratidal, (4) sabkha, (5) transitional clastic-carbonate facies, (6) fluvial overbank, (7) fluvial channel, (8) fluvial and alluvial fan, and (9) alluvial fan. All these microfacies are present in the Hillbank, but the Yalbac consists of only the carbonate and evaporite microfacies (1–4, above). Strontium-isotopic ratio analysis of samples taken throughout most of the Hillbank-Yalbac section provides new, best-fit numerical age dates. Most of the stratigraphic section sampled for this analysis is now thought to be substantially different in age from previous work Strontium-isotopic ratios indicate that the Hillbank was deposited during Late Triassic to Early Jurassic, as was the lower few meters of the Yalbac’s Y3. A significant hiatus or stratigraphic break (i.e., from 190 to 133 million years ago) occurs in the stratigraphic succession near the base of informal member Y3, and the stratigraphic section lying above that stratigraphic break, namely the Yalbac formation (informal members Y3, Y2, and Y1).

Masters Thesis: Stratigraphy and Sedimentary Petrology of the Hillbank and Yalbac Formations, Corozal Basin, Belize
Karena K. Gill

April, 2017

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.