ABSTRACT Exceptional exposures of progradational platform margin slopes of Albian age were mapped along a 5-km, near-continuous, dip-oriented transect in northern Mexico with the goal of producing an outcrop model for analogous Cretaceous reservoirs. The study area at the El Cedral is located along the western margin of the Devils River grainstone belt flanking the muddy, intrashelf Maverick Basin. Four relatively complete clinothems and portions of two adjacent clinothems are recognized in the Las Pilas Formation in the study area. The Las Pilas clinothems exhibit complex sigmoid-oblique geometries and have dimensions of 40–60 m of vertical relief, depositional dip angles of foreset beds ranging from 8 to 15°, and dip-oriented lengths of 1–2 km. Internally, in foresets of clinothems, thick- to massively bedded, sigmoid- to oblique-shaped lenses exhibit overlapping, seaward-stepping patterns toward the southeast. The primary sediment fabrics of topset, foreset, and bottomset beds of the Las Pilas clinothems are grainstones and grain-rich packstones. Coated grains are the predominant grain type, with subordinate amounts of micritized mollusk fragments and intraclasts. Porosity is predominantly moldic, but primary interparticle porosity is common. These lime sands were winnowed, coated, and micritized along the high-energy topsets of clinoforms before being exported seaward toward the breakpoint, where they cascaded onto the steep frontal slopes as sporadic grain flows or mass flows. The master bounding surfaces that define the clinothems likely represent episodes of nondeposition and abandonment of the surface, perhaps occurring during shifts in the locus of deposition laterally along the shoal margin. The architecture and composition of the Las Pilas clinoforms provide observable data for comparison with analogous subsurface reservoirs in the Aptian Shuaiba Formation (Bu Hasa field) and Cenomanian Mishrif Formation (e.g., Umm Adalkh field) of the Arabian Gulf.

This study describes the geology of a well-exposed but previously unmapped section of Paleozoic–early Cenomanian metamorphic, sedimentary, and igneous rocks in the Frey Pedro study area of the Agalta Range of east-central Honduras. The objective of the study is to use these new structural, stratigraphic, biostratigraphic, and geochemical data to better constrain the geologic and tectonic history of this part of the Chortis block during the period of time from Aptian to early Cenomanian. The study revealed that the topographic Agalta Range exposes a thick stratigraphic section (3.5 km) deposited in an Albian-Aptian intra-arc rift and on the rift shoulders. This rift feature, named here the Agua Blanca rift, presently trends northwest and is parallel to three other belts of deformed Cretaceous rocks in Honduras (the Comayagua, Minas de Oro, and Montaña de la Flor belts) that also may correspond to Cretaceous intra-arc rifts produced during the same phase of intra-arc extension. These other three deformed belts are west of the Agalta Range and also form topographically elevated mountain ranges. Albian-Aptian calc-alkaline volcanic rocks and pyroclastic flows of the Manto Formation record arc affinity, while the volcaniclastic wedges of the Tayaco Formation record syn-rift deposition. These rift and arc-related units occupy the stratigraphic position between two major, extensive, shallow-water carbonate units, the lower and upper Atima Formations, also of middle Cretaceous age. Thickening of volcaniclastic rocks of the Tayaco Formation strata in the Agua Blanca rift accompanied erosion of the adjacent rift shoulders and eruption of Manto calc-alkaline volcanic rocks both within and adjacent to the Agua Blanca rift. The Agua Blanca intra-arc rift was inverted by a regional shortening event presumably in the Late Cretaceous. Rocks within the rift were intensely shortened, while rocks on the rift shoulders were shortened less because they are underlain by more competent metamorphic basement rocks. In order to better understand the Aptian–early Cenomanian tectonic setting for intra-arc rifting and subsequent rift inversion on the Chortis block, we reconstructed the Chortis block relative to the terranes studied by previous workers in southern Mexico. Five geologic and tectonic features were selected for realigning the two now widely separated areas: (1) areas of Precambrian basement outcrops, (2) areas of similar Mesozoic stratigraphy, (3) aligned trends of Mesozoic volcanic arc rocks exhibiting a similar arc geochemistry, (4) aligned trends of Late Cretaceous folds and thrusts, and (5) alignment of magnetic boundary.