Late Jurassic deformation in Honduras. Proposals for a revised regional stratigraphy

Abstract

The lithostratigraphical subdivision of Mesozoic rocks in Honduras poses problems that are still imperfectly resolved. Observations made by the authors in Honduras between 1988 and 1992, show a clear separation between an older detrital unit, locally known as the Agua Fria Formation which is relatively homogeneous and can be found throughout the so-called Chortis Block, and a younger one composed essentially of cyclic detrital deposition (conglomerates and other molasse type rocks grading into pelites and silstones), usually called red beds (“capas rojas”). The older detrital unit, here called the Honduras Group, is an Early to Middle Jurassic folded and locally metamorphosed unit. The younger unit known as “capas rojas” starts with a molasse type rock which is not metamorphosed and presently have no generic name, and was deposited in extensional basins during a second sedimentary cycle of Early (and younger?) Cretaceous age. Between the two cycles a clear deformation phase took place in the late Jurassic. At La Chacra, near Comayagua in the centre of the country, outcrop conditions and exploration data enabled the observation of a composite section through the lithostratigraphic succession of the two Mesozoic cycles. The following succession was observed from bottom to top: • • Schist and conglomerate of the Agua Fria Formation, turned to a near-vertical position, which was intruded by granite and mineralized veins that are considered to have a late magmatic (Late Jurassic) age. • • A paleo-erosion surface over the Agua Fria Formation, shown by the presence of discordant weathering phenomena and a palaeosol. • • Detrital deposits (calcareous sandstone) and carbonate rocks of the Cretaceous cycle (molasse and red beds of the second cycle), which unconformably overlie the rocks of the first cycle. These observations confirm the existence, in Honduras and throughout the Chortis Block, of two distinct depositional cycles of Mesozoic age that were separated by a major crustal-deformation phase of Late Jurassic age. The same features are known from central Mexico, whence the Chortis Block seems to have come according to generally admitted crustal-block reconstructions of Central America. Early Cretaceous deposits in Honduras fill Cretaceous graben of the central zone. Although superficially resembling the (Triassic-Jurassic) Todos Santos Formation in Mexico and Guatemala, they cannot be compared with this unit because of their different age and origin, and will require better definition and a new name in Honduras.