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ABSTRACT

Geologic mapping on the Chortís block has defined two major Mesozoic units which are older than the well-known, lower Cretaceous Yojoa Group. The two units have been considered part of the same stratigraphic element, the Honduras Group. The Agua Fría Formation, a dominantly clastic mixed continental-marine unit, has been dated as Middle Jurassic based on recent discoveries of ammonites and plant fossils. Dates as old as Rhaetian for similar rocks have been reported in the literature, but have not been confirmed. No unfaulted contact between the Agua Fría Formation and the Yojoa Group has been found. The Yojoa Group is commonly underlain by a conglomeratic unit, the unnamed siliciclastic member of the Honduras Group, that is lithologically distinct from the Agua Fría Formation. Available palynological data indicate that the unnamed siliciclastic member is restricted to the lower Cretaceous. The Honduras Group as currently defined is an inappropriate stratigraphic term. The two units of the group are not contiguous and have significant differences in lithology, and, especially, in age. Inasmuch as its usage is causing stratigraphic confusion. I recommend abandoning the term Honduras Group. The Agua Fría Formation is left as an independent stratigraphic element. The unnamed siliciclastic member is herein named “the Tepemechín Formation” after a river south of Lake Yojoa.

Although thicknesses as great as 1200m have been reported for the Agua Fría Formation, it is limited to rather narrow basins probably of strike-slip origin. By contrast, the Tepemechín Formation is widespread yet thinner and was deposited immediately prior to the deposition of the Yojoa Group limestones. Sedimentologically, the formation is similar to known rift deposits. Rifting probably occurred in the earliest Cretaceous and was followed by subsidence represented by the limestone. Thus, the Chortís block experienced strike-slip faulting in the Middle Jurassic followed by rifting throughout the block in the Early Cretaceous whereas neighboring regions experienced “pure” rifting from the Middle Jurassic to the Cretaceous as evidenced by thick deposits of continental redbeds.

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