ABSTRACT

Extensive coastal lagoons border the narrow, high-energy shelf of NE Yucatan. The lagoons contain up to 11.5 m of fine-grained carbonate sediments, which record lagoonal development during the last 6000 years of sea level rise. Comparison of the larger lagoons of Nichupte and Blanca reveals that, although they are in close proximity to one another, their sedimentology and depositional histories are different.

Semi-restricted conditions prevailed in the early stages of both Nichupte and Blanca lagoons. Nichupte Lagoon developed in the lee of Pleistocene eolian dune remnants and became nearly isolated from open-marine conditions with subsequent deposition of Holocene tombolos and eolian sands. In contrast Blanca Lagoon formed behind a large Holocene spit and remained open to normal marine waters.

Aragonitic sediments containing mollusks, coralline algae, Porites, and Halimeda accumulated in Nichupte Lagoon during its early history. An abrupt change to Mg-calcite rich mud and an assemblage characterized by foraminifers, mollusks, and ostracods occurred 1000-1500 y.b.p. when the lagoon was essentially cut off from open-marine circulation. Most of the sediments of Nichupte have been produced in situ by indigenous biota. Mg calcite of the near-surface sediment is mainly derived from breakdown of Melobesia which heavily encrusts Thalassia in the lagoon. Biochemically precipitated calcitic mud occurs in the fresh-water marsh landward of Nichupte.

1 First printed 1974

This paper is based on an unpublished Ph.D. dissertation done at Rice University under the supervision of Dr. J. L. Wilson.

End_Page 85-------------------------

The aragonitic muds of Blanca Lagoon are composed principally of codiacean algal needles. The younger muds must be transported into the lagoon around the barrier-spit, because the lagoon is presently devoid of codiacean algae. Sediment from the open-marine shelf is continuously carried into the lagoon around the end of the spit, and additional shelf material is supplied by washovers during occasional storms.

Comparison of the sedimentology and of depositional histories of Nichupte and Blanca lagoons shows that shallow-water carbonate mud can form in a variety of settings, even in small geographic areas. Care should be taken when interpreting ancient micrites to avoid overgeneralized interpretations, which may lead to erroneous conclusions.