Subsurface biogeology of the Alacranes Reef, Yucatán

Abstract

A complementary analysis of previous studies on the Alacranes reef complex has been conducted to determine sedimentary environments in terms of physiographic elements and their inhabiting biotic communities. Based on the established successional sequences across different physiographic elements, a provisional conclusion has been reached: partial successions converge into four climax communities as follows: Reef front and shallow waters of the windward slope: Acropora palmata association. Front and shallow waters of the flanks in protected reefs of the upper leeward slope: Acropora cervicornis association. Lagoon floors and lower windward and leeward slopes: massive coral and gorgonian association. These three climax communities occupy biotopes characterized by decreasing kinetic energy levels in the order listed. Another climax community, the Thalassia seagrass bed, occupies extremely shallow protected waters.

Surface bottom sediments, represented by samples collected for this purpose, were studied through grain-size and constituent analyses. The results, along with those previously published by Hoskin, Cann and Folk, and Robles, were utilized to establish criteria for paleoecological reconstructions based on materials from an exploratory well on Isla Pérez. Broadly speaking, grain-size data distinguish very shallow bottoms (reef crests) from lagoonal and Campeche Bank (Sonda) bottoms. Constituent analyses, in turn, differentiate lagoonal bottoms from Campeche Bank bottoms.

The grain-size and constituent study of subsoil samples from the drilling revealed that the gravel fraction is inadequately represented in modern bottom samples, leading to a systematic error in sample composition, as different organisms dominate coarse gravels compared to sandy fractions. Furthermore, the taxonomic composition of gravel-forming organisms allows for the differentiation between protected and exposed reef communities—data used for paleoecological reconstruction. Future work should develop more effective field methods for gravel sampling.

An unexpected result in comparing modern bottom grain size with subsoil materials is the marked multimodality of the latter, which hindered an adequate comparative interpretation of their parameters, as the geological significance of distribution parameters that deviate significantly from the normal probability curve is highly doubtful.

The exploratory drilling traversed a sedimentary column divisible into the following sections: From sea level to 33.5 m deep, loose, unlithified sediments essentially identical to modern reef sediments were obtained, consisting of silt, sand, loose calcareous gravel, and coral heads. Overall, they comprise nearly 80% coral fragments, with the remainder being Halimeda remains, mollusks, and other organisms. These are interpreted as modern reef deposits, specifically as marginal or protected reef deposits at the drilling site.

The second section, from 33.5 m to 60.4 m deep, comprises sediments of the same composition as the above, also interpreted as forming in a protected reef or its vicinity. However, they differ by appearing in well-lithified limestone layers with signs of redissolution, interbedded with loose sediments, indicating a stage of subaerial exposure.

The third section consists of calcareous sediments with mollusk remains and other organisms, but corals are virtually absent. Here too, well-lithified limestone layers with signs of redissolution alternate with loose sediments; this portion is interpreted as the top of the sediments constituting the Campeche Bank (Sonda de Campeche).

Comparative morphology between Alacranes Reef and other reef complexes of the Campeche Bank and the Veracruz Gulf coast suggests that Alacranes formed through the lateral growth of a reef that eventually merged with others in the same complex, enclosing a portion of the inter-reef area to form the current lagoon. The lagoon tends to disappear through a filling process caused by the proliferation of elementary protected reefs, which tend to convert the complex into a table reef.

Chemical and X-ray diffraction analyses have shown that dolomite and dolomitic limestones are completely absent in both the modern reef and subsoil materials. The aragonite-calcite ratio varies with the nature of the constituent organisms but not with the age of the sediments; in other words, no appreciable aragonite-calcite inversion has occurred.

Estimates of the global composition of modern coral reefs based on the percentages of organism remains in sandy fractions lead to figures far from reality, as the global composition is dominated by gravels and blocks formed almost exclusively by corals. The estimate for Alacranes Reef, accounting for gravels, yields a total coral percentage higher than other constituents, whereas in sandy fractions alone, Halimeda dominates over the rest.

No oolites or oolitic limestones were found in the lithified limestones of the ancient reef or the Bank deposits. Limestones with sparry calcite mosaics (sparites) are also absent. These lithic elements do not exist in the modern or recent reef but are present as relict elements in current Campeche Bank bottoms.

In the drilled section of the ancient reef and Bank deposits, loose sediments are preserved between well-lithified limestone layers, containing calcareous nodules considered the result of incipient lithification. These loose sediments and their nodules are very similar to the chalky deposits locally called "sashcab," common in the Carrillo Puerto and Bacalar Formations outcropping in northern and eastern Yucatán. Possibly, some of the lithoclasts studied by Harding are actually nodules like those mentioned, in which case they would be intraclasts rather than lithoclasts.

Alacranes Reef, like others on the Bank, sits on a terrace between the 28 and 35 fathom isobaths, assumed to have been carved during an eustatic sea-level drop at the end of the Tazewell (Wisconsin) or the beginning of the Holocene transgression (11,000 years b.p.). The portion called the "ancient reef" would have grown as this transgression progressed until reaching the level of another terrace at the current 18-fathom isobath. This, in turn, would have formed by a temporary stabilization of the coastline or even a regression that momentarily exposed the reef (8,000 years b.p.). Upon the resumption of the transgression, the "modern reef"—represented in the first portion of the drilled thickness—would have continued building over the "ancient reef" until reaching the current level (5,000 years b.p.).

Based on certain assumptions, it is estimated that the "ancient reef" grew over a period of 3,000 years, and the recent reef took approximately as long to reach its current size. These figures presuppose a vertical growth rate consistent with those obtained by Hoffmeister and Multer in Florida. Some of the assumptions stated in the final paragraphs may be confirmed or discarded once aragonite-calcite petrographic determinations and absolute age dating by the carbon-14 method are completed.

Abstract from:

Gómez-Caballero, J. A. (2005). Historia e índice comentado del Boletín del Instituto de Geología de la UNAM. Boletín de la Sociedad Geológica Mexicana: Volumen Conmemorativo del Centenario Aspectos históricos de la Geología Mexicana, 57I(2), 149-185. http://dx.doi.org/10.18268/BSGM2005v57n2a3