THE UPPER CENOZOIC STRATIGRAPHIC RECORD FROM MAYAGUANA ISLAND, SE BAHAMAS : A PRODUCT OF TECTONIC ACTION, EPISODIC FLOODING, AND ANTECEDENT TOPOGRAPHY

The peculiar occurrence of Miocene, Pliocene, and Lower Quaternary rock units along the north coast of Mayaguana suggests that this bank stood in a high position during the Neogene, that it was only flooded by major transgressions during this time interval, and that is was tilted towards the South or affected by a pre-Miocene topography.

The geology of Mayaguana, a small island in the SE Bahamas, was investigated using a multi-method approach including stratigraphic logging, sedimentological and petrographic analyses, foraminiferal biostratigraphy and Sr-isotope dating.

The north coast of Mayaguana reveals Upper Cenozoic rock bodies that are not exposed on any other Bahamian islands. These include reefal, lagoonal and beach deposits of Early Pleistocene age (ca. 1.1 Ma BP, average ⁸⁷Sr/⁸⁶Sr ratio = 0.709131), a partly dolomitized coralgal framestone dating from the Pliocene (ca. 2.6 Ma BP, average ⁸⁷Sr/⁸⁶Sr ratio = 0.709067), crossbedded microsucrosic dolostone of Messinian age (ca. 6 Ma BP, average ⁸⁷Sr/⁸⁶Sr ratio = 0.708988) and bioclastic packstone dated by both biostratigraphy (miogypsinids) and Sr-isotopes from the Burdigalian (ca. 18.5 Ma BP, average ⁸⁷Sr/⁸⁶Sr ratio = 0.708546). Subtle karstic surfaces, representing hiatuses up to 12 Ma long, separate these stratigraphic units from one another.

This peculiar stratigraphic record is related to the tectonic setting of Mayaguana (proximity to a plate boundary). Tectonic action maintained the bank at a high elevation during Neogene and it was only flooded during the highest sea-level stands of the past 20 Ma (Burdigalian, Messinian, mid-Pliocene and Early Pleistocene). Between these episodes, the bank top was emergent and subjected to dissolution. The value of this elevation cannot be constrained due to large uncertainties related to Neogene eustasy (e.g. between +20 and +150 m for the Burdigalian sea level). The asymetrical distribution of these old rocks can be explained either by a southward tilt of the bank or by an antecedent (pre-Miocene) topography which appears to be corroborated by recent coring in the southern part of Mayaguana.