Fracture Development in the Interior of a Stable Carbonate Platform: New Evidence From the Distribution of Karst Features on Andros Island, Great Bahama Bank
The Bahamas platform is widely recognised as the most stable continental block in the Caribbean. It is underlain by a series of normal and wench faults, which are inherited from an upper Jurassic rifting phase but presumed inactive since. These deep faults appear to control the geometry of the steep platform margins and deep-water re-entrants. At the surface major fracture systems, extending up 40 km, occur parallel to the platform margins and show diagenetic alteration by circulating groundwater. Similar systems are described in ancient platform margins in the Canning Basin and Guadalupe Mountains, and in the subsurface, for example in Tengiz where they are a critical control on flow. Here we present a statistical analysis of the distribution of large scale karst features distributed across Andros island, that provides the first evidence for extensive fracturing up to 35 km from the platform margin.
Some 177 circular collapses (“cenotes”) were identified from satellite imagery. At the surface these features have a diameter of 105±60 m, with the largest spanning 370 m. Field survey shows they have a maximum depth of c.110 m, with floors of collapse boulders and soft sediment suggesting active autochthonous infill. Underwater exploration of a subset of cenotes suggest they tend to increase in diameter with depth, possibly due to the effects of mixing-zone dissolution and associated collapse. Only a few of the smaller cenotes are known to connect with lateral cave passage.
Perpendicular regression reveals the distribution of cenotes is significantly non-random, and identifies many lineations, defined by rows of up to 6 cenotes. The distribution suggests that cenote formation may be focussed at the intersections of lineations. Most lineations within 10 km of the eastern bank margin are oriented parallel to this margin, with a subset perpendicular. However, further inland, the orientation of many lineations echoes that of the N60oW trending Sunniland (Bahamas) Fracture Zone, a long-lived transform feature related to Jurassic rifting that extends across the Floridian Peninsula and crosses the Great Bahama Bank to the north of Andros Island. The vast majority (98%) of identified cenotes occur within 35 km of the western bank margin. The absence of cenotes at greater distances from the margin may reflect increased infill by sediment generated on the shallow modern bank, or alternatively may be controlled by the location of margin of the underlying Cretaceous Andros Bank.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019