Subsea Cementation Preserves Columnar Bahamian Microbialites and Provides Foundations for Multiple Generations
The abundant giant columnar microbialites in tidal channels between the Exuma Islands are composed of ooid sand trapped and bound by cyanobacterial. Metazoans and benthic algal are largely excluded by twice daily strong tidal currents. These same currents also provide the repeated flushing of microbialites necessary for cementation. Syn-sedimentary fibrous aragonite cement precipitated within microbialites from oceanic water allows these columns to grow to two meters; it provides the hardground foundations for multiple generations of columns; and it solidifies the centimeter- thick petals that surround some columns like artichoke leaves.
The growth rate growth of aragonite from 14C dates of botryiodal growths and an eight month experiment range from 10-100 µ/yr (Grammer et al, 1999). This rate permits an estimate of the period required to completely fill pores of 500µ in an ooid sand as from 5 to 50 yrs. or fewer for point contact cement! Such rapid cementation of microbialites explains how they can grow in strong tidal currents and how fragile petals around microbiolites are preserved.
The same fibrous cement of microbialites occurs widely in oolitic sands on the eastern Great Bahama Bank to form extensive submarine hardgrounds and is present in the skeletal sands of living coral reefs. All these contemporary, extensive occurrences of syndepositional carbonate precipitation from seawater of near oceanic composition (reef cements, microbialites, submarine hardgrounds) need to be considered when inferring past levels of carbonate super-saturation
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009