Physical Properties of Carbonate Turbidite Sequences Surrounding the Bahamas—Implications for Slope Stability and Fluid Movements

Abstract

Along the slopes and in adjacent basinal areas of Bahamian platforms, periplatform ooze forms the host sediment for variable proportions of carbonate turbidites. In unlithified sections, these turbidites appear as unconsolidated layers intercalated with stiffer ooze. Within an individual turbidite, differences in grain size result in variations in consolidation and physical properties. With decreasing grain size, water content and porosity decrease, and two distinct surfaces develop at the lower and upper boundaries of the turbidite. These surfaces are potential instability horizons where mass-wasting can occur. Therefore, a relation between turbidites and slumping frequency is proposed. The higher proportion of turbidites in sediments deposited on low-angle, accretionary terrains, such as the toe of the northern slope of Little Bahama Bank, probably facilitates frequent, small-scale slumping and creeping, as seen in seismic profiles. In contrast, slumping is less frequent along the steeper (12°) bypass slope in Exuma Sound, where turbidites are rarely found.Where the background sediment was initially a pelagic nannofossil ooze rather than a periplatform ooze, mineralogical composition results in lithification differences. In lithified sections having chalk as the background sediment, turbidites display a higher sonic velocity, indicating that they are the more competent beds. This lithification variation is the result of differential diagenesis between the platform-derived turbidites, enriched in metastable carbonates, and the calcitic nannofossil ooze of the background sediment.The different lithification owing to dissimilar mineralogical composition could also influence fluid migration in carbonate sequences. In a periplatform sequence, the more porous turbidite might be the fluid conduit, whereas in a pelagic sequence the chalky background sediment allows for fluid migration.

Margin collapse and slope failure along southwestern Great Bahama Bank

“…In the unlithified to partially lithified portions, consisting of mudstone, wackestone, packstone, and grainstone with interbedded floatstone, shear strength and overburden stress generally increase downcore but both properties display large variability. For example, in cores at ODP sites 1008 and 1009, shear strength varies from 5 to 75 kPa (Eberli, 1988). However, the variation shows a pattern; shear strength is the lowest above the hardgrounds and increases upsection within the coarsening-upward succession.…”

“…During the Pliocene however, numerous small-scale downslope movements and slump masses occurred on a detachment surface (Harwood and Towers, 1988). Turbidites within the periplatform ooze can potentially act as detachment surface based on a core study from slopes along the Great and Little Bahama Bank (Eberli, 1988). In the cores from these slopes, the coarse bottom layer of the turbidites contains more interstitial water, which is prone to overpressure and may act as a detachment layer.…”

“…Cores drilled along the western slope of GBB during ODP Leg 166 retrieved a slope strata in which coarsening up-wards successions of 10-20 m thickness are separated by well-cemented intervals (Eberli, 1988). The well-lithified intervals (i.e., hardgrounds) are found almost exclusively in glacial lowstand deposits and interpreted to have formed at or near the seafloor (Malone et al, 2001).…”

Acoustic Properties of "Young" Carbonate Rocks, ODP Leg 166 and Boreholes Clino and Unda, Western Great Bahama Bank

“…The differential diagenesis of Bahamian periplatform sediment and calciturbidites and its effect on petrophysical properties, using well logs and multisensor track data, was earlier documented by Eberli (1988) for sediments recovered during ODP Leg 101, north of Little Bahama Bank. Anselmetti and Eberli (1993), measuring properties directly on core plugs, documented a strong link between acoustic properties and primary lithology, nearly clay-free platform-top to reefal and proximal slope sediments, and diagenetic alterations for the Clino and Unda boreholes.…”

Sedimentology and Geochemistry of the 2930 Ma Red Lake–Wallace Lake Carbonate Platform, Western Superior Province, Canada

“…The intraclastic carbonate lithoclasts and marl contain fragments representing mixtures of the constituents of the ribbon rock lithofacies and these lithofacies are closely related. Eberli () found the frequency of slope failure was increased and the angle of repose needed to produce slope failure decreased when peri‐platform oozes hosted platform derived detrital carbonate as opposed to uniform sediments. Similarly, the density contrasts between different beds in the ribbon rock likely resulted in more prolific slope failure and slumping on the upper platform slope.…”