Dolomitization of the Oligocene–Miocene Bluff Formation on Grand Cayman, British West Indies

1990 ◽  
Vol 27 (8) ◽  
pp. 1098-1110 ◽  
Author(s):  
Suzanne M. Pleydell ◽  
Brian Jones ◽  
F. J. Longstaffe ◽  
H. Baadsgaard
Keyword(s):  
West Indies ◽  
Middle Miocene ◽  
Geochemical Data ◽  
High Porosity ◽  
British West Indies ◽  
Terra Rossa ◽  
Grand Cayman ◽  
Skeletal Material

The Oligocene–Miocene Bluff Formation on Grand Cayman is formed of hard, white, finely crystalline dolostone in which the precursor textures and fossil microstructures are commonly preserved. The dolostones have a high porosity (up to 25%) because of leaching of skeletal material, which was originally aragonite, and dissolution of the bedrock during the various phases of exposure and karst development.A major disconformity divides the Bluff Formation into the Cayman Member (Oligocene) and Pedro Castle Member (Middle Miocene). Cavities in the Cayman Member are commonly filled or partly filled with caymanite, dolomitized skeletal grainstone, terra rossa, and flowstone. Available evidence suggests that the caymanite and skeletal grainstone were emplaced prior to deposition of the Pedro Castle Member in Middle Miocene times, whereas the emplacement of the terra rossa and flowstone postdates dolomitization of the Bluff Formation.Petrographic and geochemical data suggest that there was only one phase of dolomitization that was mediated by normal seawater. There is no signifiant difference among 87Sr/86Sr ratios of the dolostones of the Cayman and the Pedro Castle members. The average 87Sr/86Sr ratio of 0.70905 for these dolostones is significantly lower than the average 87Sr/86Sr ratio of 0.70917 obtained from modern seawater around Grand Cayman. The 87Sr/86Sr ratios for the dolostones of the Bluff Formation, ranging from 0.70900 to 0.70914, suggest the dolomitization occurred 2–5 Ma ago. The underlying cause of the pervasive dolomitization is uncertain. Although it appears that "normal" seawater was responsible for that dolomitization, there is little evidence pointing to why it occurred 2–5 Ma ago.

Manganese precipitates in the karst terrain of Grand Cayman, British West Indies

1992 ◽  
Vol 29 (6) ◽  
pp. 1125-1139 ◽  
Author(s):  
Brian Jones
Keyword(s):  
West Indies ◽  
Climatic Conditions ◽  
British West Indies ◽  
Time And Space ◽  
Terra Rossa ◽  
Karst Terrain ◽  
Grand Cayman ◽  
Ongoing Process ◽  
Variable Morphology

Ten morphologically distinct forms of Mn precipitates (probably birnessite) occur in a wide variety of paleokarst features that characterize dolostones of the Bluff Formation on Grand Cayman. These Mn-rich precipitates, which commonly contain variable amounts and selections of Al, Si, Fe, K, Ti, Ni, Na. Mg, and Ca, are not evenly distributed throughout the Bluff Formation of Grand Cayman. Thus, some areas have abundant Mn precipitates, whereas other areas are devoid of such precipitates. Mn precipitates, of variable morphology, have teen found in stalactites, karst breccia, caymanite, terrestrial oncoids, and root calcretes and along fractures and cavities in the dolostone. There does not appear to be any correlation between the morphology and composition of the Mn precipitate and its host substrate.The Mn and its associated elements may have been derived from the terra rossa and (or) swamps that formed on the surface of the Bluff Formation at various times. The scattered distribution of the soils and swamps, with respect to time and space, accounts for the patchy occurrence of the Mn precipitates presently found in the Bluff Formation. Although some of the Mn precipitates are abiogenic in origin, others appear to have formed through the direct or indirect intervention of various microbes.Paragenetic analysis of the Mn precipitates and their associated cements shows that Mn precipitation has not been a constant, ongoing process. Indeed, Mn precipitation does not appear to be occurring at the present day. Available evidence suggests that Mn precipitation occurred at various times when climatic conditions were suitable for Mn mobilization.

Biogenic structures and micrite in stalactites from Grand Cayman Island, British West Indies

1987 ◽  
Vol 24 (7) ◽  
pp. 1402-1411 ◽  
Author(s):  
Brian Jones ◽  
A. Motyka
Keyword(s):  
West Indies ◽  
British West Indies ◽  
High Concentration ◽  
Cayman Island ◽  
Grand Cayman ◽  
Bacterial Origin ◽  
Biogenic Structures

Stalactites from modern and old caves developed in the Bluff Formation of Grand Cayman Island contain laminae and bulbous masses of micrite intercalated with sparry calcite. The micrite, as well as some of the sparry calcite around it, contains small (up to 25 μm long) ovate to spherical bodies that have a high concentration of either manganese or iron. Such bodies may be of bacterial origin. The micrite contains numerous calcified filaments that are probably of algal origin. Calcification of the filaments occurred either during life or shortly after death of the algae. The algae played an important role in trapping and binding the micrite. Furthermore, the algae may be directly or indirectly responsible for the formation of much of the micrite.

Tunicate spicules and their syntaxial overgrowths: examples from the Pleistocene Ironshore Formation, Grand Cayman, British West Indies

1990 ◽  
Vol 27 (4) ◽  
pp. 525-532 ◽  
Author(s):  
Brian Jones
Keyword(s):  
Cross Section ◽  
Outer Surface ◽  
West Indies ◽  
Careful Consideration ◽  
British West Indies ◽  
Tangential Plane ◽  
Growth Rings ◽  
Morphological Attributes ◽  
Grand Cayman ◽  
Composite Crystals

Pleistocene limestones of the Ironshore Formation, deposited approximately 125 000 years ago, contain stellate tunicate spicules that are morphologically similar to those of the modern Trididemnum strigosum Kott, 1980 from Australia. The aragonite spicules, up to 0.1 mm in diameter, have a 1–4 μm diameter nucleus formed of anhedral crystals (<1 μm long), surrounded by radially disposed composite crystals that have a pseudohexagonal cross section. The spines, which have a hexagonal cross section, are formed of subcrystals.During diagenesis the tunicate spicules were modified by (i) the development of concentric laminae of aragonite needles (up to 2.2 μm long and 0.2 μm wide), which are tangential to the outer surface of the spicule but randomly orientated in the tangential plane, and (or) by (ii) syntaxial aragonite overgrowth of the crystals in the spicule. If the spicules were surrounded by densely packed mud, no overgrowths occurred. Conversely, significant overgrowth occurred if the spicules were surrounded by loosely packed mud or faced into an open cavity. Where completely encased by syntaxial overgrowths, the original spicule is difficult to recognize. Interruptions in the growth of the syntaxial overgrowths produced concentric growth rings, which are marked by a physical discontinuity or a thin zone of concentrically arranged aragonite crystals. Some spicules are encased by two or more zones of aragonite overgrowths with well-developed growth lines. Available evidence suggests that the syntaxial overgrowths on the tunicate spicules probably developed soon after deposition of the sediments while they were still in a marine setting.The recognition of tunicate spicules in ancient limestones is important because they provide evidence of soft-bodied animals that are rarely documented from ancient faunas despite their abundance in modern oceans. Although superficially similar to inorganically precipitated aragonite spherulites and Microcodium, the tunicate spicules can be recognized by careful consideration of their morphological attributes.

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