Caribbean Journal of Earth Science Volume 34 (2000)

Editor: Simon F. Mitchell

 


The age of the Port Morant Formation, south-eastern Jamaica

S. F. Mitchell, R. K. Pickerill, A. B. Blackwell and A. R. Skinner

Abstract - Two coral samples from the Port MMorant Formation in south-eastern Jamaica have been dated using electron spin resonance (ESR). Petrographic analysis showed some dissolution of the primary coralline aragonite as well as secondary mineral precipitation within the coral pore space in one sample, which also showed large associated uncertainties for accumulated doses calculated from the ERS growth curves. Therefore, the age, 125 7 kyr, for the sample probably represents a minimum age, being a weighted average of the date of the original skeletal aragonite and that for the secondary mineral phase. The second specimen yielded a single age of 132 7 kyr. Although we regard this as an accurate age, no single date can be considered to be totally reliable by itself. The ages suggest that at least part of the Port Morant Formation was deposited during the latest Isotope Stage 6 and probably earliest Stage 5e. More dates from the unit, however, are necessary to confirm this conclusion. Caribbean Journal of Earth Science Volume 34(2000), 1-4.


Ore mineral associations and industrial minerals in the ultramafic rocks of Jamaica and Tobago

P. W. Scott, T. A. Jackson and A. C. Dunham

 

ABSTRACT - The ultramafic rocks in Jamaica are duniites with minor lherzolite, most of which are at least partly serpentinised. They are part of a dismembered ophiolite complex. In Tobago, dunites, wehrlites, pyroxenites and hornblendites constitute the lower part of a Cretaceous plutonic complex of island arc affinity The chromite in Jamaica is high in Al and Mg, whereas that in Tobago is rich in Fe, as in Alaskan-type intrusives. Ni-Cu-PGE assemblages in Jamaica consist of pentlandite, with paragenetically later low temperature heazlewoodite, awaruite and native copper, the latter sometimes Pt and Pd enriched. The low temperature assemblage is probably associated with the serpentinisation event. In Tobago there is a sulphide assemblage of pentlandite, pyrrhotite, pyrite, chalcopyrite, much less affected by later alteration, and Pt, and RhPtIr phases. The dunites in Jamaica have sufficient MgO to be considered a potential source of industrial olivine. The higher Fe in the olivine from Tobago indicates that olivine cumulates in plutonics from island arc settings are a less suitable source of the mineral. Although oxide minerals develop during tropical weathering in the Blue Mountains of Jamaica, the formation of Ni-laterites is unlikely because of the high topographic relief. In Tobago the prospect for Ni-laterites is low as there is much less Ni in the olivines. Chrysotile asbestos, talc and magnesite are absent in both ultramafic complexes. This is likely to be a consequence of the lack of secondary serpentine recrystallisation to form fibrous chrysotile veins, the deep tectonic level and lack of hydrothermal circulation for magnesite formation, and the absence of metamorphic/ metasomatic events and/or late stage extension tectonics which might have yielded talc. Caribbean Journal of Earth Science Volume 34(2000), 5-16.

 


 

Field guide to the geology of the University of the West Indies campus, Mona

 

S. K. Donovan and T. A. Jackson

Abstract. This is the first published field guide to utilise urban geology as a tool for undergraduate education in Jamaica. The campus of the University of the West Indies, Mona, is well-suited for this purpose, being situated on the Liguanea Plain, surrounded by the Port Royal, Dallas and Long Mountains, and the Hope River, and including buildings that utilise a broad range of local rock types in their construction. Five stops are discussed in detail and suggestions made for other sites on campus where further observations can be made.. Caribbean Journal of Earth Science Volume 34(2000), 17-24.


 

Serratolamna serrata (Agassiz) (Pisces, Neoselachii) from the Maastrichtian (Late Cretaceous) of Jamaica

 

C. J. Underwood and S. F. Mitchell

Abstract. Three teeth of the shark Serratolamna serrata (Agassiz) from the Guinea Corn Formation (Central Inlier) represent the first recorded occurrence of fossil sharks from Cretaceous rocks of Jamaica. This occurrence increases the known palaeogeographical distribution of S. serrata, which appears to have been global. This species is largely known from the Upper Maastrichtian, which helps confirm the stratigraphic position of the Guinea Corn Formation. Caribbean Journal of Earth Science Volume 34(2000), 25-30.


Geological Society of Jamaica, 2000.