Agglutinated and planktonic foraminifera of the Nariva Formation, Central Trinidad, as indicators of its age and paleoenvironment

Abstract

The Nariva Formation comprises mostly non-calcareous rocks with few calcareous foraminifera, but it is sandwiched geographically in a narrow (<10 km wide) band between the richly calcareous Brasso and Cipero Formations to the north and south respectively. Sixty-one samples were collected from the Nariva Formation at nine transient exposures along a transect trending NNE-SSW across southwestern Central Trinidad. The sparse planktonic foraminiferal assemblages showed the exposures to range in age between Oligocene and early middle Miocene, though many of the samples were of earliest middle Miocene age and coincident with a tectonically-induced transgressive-regressive cycle in the Brasso Formation. The Nariva assemblage at the most southerly exposure, at which the Nariva and Cipero Formations interfingered, was calcareous and of lower bathyal aspect, containing many Planulina wuellerstorfi. The assemblage at this exposure did not differ statistically between the two formations, and was thus said to be of "Cipero aspect", although the presence of rare Elphidium sp. in the Nariva parts of the exposure attests to some downslope transport. Recovery from most other exposures was typically small. Richer, predominantly agglutinated assemblages, however, were obtained from the Tarouba Community Centre, Ben Lomond Quarry, Raphael Street, and Tabaquite Heights. The assemblages were of "Brasso aspect", comprising species illustrated previously from the Brasso Formation. Three exposures were sampled at the Raphael Street site, which covered about ten hectares and had been cleared for development. The rocks at this site are patchily but richly stained with hematite and possible manganese derived from hydrothermal activity. Exposure 1 (13 samples) was subdivided into an almost barren interval (Exposure 1a, 8 samples) and an interval containing abundant Cribrostomoides carapitanus and Trochammina cf. pacifica and lesser Jarvisella karamatensis, Arenogaudryina flexilis and Glaphyrammina americana (Exposure 1b, 5 samples). This assemblage is thought to indicate low dissolved oxygen concentrations at middle to lower bathyal paleodepths. That the Brasso Formation was deposited at neritic to shallower middle bathyal paleodepths, while the Cipero Formation at lower bathyal to abyssal depths, demonstrates that the Nariva Formation was deposited on an eastward-facing paleo-slope. Exposures 2 (9 samples) and 3 (5 samples) are dominated by Simobaculites saundersi Wilson and Kaminski, n. sp., the walls of which do not incorporate calcareous particles. Many samples yielded large quantities of gypsum that, in view of (a) the paleodepth and (b) the rich planktonic foraminiferal recovery from the adjacent Brasso and Cipero Formations, is thought to reflect syndepositional dissolution of foraminiferal calcium carbonate and its precipitation as calcium sulphate. Because both the Nariva Formation and the upper Miocene to lower Pliocene Lower Cruse Member (southern Trinidad) yield primarily organically-cemented agglutinated foraminifera, a comparison is made between them. SHE analysis and different mean values of the Assemblage Turnover Index (ATI) shows the community structures in the two formations to differ. Because there is no sign of hydrothermal activity associated with the Lower Cruse Member, despite their taxonomic similarity we conclude that the agglutinated assemblages in the Nariva and Cruse Formations lived in markedly different paleoenvironments.