Naparima Hill

The Naparima Hill Formation

Originally defined at its type locality of San Fernando Hill in the western Central Range, this formation has been found to extend in the subsurface to the south, west and east.

Spot geological map (Kugler)

The Hill is a Late Cretaceous (Campanian) inlier surrounded by Paleocene Lower Lizard Springs Formation on its northeast and Oligo-Miocene Cipero Formation on its southern boundary.

Cross section through Naparima Hill after Kugler (1939)

Cross section through Naparima Hill with wells (after Kugler)

Compositionally it is an organic rich siliceous mudstone, and thus represents a significant departure from the pre-existing shale dominated formations. Of all the Cretaceous and Paleogene formations this has probably received the most study because if its economic importance as the main source rock for the majority of the hydrocarbons produced in the region (Talukdar et al 1991).


Fine grained siliceous mudstones of this form and extent are likely to be pelagic deposits. As mentioned earlier this unit is generally massive and though this is most probably a result of continuous deposition the abundant bioturbation will have had some homogenising effect. The burrows usually appear as dark wisps, but lines between the walls of larger burrows reveal their true nature. The most common trace fossil is Zoophycos, a form indicative of relatively deep quiet water depositional environments.
Higher silica percentages in the Naparima Hill of the eastern Central Range (Chert Hill Member) might be indicative of a greater depth of deposition (>200m).

The Naparima Hill is absent to the north of the Central Range and Warm Springs Faults.

View of Naparima Hill looking west
Chert Nodule, heavily fractured, some of the chert has a light blue stain.

Bioturbated argilline

The Naparima Hill Formation is made up of a well-bedded indurated black to light-grey mudstone (argilline) to siliceous claystone weathering to cream and white colours. The rock is very’ brittle and highly fractured and may contain chert nodules in parts. The average thickness of the formation may amount to about 400 m but thicknesses of 700 m are known from well data. A relatively rich, mostly benthonic. foraminiferal fauna can be obtained from some of the more shaley beds.

The Naparima Hill Formation is made up of a well-bedded indurated black to light-grey mudstone (argilline) to siliceous claystone weathering to cream and white colours. The rock is very’ brittle and highly fractured and may contain chert nodules in parts. The average thickness of the formation may amount to about 400 m but thicknesses of 700 m are known from well data. A relatively rich, mostly benthonic. foraminiferal fauna can be obtained from some of the more shaley beds.

The fracturing is due to the low clay content and is a result of Middle Miocene deformation which formed the Naparima Hill thrust and associated anticline. In strike view the beds are disturbed by numerous anastomosing thrust faults. In dip view large scale dip panels are evident indicating the structure was controlled by brittle fault bend folding.

Fractured siliceous mudstone (Argilline)

Fractured Argilline, thinly bedded mudstone, chert nodule visible