Field Meeting in the Inlier ofBenbow and Guy's Hill, Jamaica

JAMAICA GROUP

25 November 1956

Report by the Directors: L. J. CHUBB and B. V. BAILEY

Received 11 June 1957

FROM KINGSTON the main party drove westwards over the Liguanea alluvialplain to Spanish Town, the old capital of Jamaica, and thence northwardsthrough the magnificent limestone gorge of the Rio Cobre to the interiorvalley or polje of St. Thomas-in-the-Vale. At Linstead several countrymembers were waiting and the augmented party, numbering twenty-eightmembers and friends in nine cars, took the Guy's Hill road, runningtowards the north-east. About four miles beyond Linstead the road leavesthe interior valley and again enters upon the White Limestone formation,which dips south, so that the first beds seen near milepost 18 belonged tothe Lower Miocene zone of Amphisorus matleyi Vaughan. Within the nexttwo miles the whole thickness of the White Limestone was traversed, andnear milepost 20, Eocene beds with Dictyoconus were seen. The MiddleEocene Yellow Limestone formation which normally underlies the WhiteLimestone is here absent, owing to overlap or faulting.

Beyond this there is a gap of 100 yards with no exposures, and then apurple conglomerate was seen which, while it retains its structure, is sodecomposed that both pebbles and matrix may be cut with a knife. Matley(1942) interpreted this as the basal conglomerate of the White Limestone,but more recent work suggests that it is Cretaceous in age, and that therelationship between the Cretaceous and Tertiary is a faulted one.

Beyond this the road passed over a series of Cretaceous andesitic lavasand tufaceous sediments dipping north, vertically intruded by a variety ofigneous masses which are mainly acid and include granites and aplite veins.Barytes was found in thin stringers on the west side of the road near to andbeyond Inilepost 21, in the neighbourhood of Gospel Hall Church. Thebest veins are two or three inches thick. This area of precipitous topo­graphy, traversed by a winding road cut into the steep hill-sides, is knownas the Devil's Race Course. The last intrusions were seen near Inilepost 22.

A quarter-mile farther on a road-cutting showed a thin-bedded conglo­merate succeeded by a dark limestone containing lapilli and crowded withsmall rudists, above which were some highly ferruginous purple to redshales, and two thin limestones separated by calcareous shales. These were

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JAMAICA GROUP FIELD MEETING 33

followed by a thick series of purple tuffs which extend to beyond milepost23. All this series is conformable and dips NNE.

In the Benbow district, a little beyond the Rock Hall road junction,about half a mile beyond milepost 23, the purple beds pass under theBenbow Limestone, which is almost black, compact and bituminous, andis here about 900 feet thick. No fossils have been found in this section;Sawkins (1869) mapped it as Cretaceous, while Matley (1942) regarded itas certainly of Tertiary age. Northwards the Benbow Limestone passesunder undoubted Middle Eocene beds, a thick series of tuffs interbeddedwith limestones and sandy shales of the Yellow Limestone formation.which is well developed on the north side of the inlier. Near the office ofthe Public Works Department in Guy's Hill village many of the fossilscharacteristic of this formation were collected, such as Lucina sp. andTerebellum proeerum Merian (T. subdistortum Trechmann).

The party proceeded eastwards through Arcadia to Springfield, andthence southward to Seafield, this part of the route being over the YellowLimestone. At Seafield lunch was taken, and members then walked to thespot where the Geological Survey recently drilled an exploratory boreholeto a depth of over 600 feet.

A strong magnetic anomaly had been found, in the course of an airbornemagnetometer survey, in a belt running nearly E.-W. north of and parallelto the outcrop of the Benbow Limestone, and the purpose of the hole. wasto investigate this. Samples of the core were shown to the members. Afterpassing through 105 feet of Eocene limestone the drill penetrated a volcanictuff very rich in magnetite grains, and similar agglomeratic tuff continuedto the bottom of the hole. Throughout its thickness the material is somagnetic that it strongly affects a compass needle, and small chips areeasily picked up with a magnet. This tuff overlies the Benbow Limestone.

Proceeding southwards the party soon re-entered the Benbow Inlier, thefirst exposures seen being the surface outcrops of the rocks penetrated inthe borehole. A mile south of Seafield, at Golden Grove, some thin lime­stones were seen interbedded in the tuff. From near here the well-developedhogback ridge formed by the eastward extension of the Benbow Limestonewas seen, and this limestone was examined again at the river fording aquarter-mile farther south and in the fields west of the road, where the dip­slope is well exposed. Here many fossils were found including smallrudistes and gastropods such as Actaeonella sp. and a large species ofNerinea, proving that the Benbow Limestone is Cretaceous in age, asSawkins believed.

Continuing south-westwards to Phillipsburg the party again traversedthe purple tuffs with interbedded limestones which underlie the BenbowLimestone. Thence the route ran WNW. to Johnson Hill, where a thinlimestone was seen containing a marl band crowded with a small species of

PROC. GEOL. ASSOC .• VOL. 69, PART I, 1958

34 L. J. CHUBB AND B. V. BAILEY

Nerinea, and other Cretaceous gastropods. Beyond this the outcrop of theBenbow Limestone was again crossed, and again specimens of the largespecies of Nerinea were found both in the uppermost beds of the mainlimestone, and in a higher thin limestone at Cocking Hill, a quarter-milefarther north.

From this point the party returned to Kingston via Benbow and theDevil's Race Course.

REFERENCESMATLEY, C. A. & F. RAw. 1942. A Road Section near Guy's Hill, Jamaica. Geol.

Mag., 79, 241-52.SAWKINS, J. G. 1869. Reports on the Geology of Jamaica. Mem. geol. Surv. U.K.TRECHMANN, C. T. 1942. Metasomatism and Intrusion in Jamaica. Geol. Mag., 79,

161-78.