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Field Meeting in the Central Inlier of Jamaica
Field Meeting in the Central Inlier of Jamaica JAMAICA GROUP 24 November 1957 Report by the Director: J. B. WILLIAMS TWENTY-ONE MEMBERS and
friends assembled in Kingston and drove in six cars to Christiana, a distance of nearly seventy miles, where they met the Director. The party proceeded through Spaldings and White Shop to the road that follows the Rio Minho eastwards into the Cretaceous inlier. While still on the Yellow Limestone (Eocene) Shales, a stop was made to explain the panoramic view of the inlier, the northern flank of which is particularly well seen from this part of the periphery. The Central Inlier is a composite anticline with an E.-W. trend. Main Ridge, which was seen towards the south-east, constitutes the core of the principal upfold. Several faults trend in the same general direction, and these structural elements determine the course of the Rio Minho. The down-dip migration of this river has resulted in the opening up of a broad valley in the tuffs and conglomerates, overlooked by the horst-like mass of Main Ridge to the south. The view north-eastwards towards the 'feather edge' of the Tertiary Limestones, with 'cockpit' silhouette on the skyline, provides a graphic demonstration of the immense amount of material which has been removed from this enormous valley inlier. The opportunity was also taken at this point to augment the geomorphological grandeur of the scene with an explanation of the succession (see map, Fig. 1). The flanks of the anticline are covered unconformably by the Tertiary Yellow and White Limestone Formations, the dips being steepest on the south side. On this flank the predominant faults trend NW.-SE., the same direction as the Yallahs-Wagwater tectonic belt and the Blue Mountain ridge. The formations described by Sawkins (1869) as the Trappean and Metamorphic Series have been identified as Upper Cretaceous, and are now subdivided as follows: Upper Tuffaceous Series Rudist Limestone Series Lower Tuffaceous Series
Tuffs, tuffaceous sandstones and conglomerates. Nodular and massive limestones intercalated with and grading into calcareous shales. Red weathering argillaceous beds and loose conglomerates. 254
255
JAM AI CA G R O U P F IE L D ME ETING
.... .... .. ...... .. ..
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EJ IT
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~
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GEO LOGICAL
~ ",,"' ~
WESTERN
MAP
OF TH E
PART OF THE
CENTRAL
INLIER
Uppor Creloeeoul
T"Uoctou. S.r ies
Published by permission of the Director of Geoiooicoi Survey Deportmentt Jamaica ·
Fig . 1
256
J. B. WILLIAMS Unconformity
Lower Rudist Limestone Inoceramus Shales Basal Conglomerate
Nodular limestone with Rudists. Brown tuffaceous and calcareous shales with molluscan fauna. Pebbles of igneous rocks in a tuffaceous matrix.
The whole succession, especially in the eastern part of the Central Inlier, is intruded by dykes and extensive masses of andesitic and granodioritic rocks, the latter being the latest intrusions. Some of these are associated with small-scale copper mineralisation. The basal conglomerate, consisting of pebbles of andesitic and basaltic rocks, is overlain by a thick series of tuffaceous shales with Inoceramus, which occur only in two areas of the inlier, namely, on the southern slope of Juan de Bolas and in the Arthur's Seat district. This series which is associated with the Lower Rudist Limestone is here much disturbed and cut up by faults. The Lower Tuffaceous Series, which overlies these beds unconformably, consists of a conglomerate series markedly coarser towards the base. The estimated minimum thickness is about 2500 feet. The beds are totally devoid of organic remains and comprise pebbles and boulders of basic and intermediate igneous rocks in a matrix of tuffaceous material and rock detritus. In several localities, notably at Effort Bridge, these beds have been intruded and metamorphosed by minor basic intrusions, resulting in some cases in the conversion of the sedimentary beds into dense grey hornfels, closely resembling normal volcanic rocks. At the base of the Upper Rudist Limestone, conformable to the conglomerates and overlying them, is a shale horizon about 500 feet in thickness. The lowest beds consist of unfossiliferous red-weathering clay-shale overlain by fine laminated clay-shales which contain a fauna of small molluscs together with plant remains, and show traces of ripple marking. Above the shales is the Rudist Limestone, a formation widely exposed north of the Rio Minho between Guinea Corn and Alston, and more narrowly on the southern side of the Inlier. On the northern side, east of Frankfield, the limestone splits into very thin beds separated by a considerable thickness of shaly, sandy and calcareous material with some clay and lignitic intercalations with plant remains. The thickness of the main limestone horizon is somewhat variable but averages 250 feet in the western outcrops. The lower part is massive and blue-hearted and yields Praebarrettia and Durania nicholasi; the upper division is characterised by the presence of Titanosarcolites and many other Rudists. The Upper Tuffaceous Series outcrops in a zone marginal to the Rudist Limestone, rather narrowly to the south, but occupying a wide area on the western and northern sides of the Inlier. It rests unconformably on various
JAMAICA GROUP FIELD MEETING
257
horizons of the underlying beds, and consists of two lithological types, whose relative stratigraphic position is, however, uniform. The basal division is a thick series of conglomerates which in places become extremely coarse. In a matrix which is predominantly tuffaceous are well-rounded boulders and pebbles of several types of volcanic rock; porphyritic andesitic is the type most commonly encountered. The characteristics of this formation suggest that it is the result of rapid erosion and deposition in a tectonically unstable environment. The upper part of the series is tuffaceous, extremely homogeneous with few pebbles or boulders, and is of greyish-purple colour, weathering red at high levels. From the vantage point below White Shop, the party proceeded eastwards. The first formation encountered beyond the Tertiary boundary was the Rudist Limestone, which is well exposed in road sections along the road to Grantham; several sections were examined, in particular that of the type locality at Logie Green, and a number of rudistids were collected. Below the limestone sporadic exposures of the finely laminated red shale of the Lower Series were seen which are repeated along the road by a number of small faults. Passing through Grantham, which is on a terrace of the Rio Minho, the traverse continued through Guinea Corn, where the steeply dipping Rudist Limestone of the northern limb of the anticline is well exposed in the incised meanders of the Rio Minho. The convolutions of the river in this area conveniently expose the junction between the limestone and the overlying tuffs, and the party spent some time here examining various sections, after which a picnic lunch was taken by the riverside. After lunch the grey tuffs and conglomerates of the Upper Tuffaceous Series were examined at various places along the road to Frankfield, where the road going southward over Main Ridge was taken. Between here and the top of the hill at Nine Turns road-cuttings provide an almost continuous section of the Upper and Lower Tuffaceous Series and the intervening Rudist Limestone. Nine Turns is situated at the western end of Main Ridge, a typical hogback rising to 2700 feet, formed of the tuffs and conglomerates of the Lower Tuffaceous Series. This folded mass of waterlaid pyroclastic material is penetrated at several points by zones of invasive metamorphism, associated with basic and intermediate dykes and sills, which have had the effect of reinforcing the ridge against erosion and assuring its topographical dominance of the inlier. The best exposures of sediments altered in this way can be seen in the bed of the Thomas River, at Effort Bridge, and accordingly a lengthy stop was made here to examine the compact greyish hornfels with its strings of 'ghost' pebbles, extensively penetrated by dykes and sills of intermediate basic composition. Southwards from the river roadside sections show a gradual diminution of metamorphic effect, and half a mile north of Smithville the conglomerates are entirely unaltered. Immediately north of
258
J. B. WILLIAMS
that village a stop was made to demonstrate the very good section in the shales underlying the Rudist Limestone, provided by the bed of a small stream, which conveniently exposes extensive ripple marking, and in the thin overlying grey shales a proliferation of small molluscs. After examining this section, the party returned to Kingston independently.
REFERENCES GEOLOGICAL SURVEY DEPARTMENT, JAMAICA. 1955. Annual Report for 1953-54. - - - . 1957. Annual Report for 1954-55. SAWKINS, J. G. 1869. Reports on the Geology of Jamaica. Mem. geol, Surv. U.K. (with map dated 1865).
friends assembled in Kingston and drove in six cars to Christiana, a distance of nearly seventy miles, where they met the Director. The party proceeded through Spaldings and White Shop to the road that follows the Rio Minho eastwards into the Cretaceous inlier. While still on the Yellow Limestone (Eocene) Shales, a stop was made to explain the panoramic view of the inlier, the northern flank of which is particularly well seen from this part of the periphery. The Central Inlier is a composite anticline with an E.-W. trend. Main Ridge, which was seen towards the south-east, constitutes the core of the principal upfold. Several faults trend in the same general direction, and these structural elements determine the course of the Rio Minho. The down-dip migration of this river has resulted in the opening up of a broad valley in the tuffs and conglomerates, overlooked by the horst-like mass of Main Ridge to the south. The view north-eastwards towards the 'feather edge' of the Tertiary Limestones, with 'cockpit' silhouette on the skyline, provides a graphic demonstration of the immense amount of material which has been removed from this enormous valley inlier. The opportunity was also taken at this point to augment the geomorphological grandeur of the scene with an explanation of the succession (see map, Fig. 1). The flanks of the anticline are covered unconformably by the Tertiary Yellow and White Limestone Formations, the dips being steepest on the south side. On this flank the predominant faults trend NW.-SE., the same direction as the Yallahs-Wagwater tectonic belt and the Blue Mountain ridge. The formations described by Sawkins (1869) as the Trappean and Metamorphic Series have been identified as Upper Cretaceous, and are now subdivided as follows: Upper Tuffaceous Series Rudist Limestone Series Lower Tuffaceous Series
Tuffs, tuffaceous sandstones and conglomerates. Nodular and massive limestones intercalated with and grading into calcareous shales. Red weathering argillaceous beds and loose conglomerates. 254
255
JAM AI CA G R O U P F IE L D ME ETING
.... .... .. ...... .. ..
i.~] } j~ ; f ~ ~ i~ :~ ~ ~.~ ~ ~~~ J~T IT ~ TI: ~ ~ ::: :..
EJ IT
Allulfium
a
Rtc.nt
T,rro e. Grov. 11
Whil' Limll ton.
§
Upp.r y, llow Limilion.
g
Low., Yillow l imn lone
C
,",pper Tuffa c. ous Seri••
~ fill)
R"di,t Limes ton.
y,lIow Umes'on. Shol.,
Lewer
"2
2
3
4 P\ilu
I
I
I
I
I
Oli90- ....eeene
White u mu tone
~
0
GEO LOGICAL
~ ",,"' ~
WESTERN
MAP
OF TH E
PART OF THE
CENTRAL
INLIER
Uppor Creloeeoul
T"Uoctou. S.r ies
Published by permission of the Director of Geoiooicoi Survey Deportmentt Jamaica ·
Fig . 1
256
J. B. WILLIAMS Unconformity
Lower Rudist Limestone Inoceramus Shales Basal Conglomerate
Nodular limestone with Rudists. Brown tuffaceous and calcareous shales with molluscan fauna. Pebbles of igneous rocks in a tuffaceous matrix.
The whole succession, especially in the eastern part of the Central Inlier, is intruded by dykes and extensive masses of andesitic and granodioritic rocks, the latter being the latest intrusions. Some of these are associated with small-scale copper mineralisation. The basal conglomerate, consisting of pebbles of andesitic and basaltic rocks, is overlain by a thick series of tuffaceous shales with Inoceramus, which occur only in two areas of the inlier, namely, on the southern slope of Juan de Bolas and in the Arthur's Seat district. This series which is associated with the Lower Rudist Limestone is here much disturbed and cut up by faults. The Lower Tuffaceous Series, which overlies these beds unconformably, consists of a conglomerate series markedly coarser towards the base. The estimated minimum thickness is about 2500 feet. The beds are totally devoid of organic remains and comprise pebbles and boulders of basic and intermediate igneous rocks in a matrix of tuffaceous material and rock detritus. In several localities, notably at Effort Bridge, these beds have been intruded and metamorphosed by minor basic intrusions, resulting in some cases in the conversion of the sedimentary beds into dense grey hornfels, closely resembling normal volcanic rocks. At the base of the Upper Rudist Limestone, conformable to the conglomerates and overlying them, is a shale horizon about 500 feet in thickness. The lowest beds consist of unfossiliferous red-weathering clay-shale overlain by fine laminated clay-shales which contain a fauna of small molluscs together with plant remains, and show traces of ripple marking. Above the shales is the Rudist Limestone, a formation widely exposed north of the Rio Minho between Guinea Corn and Alston, and more narrowly on the southern side of the Inlier. On the northern side, east of Frankfield, the limestone splits into very thin beds separated by a considerable thickness of shaly, sandy and calcareous material with some clay and lignitic intercalations with plant remains. The thickness of the main limestone horizon is somewhat variable but averages 250 feet in the western outcrops. The lower part is massive and blue-hearted and yields Praebarrettia and Durania nicholasi; the upper division is characterised by the presence of Titanosarcolites and many other Rudists. The Upper Tuffaceous Series outcrops in a zone marginal to the Rudist Limestone, rather narrowly to the south, but occupying a wide area on the western and northern sides of the Inlier. It rests unconformably on various
JAMAICA GROUP FIELD MEETING
257
horizons of the underlying beds, and consists of two lithological types, whose relative stratigraphic position is, however, uniform. The basal division is a thick series of conglomerates which in places become extremely coarse. In a matrix which is predominantly tuffaceous are well-rounded boulders and pebbles of several types of volcanic rock; porphyritic andesitic is the type most commonly encountered. The characteristics of this formation suggest that it is the result of rapid erosion and deposition in a tectonically unstable environment. The upper part of the series is tuffaceous, extremely homogeneous with few pebbles or boulders, and is of greyish-purple colour, weathering red at high levels. From the vantage point below White Shop, the party proceeded eastwards. The first formation encountered beyond the Tertiary boundary was the Rudist Limestone, which is well exposed in road sections along the road to Grantham; several sections were examined, in particular that of the type locality at Logie Green, and a number of rudistids were collected. Below the limestone sporadic exposures of the finely laminated red shale of the Lower Series were seen which are repeated along the road by a number of small faults. Passing through Grantham, which is on a terrace of the Rio Minho, the traverse continued through Guinea Corn, where the steeply dipping Rudist Limestone of the northern limb of the anticline is well exposed in the incised meanders of the Rio Minho. The convolutions of the river in this area conveniently expose the junction between the limestone and the overlying tuffs, and the party spent some time here examining various sections, after which a picnic lunch was taken by the riverside. After lunch the grey tuffs and conglomerates of the Upper Tuffaceous Series were examined at various places along the road to Frankfield, where the road going southward over Main Ridge was taken. Between here and the top of the hill at Nine Turns road-cuttings provide an almost continuous section of the Upper and Lower Tuffaceous Series and the intervening Rudist Limestone. Nine Turns is situated at the western end of Main Ridge, a typical hogback rising to 2700 feet, formed of the tuffs and conglomerates of the Lower Tuffaceous Series. This folded mass of waterlaid pyroclastic material is penetrated at several points by zones of invasive metamorphism, associated with basic and intermediate dykes and sills, which have had the effect of reinforcing the ridge against erosion and assuring its topographical dominance of the inlier. The best exposures of sediments altered in this way can be seen in the bed of the Thomas River, at Effort Bridge, and accordingly a lengthy stop was made here to examine the compact greyish hornfels with its strings of 'ghost' pebbles, extensively penetrated by dykes and sills of intermediate basic composition. Southwards from the river roadside sections show a gradual diminution of metamorphic effect, and half a mile north of Smithville the conglomerates are entirely unaltered. Immediately north of
258
J. B. WILLIAMS
that village a stop was made to demonstrate the very good section in the shales underlying the Rudist Limestone, provided by the bed of a small stream, which conveniently exposes extensive ripple marking, and in the thin overlying grey shales a proliferation of small molluscs. After examining this section, the party returned to Kingston independently.
REFERENCES GEOLOGICAL SURVEY DEPARTMENT, JAMAICA. 1955. Annual Report for 1953-54. - - - . 1957. Annual Report for 1954-55. SAWKINS, J. G. 1869. Reports on the Geology of Jamaica. Mem. geol, Surv. U.K. (with map dated 1865).