Field Guide to the Geological Evolution of the Maastrichtian Rocks of the Central Inlier, Jamaica
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Caribbean Journal of Earth Science, 36 (2002), 27-38. © Geological Society of Jamaica. Field guide to the geological evolution of the Maastrichtian rocks of the Central Inlier, Jamaica SIMON F. MITCHELL Department of Geography and Geology, University of the West Indies, Mona, Kingston, Jamaica ABSTRACT. This field guide describes the geology found in the central part of the Central Inlier. The oldest rocks present, the Arthurs Seat Formation represent the deposits of a volcanic arc. These are succeeded unconformably by a Maastrichtian (Upper Cretaceous) transgressive-regressive cycle consisting of the Slippery Rock, Thomas River and Guinea Corn formations, and the Summerfield Group. The various stops show how this transgressive-regressive cycle developed. INTRODUCTION: GEOLOGY OF JAMAICA Hauterivian to Maastrichtian. The Maastrichtian rocks of the Central Inlier, which are considered in The geological history of Jamaica is conveniently this field guide, were deposited as a major divided into four phases (Draper, 1990, 1998). transgressive-regressive cycle, and illustrate the These are: 1) a Cretaceous island arc phase; 2) a interrelationships between marine carbonate Paleocene to Early Eocene phase of rifting of the deposition and active island arc volcaniclastic arc system; 3) Middle Eocene to Miocene sedimentation. deposition of carbonate platform deposits; and 4) This field guide is based on very successful field Late Cenozoic deformation leading to major uplift trips by the Geological Society of Jamaica (a record and mountain building. The Cretaceous island-arc 43 members and friends) and the Universities of phase contains rocks ranging in age from East Anglia and Edinburgh. Summerfield Group Waterworks Formation Slippery Rock, Thomas River and Guinea Corn Formations Crofts Hill Group Ginger Ridge Granodiorite Older Rocks Stratigraphic Boundary Inferred syncline axis assuming Crooked River Minor Faults 10 km displacement along Rio Trout Hall (west) Major Faults Minho Fault Guinea Corn PE N D R O R I W E V E R FA S U RIO MIN HO FAU L LT T A St. Anns Great River Inlier AB Above Rocks Inlier B Benbow Inlier BM Blue Mountain Inlier CR AWL C Central Inlier RIVE R FAU H St. James inliers LT J Hanover inliers L Lazarotto Inlier B E C K BLUE MOUNTAIN BLOCK F F A O A U R L D T H J B Z K O R C A N L E L AB BM CLARENDON BLOCK L HANOVER BLOCK WAGWATER BELT Figure 1. Summary geological map of the Central Inlier showing principal stratigraphic units and major structural features. Inset, distribution of main Cretaceous inliers and the structural blocks and troughs of Jamaica. 27 MITCHELL THE CENTRAL INLIER and sixties through the mapping of the Jamaican Geological Survey (reported by Williams, 1959a, b The Central Inlier is the second largest inlier (the and Coates, 1964, 1965, 1968). These studies Blue Mountains Inlier is the largest) exposing largely divided the succession into alternating Cretaceous rocks in Jamaica (Fig. 1). It is largely fossiliferous beds, which attracted great attention, drained by the Rio Minho and its tributaries, which and thick intervening clastics, volcaniclastics and have cut down deeply into the Cretaceous igneous units, which were invariably undivided and succession. Although the geology of the Central attracted little or no interest. The fossiliferous, Inlier has been known in a general sense ever since predominantly carbonate sediments were heavily the early studies of geology in Jamaica (e.g., De la studied, particularly with regard to the rudist Beche, 1827; Sawkins, 1869; Hill, 1899; bivalves, the most conspicuous fossils that they Trechmann, 1924), a real understanding of the contained, and Jamaica became the Mecca for succession was only achieved in the nineteen fifties Caribbean rudist workers (e.g., Chubb, 1971; Lithostrat. Guinea Corn Ignimbrite Fluvial conglomerates w W Marine sandstones Tidal flat deposits Limestones and mud rocks r e Candidate sequence boundary v i R Major flooding surface e o h a M m a h k c e P R G SB G F n E r o D Trout Hall (west) C UC a e SB n i MC u Crooked River G LC SB B A SB r e v i R s a m 100 m o h T k c o R y r e p p i l S Figure 2. Summary stratigraphy of Maastrichtian rocks in Central Inlier (See Fig. 1 for location of sections). The succession represents a transgressive-regressive cycle, the wedge-shaped geometry of the marine deposits is obvious in this figure. A-G, bed letters of Mitchell, 1999. 28 FIELD GUIDE TO CRETACEOUS OF THE CENTRAL INLIER Kauffman and Sohl, 1974; Kauffman and Johnson, exposed along the road to the north of the Slippery 1988). Despite the wealth of work on the rudists, Rock River. The name was restricted to the some interest was maintained on the volcaniclastics conglomerates and interbedded sandstones and (e.g., Roobol, 1976) and the igneous rocks (e.g., mudstones by Mitchell and Blissett (2001), the Jackson, 1987). overlying thick mudstone succession being called Recent work (Mitchell and Blissett, 1999, 2001; the Thomas River Formation. Mitchell, 1999, 2000, in press) has led to a much The Slippery Rock Formation is a thick fuller understanding of the sedimentary successions succession of conglomerates and minor interbedded present and their relationship to the Cretaceous sandstones. The lowermost part consists of very island arc system. poorly-sorted, matrix-supported, unstratified conglomerates and interbedded clast-supported imbricate conglomerates and sandstones. The STRATIGRAPHY matrix-supported, unsorted conglomerates are interpreted as debris flow deposits, the clast- The succession examined consists of a major supported conglomerates and sandstones, as braided Maastrichtian (Late Cretaceous) transgressive- stream deposits. regressive cycle that was deposited in a back-arc The lower part of the Slippery Rock Formation basin situated at some distance (perhaps 50 km) passes rapidly upwards into a succession of from the arc volcanoes. The stratigraphy is erosively based conglomerates with imbricate clasts described below (Fig. 2). and trough cross-bedding. These conglomerates show distinctive 1-4 m thick rhythms. The basal part of these rhythms is composed of clast- Arthurs Seat Formation supported conglomerates with strongly erosive bases. This passes upwards into muddy, pebbly The name Arthurs Seat Formation is now applied to sandstones that have insipient to well-developed the rocks of Main Ridge formerly called the Main nodular calcretes. Locally, the nodular calcretes Ridge Formation (Mitchell, in press). The Arthurs preferentially cement burrow systems or root Seat Formation consists of a thick succession of structures. The calcretes and cemented burrow largely unbedded poorly sorted conglomerates. The systems are variably truncated by the base of the clasts mainly consist of andesitic lithic fragments overlying rhythm. The rhythms are interpreted as and crystals, although other lithologies are also braided stream channel deposits, with soil (calcrete) present. The succession contains many andesitic horizons developed adjacent to channels and and less frequently amygdaloidal basaltic lava burrows probably formed by land crabs. flows and is intruded by a variety of dykes The uppermost part of the Slippery Rock (hornblende and pyroxene andesites, amongst Formation is characterised by poorly-sorted pebbly others). The formation is currently being mined for sandstones with localised pebble stringers gold. alternating with 10-20 cm thick laminated sandstone Fresh exposures of the Arthurs Seat Formation beds. The former are interpreted as braided stream in rivers may be extremely hard, elsewhere deep deposits, the latter, as sheet-flood deposits. tropical weathering has altered the primary igneous The facies represented in the Slippery Rock minerals to clays and locally intense leaching, Formation, namely braided stream/debris flow particularly on ridges, has produced bauxitic soils. deposits – braided stream deposits – braided The Arthurs Seat Formation represents the stream/sheet flood deposits indicates that this deeply eroded remnants of an early island arc represents an alluvial fan system. The former system, the presence of dykes in the Arthurs Seat interpreted as proximal facies the latter as Formation and their lack in the overlying post- distal. Arthurs Seat sediments indicating the significant In the western part of the Central Inlier the erosion and time gap that exists between the two uppermost sandstones and conglomerates of the successions. Slippery Rock Formation contain scattered marine fossils (bivalves and gastropods) and trace fossils (?Arenocolites isp. and Taenidium serpentinum Slippery Rock Formation Heer). This marine influx is succeeded by a thick series of red and grey shales with minor sandstones The name Slippery Rock Formation was introduced and limestones (Thomas River Formation). In the by Robinson and Lewis (in Robinson et al., 1972) mid Central Inlier the Slippery Rock Formation is for the conglomerates, sandstones and siltstones succeeded by the Guinea Corn Formation. 29 MITCHELL Thomas River Formation Guinea Corn Formation Williams (1959a, b) recognised 500 ft of shales, The name Guinea Corn Limestone was first used by which he placed either, at the base of Rudist Chubb (in Zans et al., 1963) for the limestones in Limestone (Williams, 1959a, p. 256) or, in the the upper part of the Titanosarcolites Series in the upper part of the Lower Tuffaceous Series Central Inlier. Coates (1965) described a new (Williams, 1959b, p. 12). ‘Williams’ shale unit’ section in the Guinea Corn Formation exposed in consists of a thick succession of red and grey Pindars River near the town of Crawle River and mudstones that contain thin ripple cross-laminated defined a type locality for the Guinea Corn sandstones and was named the Thomas River Formation at Grantham. Mitchell (1999) accepted Formation by Mitchell and Blissett (2001). The this as the type locality and indicated a thickness of lower part of the formation is characterised by 140 to 165 m for the formation around Grantham. monotonous red siltstones with thin layers of The boundary between the Thomas River concretionary limestones which are interpreted as Formation and the Guinea Corn Formation is sharp calcretes.