Sedimentology and Tectonic Evolution of the Cretaceous ... - UWI, Mona

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Mitchell, S. F., 2003, Sedimentology and tectonic evolution of the Cretaceous rocks of central Jamaica: Relationships to the plate tectonic evolution of the Caribbean, in C. Bartolini, R. T. Buffler, and J. Blickwede, eds., The Circum-Gulf of Mexico and the Caribbean: Hydrocarbon habitats, basin formation, and plate tectonics: AAPG Memoir 79, p. 605 – 623.

Sedimentology and Tectonic Evolution of the Cretaceous Rocks of Central Jamaica: Relationships to the Plate Tectonic Evolution of the Caribbean Simon F. Mitchell Department of Geography and Geology, University of the West Indies, Kingston, Jamaica

ABSTRACT

D

etailed mapping and logging of successions in the Central Inlier of central Jamaica has been undertaken. The succession contains three angular unconformities that allow the division of the succession into four units. The oldest rocks, the Arthurs Seat Formation, consist of a lower series of tholeiitic basaltic lavas and an upper unit of poorly sorted conglomerates with some calcalkaline lavas. They represent lavas and proximal volcaniclastic deposits of a late early or early late Cretaceous volcanic center. The late Santonian to early Campanian Crofts Hill Synthem (Peters Hill Formation, Back River Formation [new], and Dawburns Content Formation [new]) rests unconformably on the Arthurs Seat Formation. It represents a deepening-upward succession, beginning with rudist limestones and ending in turbiditic sandstones and shales. The unconformity at the base of the Crofts Hill Synthem is interpreted as the result of a rifting event that created an intra-arc to back-arc basin, in which the rocks of the Crofts Hill Synthem were deposited. The Crofts Hill Synthem is cut by late Campanian thrust faults that dip toward the north. This thrusting was caused by the collision of the western part of the Greater Antilles arc with the Yucatań Peninsula. The Maastrichtian Kellits Synthem represents a transgressive-regressive cycle that rests unconformably on the rocks of the Arthurs Seat Formation and Crofts Hill Synthem. The succession begins with braided stream deposits (Slippery Rock Formation) and passes upward through tidal-flat siltstones (Thomas River Formation) into open-shelf limestones (Guinea Corn Formation). The succeeding Summerfield Group represents a progradational volcaniclastic braid-plain delta complex (Green River, Peckham, and Mahoe River Formations) and is succeeded by a thick succession of ignimbrites (Waterworks Formation). The volcaniclastic sediments and ignimbrites are interpreted to have been shed from a newly emergent volcanic center in eastern Jamaica, possibly the Above Rocks magma chamber. The 605

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Kellits Synthem is overlain unconformably by the limestones and clastics of the middle Eocene Yellow Limestone Group. This unconformity is interpreted to represent the initiation of northeast-southwest-directed extension that saw the formation of the Wagwater Trough in eastern Jamaica.

INTRODUCTION The northern margin of the Caribbean Plate (Figure 1) is associated with a 200-km-wide zone of leftlateral strike-slip shearing related to left-lateral strikeslip displacement along the Cayman Trough (e.g., Holcombe and Sharman, 1983; Leroy et al., 1996). Restraining bends associated with major east-west strike-slip faults in this shear zone (e.g., Mann et al., 1985) are responsible for areas of uplift, of which Jamaica is the only subaerial example on the Nicaragua Rise at the present time (e.g., Arden, 1969; Robinson, 1994). It is this Miocene to Holocene episode of strike-slip deformation that dominates the tectonic features of Jamaica (e.g., Wadge and Draper, 1978; Krijnen and Lee Chin, 1978; Wadge and Dixon, 1987; Draper, 1987, 1998; Figure 1). The Cretaceous rocks of Jamaica are exposed in some 27 inliers (Robinson, 1994). These are distributed across the various Tertiary structural blocks and troughs of Jamaica (Figure 2). The Cretaceous rocks include island-arc volcanics, volcaniclastics, limestones, and high- and low-grade metamorphics (e.g., Zans et al., 1963; Coates, 1968; Roobol, 1972; Draper et al., 1976; Grippi, 1980; Grippi and Burke, 1980; Schmidt, 1988; Robinson, 1994). Details of the successions in many of these inliers are known in general, but detailed mapping and section analysis has been undertaken in only a few places. Consequently, there have been

Figure 1. Setting of Jamaica on the Nicaragua Rise on the northern part of the Caribbean Plate. East-west left-lateral strike slip movement is occurring on the faults bordering the Cayman Trough. The lower and upper parts of the Nicaragua Rise are separated by the Pedro Bank Fracture Zone; the rise is separated from the Columbian Basin by the Hess Escarpment. Based on Donnelly (1994) and Leroy et al. (1996).

few detailed accounts of the pre-Tertiary structuralsedimentary history of Jamaica (compare syntheses in Arden, 1969; Robinson, 1994; Draper, 1998). Meyerhoff and Krieg (1977) suggested that the Cretaceous rocks could be divided into two cycles; the lower one, pre-Tithonian, and the upper cycle being divisible into three subcycles (Tithonian?)-Albian; Coniacian(?) – early Campanian; late Campanian – Maastrichtian). Models have been proposed to explain the distribution of sedimentary rocks (Coates, 1968; Schmidt, 1988), metamorphic rocks (Draper et al., 1976; Draper, 1986), and igneous rocks ( Jackson and Smith, 1976; Isaacs and Jackson, 1987; Jackson, 1987; Jackson et al., 1989) in Jamaica. In the present study, the succession in the Central Inlier of central Jamaica that exposes a Cretaceous to ?Paleocene suite of sedimentary and igneous rocks is described. The Central Inlier is the key to unraveling Jamaica’s Cretaceous geological history for two reasons: the succession exposed in the inlier extends from the ?Lower Cretaceous to the Paleocene; and the succession contains many levels with abundant fossils allowing age determination.

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Sedimentology and Tectonic Evolution of the Cretaceous Rocks of Central Jamaica / 607

Figure 2. 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.

STRATIGRAPHY OF THE CENTRAL INLIER

PREVIOUS RESEARCH Early studies of the Cretaceous succession in the Central Inlier recognized suites of igneous, sedimentary, and metamorphic rocks (e.g., Sawkins, 1869; Hill, 1899; Trechmann, 1922, 1924). With reestablishment of the geological survey in the 1950s, geological mapping of the inlier began (Williams, 1959a, b; Chubb in Zans et al., 1963). The basis for the current stratigraphic scheme for the inlier dates from the studies of Coates (1964, 1965, 1968). Coates (1968) produced a detailed map of the middle section of the inlier, and he recognized an important east-west fault system (Crawle River Fault) separating distinct northern and southern successions. Separate stratigraphic schemes were introduced for each of these areas. Robinson et al. (1972) recognized that the two successions were not separate, but that lateral facies changes explained some of the differences in the two successions. They introduced the names Main Ridge Formation and Slippery Rock Formation. A small granodiorite intrusion is present in the eastern part of the inlier (Figure 2). It was described by Zans et al. (1963) and Porter (1970, 1972). Studies of the igneous rocks in the inlier have been presented by Jackson and Smith (1976), Isaacs and Jackson (1987), Jackson (1987) and Jackson et al. (1989). Recent 1:12,500-scale geological mapping of the inlier has demonstrated the complexity of lithostratigraphic units present, and the stratigraphy of the late Maastrichtian is now well established (Mitchell, 1999, 2000; Mitchell and Blissett, 1999, 2001).

The rocks of the Central Inlier can be divided into stratigraphic units that can be defined by angular unconformities (Figure 2). The term synthem (Chang, 1975; Salvador, 1994) is applied to those stratigraphic units that are bounded above and below by unconformities. Although Murphy and Salvador (1999) suggested that the term sequence might be used, the term synthem is used here, since the unconformity-bounded stratigraphic units in the Central Inlier have little relationship with changes in relative sea level (compare definition of sequence in Van Wagoner et al., 1988). The following stratigraphic units are recognized; an older suite of volcanics (Arthurs Seat Formation), an older Cretaceous sedimentary succession (Crofts Hill Synthem), a younger Cretaceous sedimentary succession (Kellits Synthem), and a cover succession of Paleogene, shallow-water limestones and associated clastics (Yellow Limestone Group) (Table 1; Figure 2).

Arthurs Seat Formation There has been a great deal of confusion over the names and stratigraphic interpretation of the older volcanic-sedimentary successions in the Central Inlier (Table 1). This has largely come from a combination of rapid facies changes, faulting, and the localized nature of many of the previous studies. Four different names have been applied to these older volcanic rocks: Arthurs Seat Formation (Coates, 1968); Bull Head Formation (Coates, 1968); Eastern Volcanic Complex (Porter, 1970; Robinson and Lewis in Robinson et al., 1972); and Main Ridge Formation (Robinson and Lewis in Robinson et al., 1972). Coates (1968) suggested that the Arthurs Seat Formation was stratigraphically below the Peters Hill Formation and that the Bull Head Formation was above. Remapping of the inlier demonstrates that the boundary

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Table 1. Correlation of lithostratigraphic names used in the text.

between the Peters Hill and Bull Head Formations is a high-angle thrust fault (Figure 3). Consequently, the lower part of the Bull Head Formation is now regarded as equivalent to the Arthurs Seat Formation. Porter (1970) studied the eastern part of the Central Inlier and erected new lithostratigraphic units. The scheme, however, never was formally published. Robinson and Lewis (in Robinson et al., 1972) reviewed the status of the nomenclature in the Central Inlier and recognized that Coates’ Bullhead [sic] Formation contained rocks equivalent to the Arthurs Seat Formation (as interpreted here), the Slippery Rock Formation, and the Summerfield Formation. They consequently restricted the name Bull Head Formation to the lower part of the succession. Coates (1968), however, had defined a type section for the Bull Head Formation in the road leading from Brandon Hill to Bull Head Mountain (Figure 2). This is equivalent to the Mahoe River Formation of the Summerfield Group as redefined by Mitchell and Blissett (2001). Coates (1968) clearly stabilized the name ‘‘Bull Head Formation,’’ and it cannot be redefined for

rocks not represented in the type section, as Robinson and Lewis (in Robinson et al., 1972) suggested. It is recommended here that the name Bull Head Formation be suppressed to avoid future confusion. Robinson and Lewis (in Robinson et al., 1972) also introduced the name Main Ridge Formation for the rocks of Main Ridge. These are lithologically similar to the rocks of the Arthurs Seat Formation and, since the Main Ridge Formation was not formally defined, this name also should be suppressed. The older volcanic rocks in the Central Inlier are, therefore, referred to as the Arthurs Seat Formation in this report.

Description The Arthurs Seat Formation consists of a thick succession of sedimentary and volcanic rocks, although the detailed stratigraphic succession has not yet been worked out. It appears to consist of a lower sequence of lava flows succeeded by an upper succession of conglomerates with subordinate sandstones and lava flows. The whole succession has been intruded by various dikes and, in the east, by the Ginger

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Figure 3. Geological map of the area around Crawle River and Kellits, Central Inlier, Jamaica. Prominent unconformities are mapped out at the base of the Paleocene and Kellits Synthem. The unconformity at the base of the Kellits Synthem truncates a thrust fault in the underlying Crofts Hill Synthem. Grid is the Jamaican metric grid.

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Ridge Granodiorite (e.g., Porter, 1970, 1972). Many rocks in the Arthurs Seat Formation have been altered extensively by metasomatism and localized contact metamorphism. When fresh, these rocks are intensely hard, while when weathered, the characteristic igneous and sedimentary characteristics are difficult to recognize. Lava flows in the eastern part of the inlier consist of basalts and basaltic andesites, and they have tholeiitic affinities (Jackson, 1987). Lava flows in the lower part of Main Ridge consist of basalts and basaltic andesites and also have tholeiitic affinities (Betsy Bandy, personal communication, 2001). The upper part of the Arthurs Seat Formation consists predominantly of very poorly sorted, unbedded conglomerates associated with rare graded/laminated sandstones and andesitic lavas. The conglomerates are poorly sorted and either clast- or matrix-supported. Bedding is largely absent, and no grading has been seen. Grain sizes range from fine sand to boulder, with clast populations ranging from bimodal to polymodal. Grain shapes are angular to subrounded. The clasts consist exclusively of volcanic lithics and crystals derived from basaltic or andesitic parent rocks. The sandstones occur in beds as much as 50-cm thick, are well sorted, medium- to coarse-grained, and show either normal grading (from coarse-grained sandstone to fine-grained sandstone) or 1-mm-scale graded laminations. Clasts consist of lithic fragments and crystals. No fossils have been found. Soft-sediment deformation features include sheath-shaped slump folds affecting as much as 3 to 4 cm of lamination. The associated lava flows are as much as 50 m in thickness and can be mapped laterally for distances of more than 1 km. They consist of porphyritic basalts and andesites and have calc-alkaline affinities (Jackson, 1987). The Arthurs Seat Formation has been intruded by numerous dikes. The dikes include basalts and porphyritic andesites, have produced significant contact metamorphism, and are locally associated with copper and gold mineralization (Baxter, 1998; Williams and Baxter, 1998).

Age The age of the Arthurs Seat Formation is difficult to determine. No fossils have been found, but the overlying Peters Hill Limestone yields Barrettia coatesi (Chubb) of probable late Santonian age (Sohl and Kollmann, 1985). A basic dike in the Arthurs Seat Formation in the Rio Minho gave a K-Ar radiometric date of 68.7 ± 1.9 Ma (Lewis et al., 1972). The age

probably has been reset by tectonic deformation in the late Campanian. The Ginger Ridge granodiorite and contact rocks of its metamorphic aureole have given a K-Ar isochron age of 85 ± 9 Ma (Lewis et al., 1972). This indicates a Turonian to middle Campanian age and is consistent with the geological evidence. The Devils Race Course Formation of the Benbow Inlier contains primitive island arc (PIA) tholeiitic lavas (Jackson, 1987). It also contains three limestones that indicate an age range for the formation from Hauterivian to Aptian (Burke et al., 1968; Chubb, 1971; Skelton and Masse, 1988). The overlying Albian to Turonian rocks contain a calc-alkaline igneous suite (Jackson, 1987). The tholeiitic lavas in the lower part of the Arthurs Seat Formation also could be part of the PIA suite. However, unequivocal evidence of PIA affinity is not available at present. The Arthurs Seat Formation is, therefore, tentatively attributed to the upper Lower Cretaceous or lower Upper Cretaceous.

Depositional Environment The Arthurs Seat Formation displays a thick succession consisting of interlayered matrix- and clastsupported, very poorly sorted conglomerates and lava flows with subordinate graded/laminated sandstones. The poor sorting and undefined bedding in the conglomerates suggest proximal volcanic deposits of debris-flow origin (lahars). The association of these deposits with extensive lava flows indicates close proximity to a volcanic center. The graded and laminated sandstones suggest deposition in water, possibly temporary lakes produced by the damming of rivers. The succession is, therefore, interpreted to have formed in a subaerial, proximal volcaniclastic apron close to an active volcanic center.

Crofts Hill Synthem (New Name) The Crofts Hill Synthem represents a sedimentary succession that was deposited on an angular unconformity cut into the volcanic-sedimentary succession of the Arthurs Seat Formation. The synthem consists of three formations: Peters Hill (modified herein), Back River, and Dawburn Content Formations.

Peters Hill Formation The Peters Hill Formation rests unconformably on conglomerates, andesites, and basalts of the underlying Arthurs Seat Formation. The formation is as much as 25-m thick and consists of a basal conglomerate succeeded by shallow-water, rudist-bearing

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Figure 4. Summary stratigraphy of the Crofts Hill Synthem near Peters Hill, Central Inlier, showing sequence stratigraphy. LST = lowstand systems tract; TST = transgressive systems tract; HST = highstand systems tract; CS = condensed section; SB = sequence boundary; FS = flooding surface; PH = Peters Hill Formation.

limestones. The type locality is at Peters Hill (grid reference 163700 224400, Figure 3). The basal conglomerate is as much as 5-m thick and consists of well-rounded pebbles and cobbles of andesite set in a carbonate matrix. No fossils have been seen. The basal conglomerate is succeeded by as much as 20 m of fossiliferous gray limestone. The limestone is bioclastic and in beds from 20- to 100cm thick. Fossils are abundant and include the rudists Barrettia coatesi and Torreites cf. sanchezi (Douville´) (Chubb, 1971), the echinoid Metholectypus trechmanni Hawkins, and colonial corals.

Back River Formation (New Name) The name Back River Formation is introduced for the coarsening-upward package of mudstones passing up into sandstones in the middle of the Crofts Hill Synthem. The base of the formation is marked by a change from limestones to mudstones. The top is taken at the base of the overlying Dawburn Content Formation. Type section: The type section is in the Back River (grid reference 164500 222800, Figure 3). The complete thickness of the formation is exposed in the banks of this river (Figure 4). Description: The Back River Formation consists of a coarsening-upward succession of mudstones passing upward into thin-bedded sandstones, thickly bedded

sandstones, and, finally, cross-bedded sandy limestones. The mudstones consist of alternating layers, 10- to 20-cm thick, of gray, carbonate-rich (harder and paler), and carbonate poor (softer and darker) mudstone. The pale-dark rhythms pass upward into more uniform, darker mudstones without obvious banding. Fossils include the inoceramid Inoceramus balticus Boehm (Kauffman, 1966) and infaunal echinoids belonging to Hemiaster spp. The mudstones pass upward into medium-bedded, fine-grained sandstones and, finally, thickly bedded, mediumto coarse-grained sandstones. The mudstones interbedded with the medium-grained sandstones in the lower part of the succession also yield I. balticus. These sandstones are overlain sharply by sandy limestones (grainstones) that have large-scale crossbeds in sets as much as 2-m thick (Figure 4). These are well exposed in the bed of the Crawle River below Dawburn Content but are poorly visible or absent elsewhere.

Dawburn Content Formation (New Name) The Dawburn Content Formation is named for a succession of alternating graded sandstones and shales. The base is marked by the abrupt transition from massive sandstones or cross-bedded sandy limestones to shales with thin beds of graded sandstones. The top of the formation terminates against a thrust fault. Type Section: The type section is exposed in the road immediately to the north of the hamlet of

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Dawburn Content (grid reference 164600 224400: Figure 3). Description: This formation consists of monotonous repetitions of graded sandstones from 5- to 40cm thick and mudstones as much as 50-cm thick. The formation is estimated to be about 130-m thick. The sandstones show normal grading, and some have parallel laminations in their upper parts. The bases are sharp and erosive. The mudstones are dark gray and reportedly contain abundant planktic foraminifera (Chubb in Zans et al., 1963; Chubb, 1971). There are two prominent marker horizons in the section that can be traced along strike. The lowest level is characterized by the presence of extremely abundant gastropods and locally nonrudist bivalves. This level was recognized by Coates (1968) and is informally referred to here as the gastropod shales (Figure 4). Thin micritic limestones are associated locally with the gastropod shales. The second marker horizon is a 2- to 3-m-thick succession of thinly bedded micritic limestones with black, nodular cherts as much as 1- or 2-cm thick. No fossils have been seen. The unit is referred to informally as the chert limestone (Figure 4). Minor Intrusions: The Crofts Hill Synthem contains rare basaltic sills and andesitic dikes (Figure 3). Interpretation: The Peters Hill to lower Back River Formations represent a deepening-upward succession, which is interpreted as a transgressive systems tract (Figure 4). The conglomerates at the base of the Peters Hill Formation consist of rounded, pebblesized clasts in a carbonate matrix and are interpreted as a transgressive rocky shoreline succession (e.g., Johnson, 1988, 1992; Mitchell et al., 2001). The bioclastic limestones, with their rudists and scleratinian corals, are interpreted as shallow-shelf limestones deposited in the photic zone (e.g., Sanders and Pons, 1999; Mitchell, 2002). The lower part of the Back River Formation consists of alternating carbonaterich and carbonate-poor mudstones with inoceramids. This suggests a deeper-shelf environment where the carbonate-clastic fluctuations either were controlled by carbonate productivity (productivity cycles) or clastic influx (dilution cycles) (e.g., Einsele and Ricken, 1991). The middle portion of the Back River Formation consists of a 120-m-thick succession of coarseningand thickening-upward sandstones. The thickness and grain-size patterns suggest the progradation of a delta lobe (e.g., Elliott, 1986a). A similar thickening and coarsening-upward succession in the Green River and Peckham Formations of the Summerfield Group

was interpreted as a progradational, volcaniclastic, braid-plain delta by Mitchell (2000). This coarseningupward package is attributed to a progradational complex of a highstand systems tract (Figure 4). The large-scale cross-bedding in the sandy limestones at the top of the Back River Member shows many similarities with that of tidal sand-wave complexes (e.g., Allen, 1980; Buck, 1985). These deposits, therefore, are interpreted as transgressive sand waves that migrated across the abandonment surface (tentatively interpreted as a sequence boundary) of the volcaniclastic braid-plain delta (Figure 4). The thin, graded sandstone beds of the Dawburns Content Formation are typical of turbidites (e.g., Stow, 1986). The lack of shallow-water fossils and the reported presence of planktic foraminifera would agree with a deep-water deposit. This formation is interpreted as a more basinal deposit. The significance of the two marker bands is not known with certainly. The gastropod shales might represent a brief shallowing, while the chert limestone might indicate a condensed interval with reduced clastic influx. Age: The age-diagnostic fossils collected from the Crofts Hill Synthem include rudist bivalves and inoceramids. The Peters Hill Formation yields Barrettia coatesi. The rudist B. ruseae Chubb, which is a more advanced form than B. coatesi (van Dommelen, 1971), occurs in the Clifton Limestone in Hanover, western Jamaica. The Clifton Limestone has yielded the ammonite Pachydiscus jamaicensis Wiedmann and Schmidt, which was suggested to be of early Campanian age (Wiedmann and Schmidt, 1993). Kauffman (1966) recorded Inoceramus mulleri Petraschek and I. balticus cf. kunimiensis Nagao and Mutsumoto from the overlying Hanover Shales indicating a very early Campanian age, while Jiang and Robinson (1987) suggested that the Clifton Limestone was of Santonian age, based on nannofossils. The Peters Hill Limestone, with its more primitive species of Barrettia, is attributed, therefore, to the late Santonian (but possibly not the latest Santonian). The presence of I. balticus and I. balticus kunimiensis in the overlying Back River Formation suggests an early Campanian age (Kauffman, 1966). No evidence of a middle Campanian limestone with the rudists Barrettia gigas Whitfield and B. multilirata Whitfield has been seen in the Central Inlier. This limestone is widespread elsewhere in Jamaica (St. Anns Great River Inlier, St. James inliers, Hanover inliers: Trechmann, 1922; Chubb, 1971; van Dommelen, 1971), and its absence in the Central Inlier

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suggests that the middle Campanian is absent. A late Santonian to early Campanian age is adopted here for the Crofts Hill Synthem.

Kellits Synthem The Peters Hill and Arthurs Seat Formations are overlain unconformably by a Maastrichtian (and possibly as old as late Campanian and as young as late Paleocene) succession of terrestrial and marine sedimentary rocks. The lithostratigraphy of the synthem has been described by Mitchell and Blissett (2001). The succession begins with the Slippery Rock Formation, approximately 150- to 175-m thick, which rests unconformably on rocks belonging to either the Crofts Hill Synthem or the Arthurs Seat Formation. The lower part of the succession consists of red, brown, or gray pebble conglomerates in poorly defined beds as much as several meters thick. Tabular and trough cross-bedding may be present. The conglomerate beds have sharp, erosive bases. Clast types are predominantly andesitic and basic volcanic rocks. However, small proportions of sedimentary rocks (sandstones, siltstones, and limestones), metamorphic rocks (mylonites), and even a little agate may be present; quartz generally is very rare. The thick, lower succession of conglomerates passes upward into discrete conglomerate beds alternating with poorly sorted pebbly sandstones with nodular calcretes. Locally, thick red siltstones with thin conglomerates/horizontally laminated sandstones as much as 1 m thick may be present in the upper part of the formation. Toward the west, the sandstones at the top of the Slippery Rock Formation contain marine fossils (oysters) and trace fossils (Arenicolites isp. and Taenidium isp.). In the western part of the inlier, the Slippery Rock Formation is succeeded by the Thomas River Formation, which consists of as much as 175 m of red and gray mudstones. The red mudstones are unfossiliferous but contain thin horizons of nodular calcrete. The gray mudstones contain thin beds of ripple cross-laminated heterolithics showing bidirectional paleocurrents. These heterolithics are arranged into fining-upward packages (usually 10 –50cm thick, but rarely as much as 2-m thick) that show a transition from flaser to wavy to lenticular bedding. A few thin, widely spaced limestones are present (Figure 5). Fossils are common at many levels in the gray mudstones, calcareous sandstones, and limestones. Charophytes (Kumar and Oliver, 1984; Kumar and Grambast-Fessard, 1984) are abundant at

some levels, while some of the limestones contain diverse rudist bivalves including Bournonia, Biradiolites, Antillocaprina, and Thyrastylon. The Guinea Corn Formation is as much as 200-m thick. It rests on the Thomas River Formation in the west and the Slippery Rock Formation in the middle, but it is absent in the eastern parts of the inlier (Figures 3 and 5). The formation consists of rudistbearing limestones varying from massive to thinly bedded to nodular (Coates, 1965, 1977; Kauffman and Sohl, 1974; Mitchell, 1999). The limestones alternate with fossiliferous siltstones and mudstones with graded sandstones, forming rhythms (Mitchell, 2002). Rudist bivalves are abundant in the limestones and include Titanosarcolites, Praebarrettia, Bournonia, Biradiolites, Antillocaprina, and Thyrastylon (e.g., Trechmann, 1924; Chubb, 1971; Kauffman and Sohl, 1974; Mitchell, 1999). The limestones thin to the northeast, and mudstones become more prominent (Figure 5). Fossiliferous siltstones are interbedded with the limestone in the lower part of the succession. In the upper part of the succession, mudstones with graded sandstones, similar to the sandstones in the overlying Green River Formation, are present (Mitchell, 1999, 2002). The Guinea Corn Formation (or Slippery Rock Formation where the Guinea Corn Formation is missing) is succeeded by the Summerfield Group (see detailed descriptions in Mitchell, 2000; Mitchell and Blissett, 2001). The Summerfield Group consists of a shallowing-upward succession of marine to terrestrial volcaniclastic sedimentary rocks. The lower part consists of mudstones with normal graded sandstones (60m-thick Green River Formation) that pass up into massive sandstones (150-m-thick Peckham Formation). The overlying Mahoe River Formation (210-m thick) consists of thickly bedded, pebble and boulder conglomerates with rounded clasts that have welldeveloped imbrication. Matrix-supported conglomerates are more rare, and sandstones are interbedded with the conglomerates in the lower part of the succession. The Mahoe River Formation is succeeded by as much as 75 m of ignimbrites (Waterworks Formation). The ignimbrites consist of dacites and have calc-alkaline affinities ( Jackson et al., 1989). Age: The age of the Kellits Synthem has been much debated. Traditionally, the rudist Titanosarcolites has been used as an index fossil for the Maastrichtian (Chubb, 1955, 1971). Calcareous nannofossil data suggest that the Guinea Corn Formation ranges in age from late Campanian to early late Maastrichtian (Logie Green sample of Robinson, 1988). The late

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Figure 5. Summary stratigraphy of the Kellits Synthem in the Central Inlier (see Figure 2 for location of sections). The Kellits Synthem represents a transgressive-regressive cycle. The wedge-shaped geometry of the marine deposits is obvious in this figure. Ww = Waterworks Formation; GR = Green River Formation; SB = sequence boundary; G, F, E, D, UC, MC, LC, B, and A are bed divisions in the Guinea Corn Formation (after Mitchell, 1999).

Campanian age is based on the presence of Eiffellithus eximius (Stover) and Uniplanarius trifidus (Stradner) (Jiang and Robinson, 1987; Verdenius, 1993). The presence of an unconformity beneath the Kellits Synthem indicates the possibility that these elements of the flora could be reworked (cf. Scott, 1987). If only

the first appearances of nannofossils are used for the biostratigraphy, the Kellits Synthem need not be older than Maastrichtian. Underwood and Mitchell (2000) recently recorded the shark Serratolamna serrata (Agassiz) from the Guinea Corn Formation, again suggesting a late Maastrichtian age.

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A single fission-track age from apatites from the Waterworks Formation gave an age of 55.3 ± 2.8 Ma (Ahmad et al., 1987), indicating Late Paleocene (Gradstein et al., 1995). The base of the overlying Yellow Limestone Group yields larger foraminifera of early middle Eocene age (Robinson and Mitchell, 1999). This suggests only about 5 Ma between the deposition of the Waterworks Formation and the Yellow Limestone Group, during which time at least 1 km of succession was removed from the Central Inlier (the Yellow Limestone rests on the Arthurs Seat Formation to the west of Kellits; Figures 2 and 3). The erosion rates required to remove 1 km of sedimentary rocks in 5 Ma seem to be too great (cf. Kirchner et al., 2001). The late Paleocene date for the Waterworks Formation therefore is regarded as suspect here, and a latest Maastrichtian or earliest Paleocene age is preferred (Figure 6). Depositional Environment: The Kellits Synthem represents a major transgressive-regressive cycle (Figure 5). The Slippery Rock Formation shows a transition from unconfined, braided-stream conglomerates at the base to broad, braided channels with calcrete soils between channels in the middle, passing up into thick overbank deposits with sheet-flood deposits and minor channels at the top. The succession shows much similarity to transgressive alluvial fan/fan delta successions (e.g., Nemac, 1990; Postma, 1990), and provisionally is interpreted as such. Coarse-grained fan deltas and alluvial fans typically are associated with areas of high relief (e.g., Elliott, 1986a). The presence of metamorphic rocks among the clast populations suggests that erosion and tectonism must have deeply dissected the rocks of the magmatic arc. The Thomas River and Guinea Corn Formations represent progressive steps in the flooding of this continental area. The abundant charophyte assemblages indicate fresh water (Brasier, 1980), while the rudist limestones are interpreted as marine incursions. The flaser-wavy-lenticular bedded sandstones with their bidirectional paleocurrents indicate tidal flat deposits (e.g., Elliott, 1986b). The Thomas River Formation therefore is interpreted as a transgressive tidal-flat succession deposited in a restricted environment (?estuary) with fresh-water influx. The Guinea Corn Formation contains open-marine fauna (rudist bivalves, corals, red algae, echinoids) and records the flooding of the estuary to produce a shallowwater marine shelf (Mitchell, 2002). The Green River, Peckham, and Mahoe River Formations of the overlying Summerfield Group represent a shallowingupward succession. Mitchell (2000) interpreted the

succession as a progradational, volcaniclastic braiddelta, associated with a newly emergent volcanic center.

Yellow Limestone Group The Yellow Limestone Group rests erosively on the various units of the Cretaceous succession in the Central Inlier. The succession begins with the clastic Freemans Hall Formation, followed by the marine limestones of the Stettin Formation (Robinson and Mitchell, 1999). Larger foraminifera in the Stettin Formation indicate an early middle Eocene age (Robinson and Mitchell, 1999).

TECTONIC EVOLUTION OF THE CENTRAL INLIER As in much of Jamaica (e.g., Wadge and Draper, 1978; Krijnen and Lee Chin, 1978; Draper, 1987), evidence of deformation in the Cretaceous rocks of the Central Inlier is heavily overprinted by Miocene to Holocene tectonism. The presence of three angular unconformities in the Central Inlier, however, is used here to determine the tectonic history of the Cretaceous deposits. Consideration of the truncation patterns of the units beneath each unconformity allows critical indications of the deformation to be determined. The detailed succession in the Arthurs Seat Formation is not known. This, coupled with the limited extent of the exposures of the Crofts Synthem, makes characterization of the tectonic deformation of the unit difficult. The sub-Peters Hill unconformity (Figure 4) appears to be planar, with the Peters Hill Formation resting on different lithologies in the Arthurs Seat Formation. Bedding in both the Arthurs Seat Formation and the Crofts Hill Synthem dips toward the north. Dikes are common in the Arthurs Seat Formation but rare in the Crofts Hill Synthem. This suggests that dike intrusion may have been associated with a sub-Peters Hill tectonic event. The unconformity at the base of the Kellits Synthem also is considered to represent a broadly planar surface. This unconformity truncates both the Arthurs Seat Formation and the Crofts Hill Synthem (Figure 3). It also cuts across the thrust fault at the top of the Crofts Hill Synthem. The rocks beneath the unconformity strike east-west and dip, at moderate to steep angles, to the north. The upper boundary of the Crofts Hill Synthem, a high-angle thrust fault, also strikes east-west and dips to the north (Figure 3). Mapping demonstrates that the thrust fault displaces

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Figure 6. Chronostratigraphic correlation of successions in different parts of central and western Jamaica (see Figure 2 for locations). Relationship with the evolution of the Caribbean Plate is shown at right. See text for sources. Time scale is from Gradstein et al. (1995).

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the early Campanian rocks of the Crofts Hill Synthem and is overstepped by the unconformity at the base of the Maastrichtian rocks. This indicates that thrusting occurred in the middle to late Campanian. Dikes are present in the Arthurs Seat Formation and Crofts Hill Synthem but are absent in the Kellits Synthem. Similarly, hydrothermal alteration and mineralization are restricted to the rocks of the Arthurs Seat Formation and Crofts Synthem (e.g., Baxter, 1998). The lack of dykes, mineralization, and hydrothermal alteration in the Kellits Synthem suggests an interval of dike intrusion prior to the deposition of rocks above the sub-Slippery Rock unconformity. The unconformity at the base of the Yellow Limestone Group oversteps the entire Kellits Synthem to rest on the Arthurs Seat Formation to the west of Crofts Hill. The Waterworks Formation is present only in limited areas of the inlier (Figure 2). The outcrop pattern suggests that it is preserved in the core of a gentle syncline. Offsets of the depocenter in the Yellow Limestone across the Rio Minho – Crawle River Fault are estimated at approximately 10 km left lateral (Mitchell, unpublished data). This is consistent with suggested offsets along the Plantain Garden Fault (the supposed continuation of the Rio Minho – Crawle River Fault) in eastern Jamaica (Wadge and Draper, 1978; Wadge and Dixon, 1987). Restoring the offset across the fault, this would suggest that the syncline with the Waterworks Formation in its core is orientated northeast-southwest to east-northeast – west-southwest (Figure 2).

DISCUSSION Rocks of a similar age to those found in the Central Inlier are exposed also in western and northern Jamaica (Figure 6). The succession in western Jamaica (St. James and Hanover inliers) can be divided into lower and upper parts. The lower part consists of a succession of deep-water shales and sandstones with planktonic foraminifera and thin rudist limestones (Chubb, 1955; Chubb in Zans et al., 1963; Grippi, 1980; Grippi and Burke, 1980; Schmidt, 1988). The rudist limestones yield various species of Barrettia (Trechmann, 1922; Chubb, 1971; van Dommelen, 1971). The succession ranges in age from Santonian to middle Campanian (Meyerhoff and Krieg, 1977; Wiedmann and Schmidt, 1993; Verdenius, 1993). This succession correlates with the rocks of the Crofts Hill Synthem of the Central Inlier, although the base of this shale-sandstone sequence in western Jamaica is unexposed at the surface and poorly constrained

in the various boreholes (Meyerhoff and Krieg, 1977; Verdenius, 1993). Consequently, the name Crofts Hill Synthem is not extended to western Jamaica at present. A similar succession of deep-water shales with planktonic foraminifera and ammonites and a limestone with Barrettia is exposed also in the St. Anns Great River Inlier (Chubb in Zans et al., 1963; Meyerhoff and Krieg, 1977; Wiedmann and Schmidt, 1993; Verdenius, 1993; Figure 6). The upper part of the Cretaceous in western Jamaica is represented by shallow-water shales and sandstones and Titanosarcolites-bearing limestones (Trechmann, 1924; Chubb, 1955; Chubb in Zans et al., 1963; Atkinson, 1969; Meyerhoff and Krieg, 1977; Figure 6). This succession is similar to the Kellits Synthem of Central Jamaica, although an unconformity beneath the Titanosarcolites-bearing limestone succession has not been proved in western Jamaica. Several models have been developed for the geological evolution of the Caribbean region (e.g., Pindell et al., 1988; Ross and Scotese, 1988; Pindell, 1994; Kerr et al., 1999). The following Cretaceous-Paleocene events are recognized in the northern Caribbean. During the Valanginian-Barremian, a southwest-facing arc existed (Pindell, 1994). During the Aptian, there was a reversal of the arc polarity (Lebron and Perfit, 1993; Draper et al., 1996), and a northwest- to westfacing arc developed (Pindell, 1994). During the late Campanian, the western part of the arc collided with the Yucatań Peninsula and became detached from the central and eastern parts of the arc (Pindell, 1994). The central part of the arc then was obducted onto the margin of the North American Plate in the early Eocene (Pindell, 1994). The Cretaceous succession in central Jamaica can be divided into four major stratigraphic units separated by angular unconformities (Figure 6). Some of these unconformities probably extend elsewhere in Jamaica (cf. Figure 6), although they have yet to be investigated in detail. The relationship of these four stratigraphic units to the evolution of the northern margin of the Caribbean Plate is discussed below.

Metamorphic Suite The metamorphic suite of central Jamaica is exposed only in the small Lazaretto Inlier near Green Bay (Chubb, 1954; Draper, 1986). The suite consists of amphibolites and hornblende schists, and it probably has close similarities to the Mount Hibernia (highpressure, low-temperature) and Westphalia (mediumtemperature) schists in the Blue Mountains of eastern Jamaica (Draper et al., 1976; Draper, 1986). The

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former have high-pressure– low-temperature blueschist facies mineral suites. Draper et al. (1976) suggested that these rocks were formed in the trench of a subduction zone. The age of the schists similarly is difficult to determine. Lewis et al. (1972) obtained a K-Ar age from a hornblende of 76.5 ± 2.1 Ma from the Westphalia Schists. This is middle to late Campanian in age and undoubtedly indicates tectonic resetting. The boundary between these units and unmetamorphosed stratigraphic units either is faulted or is an unconformity overlain by Paleocene to Eocene deposits (Draper et al., 1976; Draper, 1986). Draper (1986) suggested, from arguments based on their tectonic setting and thermal evolution, that these schists may be of early Cretaceous age.

Older Volcanic Sequence The older volcanic sequence includes a suite of tholeiitic basalts, basaltic andesites, and keratophyres in the Benbow Inlier (Devils Race Course Formation) that have PIA affinities. The associated limestones contain rudists indicating a Hauterivian to Aptian age (Chubb, 1971; Skelton and Masse, 1998). The overlying Albian-?Cenomanian succession contains calc-alkaline basalts and andesites ( Jackson et al., 1989). The change in composition of the volcanic rocks from PIA tholeiites to calc-alkaline basalts and andesites has been attributed to a change in subduction polarity of the northern margin of the Caribbean Plate. The PIA tholeites were associated with an earlier eastward-dipping subduction zone, and the calc-alkaline basalts and andesites were associated with a westward-dipping subduction zone, the polarity reversal occurring during the Aptian (Lebron and Perfit, 1993; Draper et al., 1996). The succession in the Arthurs Seat Formation in the Central Inlier contains the calc-alkaline suite and, possibly, the PIA suite. The association of thick lava flows and proximal volcaniclastic facies in the Arthurs Seat Formation suggests a setting close to the magmatic arc, while the presence of limestones with rudists and pillow lavas in the Benbow Inlier (Burke et al., 1968) indicates shallow-marine seas around the volcanic islands.

Crofts Hill Synthem In the Central Inlier, Santonian limestones with Barrettia coatesi rest erosively on the igneous and sedimentary rocks of the Arthurs Seat Formation (Figure 6). In western and northern Jamaica, deepwater sediments similar to those of the Crofts Hill Synthem were deposited during the Coniacian to

middle Campanian (Figure 6). The onset of the deposition of these deep-water sediments occurred at different times in different inliers, although the precise timing is unknown because the bases of many of the successions are unexposed. Across central and western Jamaica, the sedimentary succession consists of deep-water shales with turbidites, planktic foraminifera, ammonites, and inoceramids (Kauffman, 1966; Meyerhoff and Krieg, 1977; Grippi, 1980; Schmidt, 1988; Wiedmann and Schmidt, 1993). A regional shallowing event is marked by the wide occurrence of a limestone with Barrettia gigas in the middle Campanian. The transition from shallowwater limestones to deep-water clastics with planktic foraminifera suggests rapid subsidence, presumably following intra-arc rifting. The rarity of primary volcanic deposits would suggest a position away from the arc massif, possibly within a back-arc basin. This suggests that a significant jump in the position of the magmatic arc corresponds with the sub-Peters Hill unconformity, and that the older magmatic arc rifted to produce an intra-arc to backarc basin.

Late Campanian Tectonic Event Evidence of this event is preserved only in the Central Inlier. Bedding in the Crofts Hill Synthem strikes east-west and dips north, and high-angle thrust faults also have east-west strikes and dip north, but at steeper angles than the bedding. This suggests an episode of north-south compression with the formation of high-angle thrust faults that inverted the back-arc basin to produce extensive land areas. This episode of deformation possibly could be related to two different events. First, the collision of the western part of the Greater Antilles arc with the Yucatań Peninsula that obducted the Jolomax accretionary prism in Guatemala (Rosenfield, 1990). It also could be associated with the proposed accretion of the lower Nicaragua Rise onto the upper Nicaragua Rise, which was postulated to explain the concurrent movement of the Chortis Block and the Caribbean Plate between North and South America (Pindell, 1994). The postulated structure of the lower Nicaragua Rise also consists of northward-dipping thrust faults (Dengo and Case, 1990). The late Campanian event was closely associated with the emplacement of granodiorite stocks. The Flint River Stock was emplaced at this time (75 ± 5 Ma, Chubb and Burke, 1963), and possibly the Ginger Ridge Stock also (85 ± 9 Ma, Lewis et al., 1972), although the errors on the K-Ar isochron age are wide

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(Figure 6). Rare basaltic and andesitic dikes are present in the sedimentary rocks of the Crofts Hill Synthem, and radiometric dates in the Westphalia Schists and Arthurs Seat Formation were reset (Figure 6).

Kellits Synthem The rocks of the Kellits Synthem were deposited on an unconformity cut into the deformed and uplifted rocks of the Arthurs Seat Formation and the Crofts Hill Synthem. The basal unit, the Slippery Rock Formation, consists of continental sedimentary rocks deposited in alluvial fans and fan deltas that must have been adjacent to mountainous areas. The Kellits Synthem represents a major transgressiveregressive cycle. Lavas are absent from the succession, and only pyroclastic flows (ignimbrites) are represented. This suggests a relatively proximal position to the volcanic center (Roobol, 1976). In western Jamaica, shales, sandstones, and limestones equivalent to the Kellits Synthem were deposited (Figure 6) but lack lavas and pyroclastic flows. Renewed plutonic activity was evident in the eastern part of the Clarendon Block with the intrusion of a suite of granodiorites. The Above Rocks granodiorite has yielded a K-Ar age of 63 ± 5 Ma (Chubb and Burke, 1963). It is tempting to suggest that the Above Rocks magma chamber was the source of the ignimbrites in the Central Inlier. Thus, the eastern Clarendon Block would have been a volcanic center in the late Maastrichtian and early Paleocene, and the western Clarendon Block and Hanover Block would have been an intra-arc basin.

Paleocene Deformation The Kellits Synthem in the Central Inlier is folded into a gentle northeast-southwest to east-northeast– west-southwest syncline. Chubb (in Zans et al., 1963) recognized an east-west anticline that affected the succession in northern St. James. The anticline is truncated by the unconformity at the base of the Yellow Limestone, and the folding is, therefore, also of Paleocene to early Eocene age. During the late Paleocene to early Eocene, northeast-southwest-directed extension occurred in Jamaica related to left-lateral strike-slip movement between the Caribbean and North American Plates (Pindell, 1994). This extension resulted in the formation of the northwestsoutheast Wagwater graben system and east-west normal faults with downthrows towards the north (Draper, 1987, 1998; Mann and Burke, 1990). The folds that developed in the late Cretaceous succession in the Paleocene– early Eocene are best interpreted as

extension-related features. The anticline in the St. James inliers is interpreted as a rollover anticline associated with a listric normal fault, and the gentle syncline in the Central Inlier is interpreted as a hanging-wall syncline.

CONCLUSIONS The geological succession in the Central Inlier can be divided into synthems based on the presence of angular unconformities. The oldest rocks, the Arthurs Seat Formation, consist of lavas and associated immature sedimentary rocks deposited close to an active volcanic center. The Crofts Hill Synthem (late Santonian to early Campanian) represents a time of subsidence, when deep-water sediments accumulated. The deposition of the Crofts Hill Synthem was succeeded by a late Campanian tectonic event resulting in the emplacement of northward-dipping thrust sheets. The event may be related to the collision of the western part of the Greater Antilles arc with the Yucatań Peninsula, or the accretion of the lower Nicaragua Rise onto the upper Nicaragua Rise. After erosion, renewed subsidence led to the deposition of the Kellits Synthem (Maastrichtian) in an intra-arc setting. Rollover anticlines and hangingwall synclines in the pre-Yellow Limestone Group rocks are related to north-southwest-directed extension in the Paleocene– early Eocene that caused the opening of the Wagwater Trough. In the early to middle Eocene, carbonates and clastics of the Yellow Limestone Group began to be deposited on the Clarendon Block.

ACKNOWLEDGMENTS Many thanks to the people who have helped in the field over the last six years, particularly Donovan Blissett and Ravidya Maharaj. Many thanks to Trevor Jackson, Edward Robinson, and Thomas Stemann for reading early versions of this paper. Steve Donovan and Gren Draper are thanked for their careful reviews of this paper. This paper is dedicated to my wife Joe-ella and my son Peter, who was born during its writing.

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