The diagenetic history of the Cariblanco concerations is complex but readily interpreted in the context of the known geologic history of the Coamo Region. AS originally suggested by Glover (1971) deposition of Cariblanco sediments took place in an euxinic basin. This is supported by the lack of benthonic fossils and any indirect evidence of benthonic organisms in the matrix; and the abundance of early diagenetic bedding-plane pyrite and pyrite in skeletal chambers of planktonic fossils. The early diagenetic history of these concretions was controlled by the interaction f advecting seawater with organic matter within the sediment and gases diffusing from the underlying sediments. The late diagenetic history as suggested by the oxygen isotopic composition of the vein-filling calcite was largely controlled by thermally driven seawater circulation.
The changes in water and sediment chemistry during organic matter oxidation and the precipitation of diagenetic/authigenic minerals and concretionary carbonates have been the subject of extensive studies (e.g., Claypool and Kaplan, 1974; Irwin and others 1977; Froelich and others, 1979; Gautier and Claypool, 1984; Curtis and Coleman, 1986; and many others). Extensive reviews of early diagenesis in offshore basins is presented by Hesse (1990), of burial diagenesis by Surdam and others (1989), and of isotopic signatures during burial by Longstaffe (1989).
The presence of pyrite disseminated through the matrix, as distinct layers parallel to bedding, and within skeletal chambers suggests that Cariblanco sediments had progressed into the sulfate reduction zone within the first few centimeters of the sediment water interface. Two lines of evidence indicate that the bulk of pyrite formation took place prior to major concretion forming calcite cementation: a) cross-cutting relationships between pyrite layers and concretion boundaries nucleation of micritic calcite cements around the pyrite crystals/clusters. The horizontal pyrite seams could be the result of enhanced precipitation at a boundary between the iron reduction zone and the sulfate reduction zone where Fe+ 2 and HS− availability was optimal for sulfide precipitation. Other alternatives that cannot be disregarded are that pyrite precipitation was periodically taking place within the water column itself or that these layers, as well as foraminiferal chambers, were areas of greater sulfate reduction, due to higher organic matter concentration, resulting in localized pyrite formation.
After significant pyrite formation had taken place cementation of the matrix commenced. The δ18O of the matrix calcite is within the range of other Santonian marine sediments (cf. Czerniakowski and others, 1983) and indicates that calcite precipitation took place from marine fluids advecting through the sediment column into the sulfate reduction zone. Although depleted δ13C calcite compositions are characteristic of concretions formed within sulfate reducing and/or methane oxidation zones, Cariblanco concretions differ from many other concretions in that they lack any systematic trends in δ13C from concretion center to concretion exterior (e.g., Gautier and Claypool, 1984; Ritger and others, 1987). The lack of systematic trends in δ13C of concretion matrix and the absence of siderite and abundant pyrite indicates that supply of hydrogen sulfide was plentiful and during matrix cementation progression into the methane generation (fermentation) zone did not occur. The lack of systematic δ13C trends in the matrix as well as the limited variability of δ13C compositions (7‰) also suggests that the source of organic or organic derived carbon needed to drive sulfate reduction was relatively constant during concretion formation.
Since the matrix δ13C composition is several per-mil below that expected for simple organic matter oxidation through sulfate reduction (-22 to -25‰; e.g., Irwin and others, 1977), an additional source of 13C depleted carbon is required. It is likely that in addition to sulfate reduction bacterially mediated methane or light weight hydrocarbon oxidation was also taking place (Longstaffe, 1989; Hesse, 1990). Two possible sources of methane and other lightweight hydrocarbons are a) fermentation reactions taking place below the sulfate reduction zone or simultaneously within the sulfate reduction zone or b) thermogenic methane (or light molecular weight hydrocarbons). If significant amounts of methane were being oxidized it is unlikely that fermentation derived methane (δ13C < -50‰) was a major source as that would result in extremely depleted δ13C compositions. Either the fermentation derived methane contribution was minimal, or thermogenic methane (δ13C range of -35 to -50‰; Schoell, 1980) was being oxidized. It is difficult to determine which of these alternatives is the most likely; though given the association of concretion with hydrocarbons, early thermogenic methane generated during early catagenesis is a preferred alternative.
With progressive, though shallow, burial the stress heterogeneity due to cementation of concretions and compacting uncemented sediments led to early fluid overpressuring within the concretions resulting in the earliest vein formation (Astin, 1986) and infilling by sediments. The shift toward heavier carbon isotopic composition of the sediment infills (-17 to -25‰) suggests a diminished contribution of oxidized methane and is compatible with organic matter oxidation through sulfate reduction as the primary or only source of HCO3-. Although this shift in δ13C could also be attributed to the influx of isotopically heavy bicarbonate produced during methanogenesis, the absence of iron-rich carbonates or siderite suggests that Fe + 2 was still being sequestered into sulfides and that sulfate reduction was the dominant organic matter oxidation reaction. The oxygen isotopic composition of SF1 suggests temperatures and fluid compositions similar to those prevalent during matrix cementation, i.e. Santonian seawater at 15 to 20°C. This suggests a strong advective transport of seawater that prevented any significant temperature increase and provided abundant sulfate to sustain sulfate reduction.
With continued burial further fractures and veins developed leading to emplacement of SF2 and SF3. The similarity of carbon isotopic composition to those of SF1 sediment infills and the presence of disseminated pyrite suggests that sediments were still within the sulfate reduction zone. The shift in the δ18O values observed in SF2 and SF3 indicates that a temperature increase ranging from 5 to 10°C took place after cementation of SF1. This minor shift in temperature was probably a result of both a slightly deeper burial and a decrease in seawater advection allowing fluids to be warmed. Additionally, a change in geothermal gradient could have been caused by the proximity (within 3 to 4 km) of the ascending magma that eventually formed the Los Panes Intrusive.
Astin (1986) suggests fractures in concretions is a result of fluid overpressuring during compaction with the fracture geometry controlled by homogeneity or lack of homogeneity in stress fields. In normal compacting basins (i.e. vertical stresses greater than horizontal stresses) vein formation in homogeneous materials, i.e. ellipsoidal or spherical concretions, should result in septarian vein formation. Under non uniform horizontal stresses, such as those present during regional doming, asymmetrical or irregular (i.e., non septarian) vein formation is expected. The pronounced differences in size and geometry of post SF1 veins and fractures found in nodules and blocks and those in ellipsoidal concretions suggests major differences in the stress fields in which these two types of concretions formed. Veins and fractures in ellipsoidal concretions developed mostly by dilation of the first set of veins while in the irregular nodules and blocks new veins and fractures were formed. These differences indicate that there are differences in the stress fields of the different concretion morphologies. These differences are most likely a result of asymmetries in horizontal stresses induced by the initiation of doming caused the ascending Los Panes Magma.
Although δ18O depleted calcite cements usually are associated with burial diagenesis (> 1 km) it is unlikely that significant burial of Cariblanco sediment took place during concretion formation. Shallowing of the Cariblanco basin occurred and deposition of the Jobo Dulce Limestone Member took place during the progressive emplacement of the Los Panes Magma and the accompanying doming and faulting of the area. Glover (1971) attributed the fine-grained textures of the diorite in the Los Panes Intrusive to rapid ascent and cooling. It is very likely that the sharp temperature gradient produced by the rapid emplacement of this hot magmatic body (within 2–3 km of the sea floor) resulted in the initiation of thermally driven seawater circulation (Forster and Smith, 1990; Lewis, 1990).
The active and progressively more vigorous circulation of seawater could have resulted in further hydraulic and thermal expansion fracturing of veins and followed by the emplacement of calcite cements. The progressive δ18O depletion of the Vein-filling calcite resulted from progressive warming of circulating seawater as magma emplacement proceeded. If calcite cements were precipitated from warm seawater without modification by rock-water interactions, the earliest calcite cements (BDC) require a temperature increase of 10 to 20°C. The latest calcite cements require a temperature increase of 30 to 40°C. The fluid inclusion data on CSP cements suggest that a progressive temperature increase was accompanied by increasing rock-water interaction with the thick sequence of volcaniclastics, pelagic limestones and igneous rocks underlying Cariblanco sediments. These interactions resulted in an δ18O enrichment of circulating seawater of up to 10‰ (Sheppard, 1986; Lawrence, 1989; Longstaffe, 1989). The likely fluid δ18O enrichment implies that a much higher temperature would be required to account for the δ18O of calcite cement. Precipitation of the late cement phases, IRC and CSP, could have taken place at temperatures ranging up to 150°C.
There are several possibilities that can account for the observed trends toward heavier δ13C compositions. These are: 1) an enhanced contribution of heavy (0 to 2‰) marine dissolved CO2; 2) a reduced contribution from organic matter derived CO2; 3) rock-water interactions resulting in a contribution from heavy rock carbonate; 4) a contribution from heavy bicarbonate derived from the carbonate reduction zone (methane formation) in areas through which seawater is flowing into the rock; and 5) contribution of heavy magmatic CO2 produced by the degassing of the ascending magma body (Taylor, 1986). With the onset of active seawater circulation and the increase in temperature it is likely that oxygenated seawater circulated through the system and more "normal" marine dissolved CO2 was contributed to the system. The iron content of FEC cements requires an increase in fluid Fe/Mn ratios. The marked difference in δ18O, δ13C, and the reactivity of SF1 with fluids precipitating FEC suggests a major change in fluid chemistry, involving an increase in Fe/Mn ratios, warmer fluids, and a less 13C depleted bicarbonate. As the Los Panes Magma moved upward and doming and faulting increased, major changes in the size, position and the sense of direction of thermally driven convection cells could have occurred at the site of concretion formation(Forster and Smith, 1990). FEC and MSP preserve a record of changing fluid chemistry or flow regime. At or near the peak of convective circulation IRC and CSP were precipitated. The minor difference in the δ13C composition of these cements may be attributed to a small decrease in the contribution of oxidized organic matter or hydrocarbons to the dissolved CO2 or to a slightly higher rock-water ratio for the fluids precipitating CSP.
The last pulses of warm fluids were responsible for the precipitation of the late clear quartz present in a few concretions. With the cooling of the Los Panes Magma, the local highs subsided and seawater circulation decreased.
The latest diagenetic alteration of Cariblanco concretions occurred when the calcite filled veinlets were formed. The leaching of iron from matrix and cements by the fluids from which the veinlet calcite was precipitated demonstrate that there are major differences between these fluids and those that precipitated cements in the major veins and fractures. As these veinlets cut across all other diagenetic fabrics and seem to be associated with the strained fabrics observed in the Vein-filling calcite cements it is likely that these were formed during the Eocene compressional folding and transcurrent faulting that affected the area (Glover, 1971; Erikson and others 1991).
Hydrocarbon generation and emplacement.
The limited carbonate data does not allow unequivocal determination of the source of the hydrocarbons in the Cariblanco concretions nor when migration occurred. The presence of hydrocarbon in the voids within larger fractures and its survival until modern exhumation indicates that emplacement occurred prior to void sealing and most likely occurred simultaneously with CSP and/or IRC cement formation. The intense anaerobic organic matter degradation that affected Cariblanco sediments likely depleted most organic matter leaving a minor fraction of more resistant terrestrial organic matter.
The emplacement of the Los Panes Magma was the probable cause of a localized elevated geothermal gradient sufficient to initiate petroleum generation deeper in the sediment column. If indeed the relatively stable δ13C signature of the concretion matrix reflects the contribution of thermogenic methane, it is likely that methane and other light hydrocarbons were produced deeper within the basin. The migration and emplacement of these hydrocarbons was likely a result of the enhanced solubility/miscibility at temperatures in excess of 125°C (Barker, 1980) and the large scale convective flow resulting from the ascent of the Los Panes magma. Accumulation of hydrocarbons at the site of concretion formation was a result of decreasing pressure and/or temperature. Most of the hydrocarbon diffused through the overlying sediments while minor amounts were preserved within some concretions.
Hydrocarbon potential and the tectonics of south-central Puerto Rico
The presence of volatile hydrocarbons within Cariblanco concretions indicate that conditions for hydrocarbon generation did exist at depth when Cariblanco and possibly Maravillas sediments were being deposited. Because of the limited available data it is difficult to evaluate whether significant amounts of hydrocarbons were ever generated within the Cretaceous Cariblanco basin and what their fate was during the intense post-Campanian fracturing, plutonism and volcanism that affected all of the Cretaceous rocks in the area. It is doubtful that, if generated, any hydrocarbons have been preserved in onshore Puerto Rico.
If we assume that Cretaceous deformation diminished southward, offshore, then any significant amounts of hydrocarbons that were generated could have been preserved in the distal
portions of the Cariblanco Basin provided they survived Eocene faulting and folding. Extensive and detailed studies of the Cretaceous sedimentary sequence in south-central Puerto Rico are clearly needed to better define the history of hydrocarbon generation in this poorly understood area of the Caribbean.