Department of Geological and Environmental Sciences, Stanford University
Richard J. Behl
Department of Geological Sciences, California State University Long Beach
For decades, the Miocene Monterey Formation has been uniquely interesting and important to stratigraphers, sedimentary petrologists, petroleum geologists and structural geologists because of its unique composition, complex diagenesis, and key significance to the petroleum system in California. The Monterey Formation is a hemipelagic, organic-rich, locally phosphatic or calcareous sequence of siliceous mudrocks that accumulated across a broad region of basins, slopes, banktops, and shelves during the middle to late Miocene. Its composition and petrography has been influenced by a complex sequence of multiphase diagenetic alteration. During burial, diatomaceous silica, deposited as metastable opal-A, undergoes two stages of dissolution and re-precipitation via metastable opal-CT to stable quartz (Murata and Nakata, 1974; Murata and Larson, 1975; Murata and Randall, 1975; Stein and Kirkpatrick, 1976; Pisciotto, 1981; Isaacs, 1981a, 1982; Behl and Garrison, 1994). Inferred temperatures of transformation in siliceous mudstone are 45-50°C and 65-80°C for opal-A to opal-CT and opal-CT to quartz, respectively (Keller and Isaacs, 1985). The transformations occur at much lower temperatures and shallower burial in the purest siliceous sediments (Behl and Garrison, 1994). Authigenic 'proto'-dolomite, forming during shallow burial, recrystallizes to more stoichiometric dolomite of higher ordering (Compton, 1988; Malone and others, 1994). Mixed-layer smectite-illite clays increase systematically in illite component from about 10% to 80% over the temperature interval of 80-115°C (Compton, 1991b). Organic matter, which can make up as much as 34% in some mudstones (Isaacs and Petersen, 1987), undergoes diagenesis and catagenesis, with catagenesis beginning at burial temperatures as low as 60°C (Hunt, 1996; Isaacs and Petersen, 1987).
The Monterey Formation is both source and reservoir of hydrocarbons (Isaacs and Petersen, 1987; Ogle and others, 1987; MacKinnon, 1989). Due to low matrix permeability, typically <1 md (Crain and others, 1985; Roehl and Weinbrandt, 1985; Isaacs and Petersen, 1987; MacKinnon, 1989), hydrocarbon migration and production is critically dependent on fractures as flow pathways. The occurrence of fractures with respect to host rock type, bed thickness, and the regional stress field has received ample consideration in the past two decades (Belfield and others, 1983; Snyder and others, 1983; Snyder, 1987; MacKinnon, 1989; Narr, 1991; Hickman and Dunham, 1992; Gross, 1993, 1995; Bartlett, 1994; Gross and Engelder, 1995; Gross and others, 1995). Eichhubl (1997) and Finkbeiner and others (1997) assessed the control of faults on fluid flow in the Monterey Formation.
There appears to be enough understanding of the diagenesis within individual chemical systems, brittle deformation, and fluid migration in the Monterey Formation to warrant a discussion of the interrelations among these processes as depicted schematically in Fig. 1. This paper may be considered a first attempt towards such a synthesis, with the expectation that an increasingly interdisciplinary perspective will improve our understanding of each component of this complex system.
The Monterey Formation provides ideal conditions to study the interaction between these processes for the following reasons: 1.) The formation has undergone for the most part a single burial/exhumation cycle which facilitates reconstruction of the diagenetic history (McCrory and others, 1995). 2.) The formation undergoes a complex diagenetic evolution during burial which potentially can be correlated with stages of vein cementation. This sequence is apparently due to the distinct thermodynamic metastability of the rock forming phases. Diagenetic alteration is thus likely to respond readily to changes in fluid content, composition, and temperature. 3.) Exposed sections have reached various depths of maximum burial, giving insight into a range of different stages of diagenetic alteration (Isaacs, 1981a). 4.) The formation is exposed in coastal outcrops along the basin margins, at the same time as central parts of the active basins are at depth close to maximum burial which allows correlation of burial diagenetic changes with measured burial temperatures in boreholes (Keller and Isaacs, 1985). 5.) The formation is a currently producing reservoir of hydrocarbons, with wells providing subsurface information on structure and fluid composition.
RELATIONS BETWEEN DIAGENESIS AND FLUID FLOW
Probably the most significant link between burial diagenesis and fluid flow are the release of structural water by dehydration reactions and the expulsion of pore fluid due to chemical/physical compaction. Rock forming mineral phases in the Monterey Formation that are bound to undergo dehydration during burial are opal-A and opal-CT, smectite, and organic matter. Opal-A contains up to 17%weight water (Hurd and Theyer, 1977) that is released during the stepwise dissolution-reprecipitation sequence of opal-A to metastable opal-CT and to stable quartz. Released structural water of smectitic clay may amount to 10-15% of the compacted bulk volume (Burst, 1969). Diagenesis and catagenesis of organic matter releases water, CO2, liquid hydrocarbons, and natural gas. The combined volume of organic matter and compounds derived from kerogen increases during catagenesis by up to 15% (Ungerer and others, 1983). This excess fluid will preferentially be expelled from the formation provided pathways for fluid drainage are available.
In addition to releasing structural water, silica diagenesis is likely to induce fluid flow by the collapse of pore space. Isaacs (1981b) measured average porosity values of 65% in opal-A diatomite, of 30% in opal-CT porcelanite, and of 10% in quartz porcelanite. Because of the temperature control on silica diagenesis, the reduction in porosity during silica diagenesis is controlled by burial temperature rather than by overburden weight, unlike the purely physical compaction of unconsolidated sediment. Fluid expulsion would thus occur preferentially in depth intervals corresponding to the temperature ranges of silica phase transformations. Because the temperature of silica transformation is controlled by detrital content, beds with differing detrital content would compact and expel pore fluid at different depths (Isaacs, 1981b; Compton, 1991a). Due to variations among beds in detrital content, the depth ranges of preferred fluid expulsion are thus likely to be smeared out and may even overlap between adjacent beds. In a single bed of given composition, however, the depth intervals of enhanced fluid expulsion may be recorded by repeated episodes of fracturing and fracture cementation. When a unit subsides through the depth intervals of silica transformation, dehydration and pore collapse are likely to increase the pore fluid pressure, particularly in the low-permeability mudstone units. The increase in pore fluid pressure would provide preferred conditions for brittle failure such as the formation of extension fractures by hydraulic fracturing or the formation and reactivating of faults. Although units of different detrital content would pass through the fluid expulsion intervals at different depths, the recorded sequence of vein cements would be the same in each rock type. Eichhubl and Boles (1998) described a sequence of four vein generations, each representing multiple stages of fracture opening and cementation. These four vein generations correlate with the silica, dolomite, and organic matter alteration in the host rock sequence. Based on this correlation, the vein generations may thus be interpreted as the result of pore fluid expulsion during different stages of burial alteration of the sequence.
The variability in composition, particularly in detrital content, on the scale of formation members or large-scale stratigraphic cycles adds an additional complexity to the fluid expulsion behavior. Silica-rich members of the formation are likely to release pore fluid at different depth intervals than clay-rich members. The passage of rock sections containing higher than average percentages of silica phases or of hydrous clay minerals may induce periods of increased pore fluid release, maybe lasting as long as 105-106 years. If fluid is released at rates faster than porous media flow allows fluid drainage, these periods of enhanced fluid release may again be recorded as distinct events of brittle deformation. In contrast to the previous case, these periods of increased fluid expulsion would affect the overlying section synchronously. Cement generations recording these fluid expulsion periods would thus deviate from the burial-related cement sequence. These "out-of-sequence" cements would appear later in the burial related cement sequence in deeper structural levels and earlier in higher levels. To date, the vein record in the Monterey Formation has not been detailed enough to provide evidence for "out-of-sequence" cement generations, however, some evidence may be seen in the replacement of dolomite by quartz in host rock adjacent to quartz cemented veins at Arroyo Burro Beach (Eichhubl and Boles, 1998). Silica replacement may be the result of upward transport of silica that originated in the underlying, more siliceous member, as it underwent opal-CT dissolution. Upward flux of silica will be more significant during the passage of silica-rich sections through the silica phase transition than during the passage of silica-poor mudstone sections across the corresponding depth range.
Diagenetic reactions affect fluid flow through the reduction in permeability due to cementation of interstitial pores and fractures. Creation of secondary porosity may increase permeability. Because of the low matrix permeability of Monterey rock types, the effect of cementation on further reduction in matrix permeability is presumably small, at least for time scales of interest in hydrocarbon production. Secondary porosity of ~25%, presumably due to the preferred removal of silica phases, has been observed in oil-saturated dolostone from the Heritage offshore oil field.
Cementation of fractures by carbonate and silica cement is widespread in dolostone and porcelanite-rich sections and less common in sections of organic-rich mudstone. Eichhubl and Boles (1998) distinguished a minimum of four generations of fracture formation and cementation, the latest stage related to hydrocarbon maturation as evidenced by inclusions of bitumen and liquid hydrocarbons in vein cement. Faults are locally cemented on a massive scale, with exposed examples found at Arroyo Burro Beach and Jalama Beach (see road log, stops 1 and 2). As pointed out by Hickman and Dunham (1992), the timing of fracture formation and cementation is critical for fractures to serve as pathways for hydrocarbon migration and production. If fracture cementation and burial diagenesis are linked processes as suggested by Eichhubl and Boles (1998), fractures have to form late with respect to the burial diagenetic sequence and preferentially just prior to hydrocarbon migration in order to remain uncemented conduits for hydrocarbon migration and production. Bitumen and liquid hydrocarbons are frequently observed to be the last phase deposited in veins suggesting that hydrocarbons may impede further carbonate and silica cementation.
Fluid flow affects diagenesis by facilitating mass transfer, by heat advection, and by changes in pore fluid pressure. The diagenetic alteration of detritus-bearing diatomite to porcelanite is considered a closed-system reaction on a macroscopic scale. Mass balance estimates by Behl and Garrison (1994) suggested that silica is neither introduced nor removed in appreciable amounts from the pore structure during dissolution and re-precipitation of opal-A to opal-CT and of opal-CT to quartz in porcelanite. Formation of opal-CT and quartz chert, on the other hand, involves the advective or diffusive addition of silica (Behl and Garrison, 1994). This addition of silica to inter- and intra-particle pore spaces accounts for the higher bulk density of chert nodules and beds when compared to surrounding diatomite or porcelanite. The higher density is not due to differences in compaction as evident by sedimentary lamination crossing from the unaltered diatomite or porcelanite into the chert nodules without changes in laminae thickness.
Mass transfer on an intra-formational scale was inferred by Eichhubl (1997) based on the strontium isotopic composition of carbonate vein and fault cement. The 87Sr/86Sr composition of dolostone beds in the Monterey Formation was found by Miller (1995) to correspond well to the Miocene sea water strontium isotopic composition for the particular stratigraphic position of the dolostone beds. The 87Sr/86Sr composition of carbonate veins, on the other hand, is typically significantly lower than the dolostone composition (Eichhubl, 1997), suggesting that strontium has been transported upward by moving fluid over vertical distances of up to the formation thickness.
Massive deposition of carbonate cement along faults is likely due to a drop in fluid pressure and the partial pressure of CO2 during upward fluid flow; excellent examples can be seen at Jalama and Arroyo Burro Beach (stops 1 and 2, road log). For realistic geobaric gradients, the tendency of decreasing CO2 partial pressure to lead to carbonate precipitation during upward fluid flow outweighs the adverse tendency of decreasing temperature to dissolve carbonate (Lundegard and Land, 1986; Eichhubl, 1997). Unlike calcite and dolomite, quartz precipitation is favored by a drop in temperature. Increased temperature with burial results in the stepwise dissolution and precipitation of opal-A to opal-CT to diagenetic quartz (Ernst and Calvert, 1969; Murata and Larson, 1975; Stein and Kirkpatrick, 1976; Isaacs, 1981a). An increase in temperature increases the rate of dissolution of metastable opal-A and opal-CT. While the aqueous solution typically remains undersaturated with respect to the dissolving phase, the solution is simultaneously oversaturated with respect to the precipitated phase. For kinetic reasons, dissolution of metastable opal-A and precipitation of stable quartz proceeds via the intermediate step of opal-CT precipitation and dissolution (Williams and Crerar, 1985; Williams and others, 1985).
RELATIONS BETWEEN DEFORMATION AND FLUID FLOW
Due to the low matrix permeability, typically less than 1 md (Crain and others, 1985; Roehl; and Weinbrandt, 1985; Isaacs and Petersen, 1987; MacKinnon, 1989), fluid flow in the Monterey Formation is controlled by the presence of extensional fractures and faults. Because of the heterogeneous interbedding of different rock types, there is tremendous local variation deformational styles from the lamination scale to that of formational members. Based on outcrop measurements of fracture density, MacKinnon (1989) ranked the following rock types with decreasing fracture permeability: chert, porcelanite, mudstone, and dolostone. "Tiger-striped chert", an alternating sequence of cm-thick chert, porcelanite, and dolomite layers (Grivetti, 1982), is one of the most prolific oil producing rock types (Exxon, Thousand Oaks, personal communication, 1995). Matrix porosity of porcelanite provides storage, whereas fractures in interbedded chert layers provide connectivity, acting as conduits for oil production.
The highest permeability estimates are for chert breccias (MacKinnon, 1989), about one order of magnitude higher than for fractured but unbrecciated chert. Because of their economic significance, breccia occurrence and formation has received ample attention (Redwine, 1981; Roehl, 1981; Grivetti, 1982; Belfield and others, 1983; Snyder, 1987; Hickman and Dunham, 1992; Behl and Garrison, 1994; Dholakia and others, 1998). In many cases, brecciation can be shown to be the result of shearing. These breccias may be categorized into 1.) breccias along faults at high angle to bedding; 2.) breccias due to shear along bedding planes or along low-angle faults; 3.) brecciation formed by interaction of sheared joints; and 4.) breccias in brittle folds where differential slip along bedding planes and buckling of brittle chert layers is the likely cause for brecciation.
Fault breccias are perhaps the most conspicuous ones, forming 1-6 m thick hydrocarbon and carbonate cemented zones along mesoscale faults. Examples are described in the road log (this volume) from Arroyo Burro and Jalama Beach (stops 1 and 2). The country rock at these locations is extensively transected by sets of extension fractures over distances of up to 200 m adjacent to these faults. Abundant liquid hydrocarbon inclusions and bitumen show that these faults are conduits for hydrocarbon migration. Meter-scale faults with displacements of less than 1 m that are common in the organic-rich mudstone sections of the Monterey Formation are frequently uncemented by carbonate cement. The formation of breccias by bedding-parallel shear was documented by Dholakia and others (1998) in outcrop and core, following a sequence of 1.) slip along bedding planes, 2.) formation of splay cracks, 3.) crack coalescence, and 4.) fragmentation, leading to 10-20 cm thick hydrocarbon-filled tabular bodies. Shear along joint surfaces at high angle to bedding may lead to localized brecciation, preferentially where neighboring and overlapping joints interact (Dholakia and others, 1998). Although frequently confined to single beds, hydrocarbon infill of these breccias suggests that shear along joints may provide fracture connectivity for hydrocarbon migration. Brecciation due to folding is characteristic of chert layers (see road log, stop 3), forming tabular breccia bodies with thicknesses in excess of 1 m (Redwine, 1981; Roehl, 1981; Grivetti, 1982). Chert breccias, whether opal-CT or quartz phase, typically have undergone complex cycles of embrittlement, brecciation, and recementation (Behl and Garrison, 1994). The relative timing of silica diagenesis and fluid flow makes the key difference between a chert breccia being an excellent pathway and reservoir for hydrocarbons or being a tightly cemented, impermeable bed (Behl and Garrison, 1994).
Breccias can be observed to form by coalescence of extension fractures with no macroscopic shear displacement (Eichhubl, 1997; Eichhubl and Boles, 1998). Extension fractures are frequently clustered, with the fracture density increasing adjacent to veins of large aperture or adjacent to faults. Clustering of these fractures and the formation of linking fractures at high angle to the predominant fracture set eventually leads to rock fragmentation and to the formation of brecciated veins of high permeability.
Faults and associated fracture systems apparently control fluid flow across bedding in the Monterey Formation. Based on the strontium isotopic composition of dolomite that forms a massive deposit along a fault at Jalama Beach (road log stop 2), Eichhubl (1997) suggested that the cross-stratigraphic distance of fluid flow along this fault is similar to the stratigraphic thickness of the Monterey Formation at this location of about 700 m. A mass balance estimate of fluid required to form the dolomite cement deposit suggested that the fault focused fluid parallel to the formation over a minimum radial distance of five times the distance of cross-stratigraphic fluid flow. This fault was apparently very effective in channeling fluid. Potential pathways of fluid flow into fault zones are bedding-confined extension fractures.
Sibson (1996) suggested that fault-fracture meshes, formed by coalescence of bedding-confined extension fractures in competent units and by interconnecting small-scale normal faults in incompetent units, may provide preferred fluid migration pathways along bedding. Competent units, hosting preferentially extension fractures, are dolostone and porcelanite, whereas mudstone units accommodate extension by normal faulting (Gross and Engelder, 1995). Fluid flow would occur preferentially along the bedding-confined extension fractures, i.e. parallel to bedding and along strike of the fracture set (Sibson, 1996). Slip along the interconnecting normal faults would continuously reopen the extension fractures, thus preventing fracture sealing due to cementation.
Finkbeiner and others (1997) recently applied the concept of critically stressed surfaces to predict fluid flow properties of faults and fractures in the Monterey Formation. They suggested that slip along fault and bedding surfaces that meet the Navier-Coulomb sliding criterion prevents cementation thus preserving the hydraulic conductivity of these surfaces. According to Finkbeiner and others (1997), surfaces conductive for fluid flow are oriented obliquely with respect to the present-day NNE-SSW oriented maximum principal stress, rather than parallel as the neotectonic joint set. If both conjugate fault sets are developed, the drilling direction that intersects the maximum number of hydraulically conductive fractures is still perpendicular to the maximum principal stress direction, however, and thus perpendicular to the neotectonic joint set. Belfield and others (1983) and MacKinnon (1989) suggested that the best production is obtained from wells perpendicular to the neotectonic joint set, a finding not necessarily confirmed by others (Exxon, Thousand Oaks, oral communication, 1996). For hydrocarbon production from stratiform breccias, the angular relations between borehole and predominant joint set may be less important than the angular relations between borehole and bedding.
In a tectonically active setting such as the California continental margin, seismic fault slip is likely to control the dynamics of basinal fluid expulsion. Carbonate cement within faults at Jalama and Arroyo Burro Beach consist of alternating bands of fluid inclusion-rich and inclusion-poor carbonate. These alternating cement bands as well as fluctuations observed in fluid inclusion homogenization temperatures and in stable isotopic composition (Eichhubl, 1997; Winter and Knauth, 1992; Martin and Rymerson, 1998) suggest variable fluid flow conditions that may be triggered by seismic slip.
Faults in the Monterey Formation may affect fluid flow as conduits or barriers. In both cases, faults could separate compartments of different fluid composition. Permeable faults may compartmentalize fluid reservoirs by channeling expelled pore fluid along the fault surfaces, thus preventing flow and fluid mixing across these faults. Cementation and deposition of solid hydrocarbons and of rock flour may reduce fault permeability significantly relative to the fractured country rock. The massively cemented fault zones at Arroyo Burro and Jalama Beach contain appreciable fracture porosity, however, despite extensive carbonate and silica cementation.
Fluid flow may be induced by tectonic folding. High rates of tectonic shortening as inferred for the Ventura-Santa Barbara coastal area (Rockwell and others, 1988; Hornafius and others, 1996) may at least temporarily increase the pore fluid pressure above hydrostatic pressures and lead to pore fluid expulsion. Elevated pore fluid pressures are observed in the Plio/Pleistocene Pico Formation of the Ventura-Rincon-Dos Cuadras anticlinorium (Yerkes and others, 1969). A tilted oil-water contact in the South Elwood offshore oil field (Hornafius and others, 1996) and in the Ventura Avenue anticline (Yeats, 1983) may also result from rapid tilting of the units due to folding. Small-scale, non-brittle deformational features that may affect fluid flow, such as pressure solution, have been documented (Pisciotto, 1978; Snyder and others, 1983; Snyder, 1987) but not investigated in a systematic way.
RELATIONS BETWEEN DEFORMATION AND DIAGENETIC ALTERATION
Preferred chertification along fractures (see road log, this volume, stop 4) and locally massive cementation of extension fractures and faults are evidence for brittle discontinuities favoring diagenetic reactions by providing pathways for mass transport. The effect of fractures on country rock alteration may be more complex then simple advection and cementation. Eichhubl and Boles (1998) found that host rock fragments of dolostone and mudstone, contained in silica and carbonate cemented veins, were altered to variable degrees by dissolution and reprecipitation of dolomite and silica pore cement. Pigmented disordered and non-stoichiometric dolomite dissolved and reprecipitated as clear dolomite of higher ordering. Dissolution/reprecipitation also led to replacement of opal-CT by quartz. This host rock alteration is not restricted to the immediate vicinity of fractures but is also observed, to a lesser degree, away from fractures. Fractures apparently acted as catalyst for host rock alteration, presumably by increasing the water-rock ratio, thus enhancing reaction rates and allowing reactions to go to completion adjacent to fractures.
As far as diagenesis affects brittle failure, two cases have to be distinguished, 1.) the effect of diagenetic alteration state of the rock on brittle failure, and 2.) possible effects of diagenetic processes on brittle failure. The Monterey Formation provides excellent examples of the influence of diagenetic alteration state on brittle failure as documented by Gross and Engelder (1995), Gross (1995), and Gross and others (1998). Well cemented, competent beds such as dolostone and porcelanite fail by forming extension fractures, whereas over- and underlying mudstone beds fail by forming normal faults. At Arroyo Burro Beach, the difference in brittle behavior reflects differences in silica diagenetic grade: Porcelanite and dolostone, both failing in extension, are in opal-CT and quartz grade, respectively. Organic-rich mudstone, failing in shear, is in the opal-A stage of silica diagenesis. The magnitude of extension as accommodated by extension fractures in the competent units is similar to the extension accommodated by normal faults in the incompetent units (Gross and Engelder, 1995). The alternation between extension fractures and normal faults leads to fault-fracture meshes as discussed above.
Differences in diagenetic alteration among adjacent layers also affect the rheologic response of the units to layer-parallel tectonic shortening. A conspicuous feature of chert-rich sections are "chert folds" (Grivetti, 1982; Snyder and others, 1983; Snyder, 1987; Bartlett, 1994; Behl and Garrison, 1994; Gutiérrez-Alonso and Gross, 1997). Chert folds are disharmonious, with up to 1 m-thick stacks of chert layers, each 5-20 mm thick, intensely folded whereas under- and overlying porcelanite or diatomite layers are planar and apparently undisturbed. Behl and Garrison (1994) explained the apparent difference in horizontal shortening by the different response of dense brittle chert, which tends to buckle during shortening, and of porous porcelanite and diatomite, which accommodate horizontal shortening by distributed pore collapse. Folding of the chert layers is typically accomplished by intense brecciation. Quantitative analysis of microfabrics show that bedding-parallel shortening of diatomite by compaction and crenulation is approximately equal to the macroscopic strain measured in adjacent folded and brecciated chert beds (Behl, 1992).
Possible effects of diagenetic reactions on brittle failure are difficult to identify based on the geologic record. Increases in pore fluid pressure due to dehydration and chemical compaction would reduce the effective rock strength, thus favoring the formation of extension fractures and faults and the reactivation of existing discontinuities. Hydraulic fracturing was invoked by Redwine (1981) for the formation of breccias in the Monterey Formation. Pore fluid pressures measured in wellbores in the Monterey Formation are close to hydrostatic, however. Bedding-parallel extension fractures which would indicate lithostatic pore pressures were described from Ellwood Beach (Bartlett, 1994) and occur at Arroyo Burro Beach in Santa Barbara. These "water sills" predate folding (Bartlett, 1994) and appear to form early in the deformational history of the Monterey Formation. They are not associated with breccias, however, and are therefore not indicative of high pore fluid pressures during breccia formation.
Diagenetic alteration around fractures may lower the fracture resistance or fracture toughness of the rock. Lowering of the fracture toughness by water-rock interaction, commonly referred to as subcritical fracture propagation in rock fracture mechanics (Atkinson and Meredith, 1987), may explain clustering of extension fractures which ultimately leads to breccia dikes and sills (Eichhubl and Boles, 1998). In addition, it is conceivable that mass transfer such as cation exchange reactions in association with clays leads to volume changes and thus to local changes in the stress field that favor fracturing. Dehydration of opaline silica had been evoked as the driving force for brecciation of cherts by early workers (Taliaferro, 1934). As shown subsequently, the transformation from opal-A to opal-CT and quartz proceeds as discrete dissolution/reprecipitation reactions, where ions are in aqueous solution during intermediate steps (Stein and Kirkpatrick, 1976; Kastner and others, 1977; Williams and Crerar, 1985). Silica diagenesis would thus lead to pore collapse if silica phases form the overburden supporting framework, and thus to compaction, but not to contraction and brecciation as envisioned by early workers (Behl, 1992).
Dissolution of framework minerals may not only control compaction of the section due to pore collapse but may also accommodate bulk horizontal shortening in response to tectonic processes. Behl and Garrison (1995) explained inconsistencies in bed length balance between folded chert layers and apparently unfolded over- and underlying beds of diatomite and porcelanite by the different response of these rock types to tectonic shortening, porous diatomite and porcelanite deforming by porosity loss and chert by buckling and folding. Bulk shortening in diatomite and porcelanite is likely to be accompanied by silica dissolution of the matrix.
The Monterey Formation, initially composed of a heterogeneous mixture of metastable silica phases, hydrated clay, abundant organic matter, and authigenic carbonate, underwent a highly complex sequence of diagenetic alteration during burial. Diagenetic alteration is potentially coupled with fluid flow and deformation in a variety of ways (Fig. 1): Dehydration reactions and chemical compaction induce fluid flow and possibly increase the pore fluid pressure which may result in the formation and reactivation of extension fractures and faults. Fractures may act as catalysts to diagenesis by opening the system to mass transport. Diagenetic alteration state of the rock sequence affects spacing and failure mode of brittle fractures and faults. Fractures and faults are preferred conduits for fluid flow in the otherwise tight formation. Diagenetic alteration of framework minerals may allow for tectonic bulk shortening, causing expulsion of pore fluid. Fluid flow accounts for mass and heat transport, favoring diagenesis by dissolution and precipitation reactions.
The complex mutual dependence of diagenesis, deformation, and fluid flow as seen in the Monterey Formation is likely to apply to other sedimentary sequences of diagenetically less reactive composition as well, even though relationships among these processes may be less obvious. The possibility of predicting any of these processes by observing related ones may, however, justify a systematic approach to these relationships in other sedimentary environments.