Introduction 

Figures 1-1 – 1-3 

	Figure 1-1. Relationship of (A) porosity and depth and (B) pressure and depth for a shale that becomes overpressured by disequilibrium compaction at the fluid retention depth (modified after Harrold et al., 2000). (C) porosity-depth profile for Brunei Darussalam (modified after Tingay et al., 2000).
	Figure 1-2. Stratigraphic section, showing Rhinestreet Shale, along with overlying Angola Shale and underlying Cashaqua Shale, at Eighteenmile Creek.
	Figure 1-3. Geologic map of part of western New York, as location map for study area.

 

Burial-induced mechanical compaction of argillaceous sediment is accomplished by the loss of porosity as sediment particles respond to increasing effective stress by reorienting into more mechanically stable arrangements and pore fluid is expulsed (Hedberg, 1936; Hamilton, 1976; Magara, 1978; Goulty, 2004). This elasto-plastic reduction in porosity of clayey sediment under hydrostatic conditions generally is expressed as some form of the exponential decay function first proposed by Athy (1930),

 

f=f_oe^-cz

 

where z is depth in meters, f_o is the initial porosity at z = 0, and c is the compaction coefficient. Indeed, the majority of porosity-depth algorithms created from empirical data (e.g., Sclater and Christie, 1980; Huang and Gradstein, 1990; Hansen, 1996; among others) define by a rapid reduction in porosity at shallow depth, followed by a reduced rate of porosity occlusion in progressively older and more deeply buried sediment.  

Under certain conditions, notably when fluid expulsion during burial is restricted due to low permeability and/or rapid sedimentation, mechanical compaction fails to keep pace with increasing vertical effective stress such that the pore pressure is greater than hydrostatic (Swarbrick et al., 2002). This phenomenon, termed disequilibrium compaction. 

This paper seeks to demonstrate that Upper Devonian rocks of the Catskill Delta Complex of western New York state were overpressured by disequilibrium compaction relatively early in their burial history. We will use compaction strain measurements from around early formed carbonate concretions in the Upper Devonian Rhinestreet black shale to calculate the final porosity achieved by gravitational mechanical compaction. These results, interpreted in the context of the Upper Devonian-Mississippian stratigraphy of this region of the Appalachian Basin, will be used to estimate the depth at which the Rhinestreet shale became overpressured, its fluid retention depth. The approach documented in this paper may find application in studies of other shale-rich basinal sequences.

 

Field and Petrographic Data 

Figures 2-1 – 5-1 

	Figure 2-1. Rhinestreet shale exposed along Eighteenmile Creek.
	Figure 2-2. Cashaqua shale-Rhinestreet shale contact along Eighteenmile Creek (note heavily jointed nature of the Rhinstreet shale and carbonate nodule horizons in the Cashaqua shale).
	Figure 3-1. Middle concretion horizon in the Rhinestreet shale along Eighteenmile Creek.
	Figure 3-2. Close-up of middle concretion horizon in the Rhinestreet shale; note differential compaction of host shale around the concretions (stick = 1.9 m).
	Figure 3-3. Small carbonate concretion in the lower concretion horizon showing differential compaction of host black shale around the concretion and laminated internal structure of the concretion.
	Figure 3-4. A small, tilted concretion in the lower concretion horizon (scale = 1 m).
	Figure 3-5. Differential compaction of host black shale around a concretion; note laminated internal structure of the concretion (scale = 13 cm).
	Figure 4-1. Secondary electron image of a Rhinestreet shale collected from a concretion strain shadow showing relatively open or porous microfabric composed of randomly arranged clay flake domains in edge-to-edge and edge-to-face domain contacts.
	Figure 4-2. Secondary electron image showing the strongly oriented microfabric in sample recovered 24 cm from a concretion strain shadow.
	Figure 4-3. Secondary electron image showing the open framework of an etched Rhinestreet carbonate concretion sample showing a framboid (F) and randomly oriented clay grain domains (c), and microscaprite (ms).
	Figure 4-4. Secondary electron image showing a moderately planar clay-grain microfabric in an etched carbonate concretion sample. Many of the clay grains are oriented subparallel to the page. A more open microfabric can be seen in the center of the image.
	Figure 5-1. Thicknesses of correlative layers inside a concretion (Td) and in encapsulating shale (Tc).

 

The Upper Devonian succession of western New York State, which includes the Rhinestreet shale, grades upward from a base of marine shales and scattered turbiditesiltstones into shallow marine or brackish-water deposits thus recording progradation of the Catskill Delta across the Acadian foreland basin (Faill, 1985; Ettensohn, 1992). Marine deposits of the Catskill Delta Complex in the northern Appalachian Plateau are arranged in several cycles, each one defined by a basal unit of uniformly laminated fissile black shale that passes upward through a transition zone of alternating black and gray shale beds into strata dominated by poorly bedded (poorly fissile) gray shale and occasional turbidite siltstone and thin black shale beds.  

The Rhinestreet shale, the thickest of the black shale units of the Lake Erie District, western New York state, comprises at least 54 m of heavily jointed, dark-gray to black fissile and thinly laminated pyritic shale, thin gray shale intervals, sparse thin siltstone beds and several intervals of carbonate concretions (Buehler and Tesmer, 1963; Lash and Blood, 2006). The Rhinestreet shale is underlain by the Cashaqua gray shale, the contact being sharp and easily recognized in the field, and passes upward through a zone of interbedded black and gray shale into the Angola shale (Buehler and Tesmer 1963).

The majority of carbonate concretions of the Rhinestreet shale are found in three stratigraphically confined but laterally persistent horizons (see lithologic log). Most concretions are oblate ellipsoids with maximum diameters and thicknesses ranging up to 2.7 m and 1.1 m, respectively. Field observations, including randomly tilted concretions and differential compaction of host sediment laminae around concretions, are consistent with early diagenetic growth in unconsolidated sediment. Further, estimates of pre-cementation host sediment porosity based on the volume percentage of calcium carbonate cement (74 to 93%) and, perhaps most importantly, the preservation of a cardhouse clay fabric observed within concretion samples studied with the scanning electron microscope, suggest that concretionary growth occurred rapidly within perhaps a meter of the seafloor (Lash and Blood, 2004a,b).  

Concretions offer a unique opportunity to quantify the effects of gravitational compaction of the Rhinestreet shale. However, to ensure that our calculations yield finite compaction strain of the host shale, we must be certain that the Rhinestreet concretions formed rapidly and, most importantly, close to the sediment-water interface. Field observations, including the wrapping of shale around concretions and the lack of center-to-edge deviation in laminae thickness, demonstrate that concretions formed rapidly at shallow depth, perhaps a meter or so below the seafloor (Lash and Blood, 2004a, b). Lash and Blood (2004a) maintain that Rhinestreet concretions formed by the passive infilling of host sediment porosity by carbonate cement (e.g., Raiswell and Fisher, 2000). Accordingly, the volume percent of carbonate cement in the concretion matrix is a proxy for the porosity of the host sediment at the time of concretion growth (Raiswell 1971; Gautier, 1982). Volume percent of 21 Rhinestreet concretion samples collected from four concretions varies from 74 to 93% (mean = 83%), a range that encompasses the high end of porosity of modern marine clay deposits close to or at the water-sediment interface (e.g., Müller, 1967; Velde, 1996) further suggesting a very shallow depth of origin. 

Scanning electron microscopic analysis of concretion and host shale samples also provides evidence for shallow concretionary growth (Lash and Blood, 2004a, b). Specifically, SEM images of mudstone samples collected from concretion strain shadows reveal a porous fabric of randomly oriented platy particles, which higher magnification proves to be face-to-face clay flake stacks or domains. Domains typically are arranged in a low-density network or cardhouse fabric of edge-to-edge and edge-to-face contacts marked by large voids relative to the thickness of clay flakes and domains (Lash and Blood, 2004b). Secondary electron images of shale samples collected only 20 to 30 cm from strain shadows, however, show a generally low-porosity microfabric defined by a moderately to strongly preferred orientation of clay flake domains (Lash and Blood, 2004b). The almost negligible degree of compaction observed in strain shadow samples demonstrates that gravitational compaction of the Rhinestreet shale was minimal before carbonate concretions had become incompressible, pointing to a shallow diagenetic origin of the concretions (Lash and Blood, 2004b). Moreover, SEM observations of concretion samples evince a generally open arrangement of detrital clay grains typical of newly deposited flocculated clayey sediment preserved by diagenetic carbonate precipitation (Lash and Blood, 2004a). However, the locally moderate planar clay grain microfabric observed in some concretion samples suggests that the sediment had started to compact, at least locally, as concretions began to form.

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Interpretation 

Figures 5-2 – 6-2 

	Figure 5-2. Porosity versus depth of burial relationships for shale and clayey sediments (after Rieke and Chilingarian, 1974) showing the estimated maximum PFRD depth range based on the mean and the upper and lower standard deviation ranges for the calculated paleoporosity of the Rhinestreet shale. The dashed normal compaction trend, which was used to define the maximum depth for the Rhinestreet shale PFRD, is from Ham (1966); all other trends predict a shallower PFRD. MBD = modeled maximum burial depth of the Rhinestreet shale.
	Figure 6-1. Calculated PFRD values for the Rhinestreet shale (fp = 37.8%) using empirical algorithms. Shaded interval defines the PFRD range based on comparison of the Rhinestreet fp with published normal compaction curves for shale. BB = Baldwin and Butler  (1985); FD = Falvey and Deighton (1982); GL = Gallagher and Lambeck (1989); HA = Hansen (1996); HGY = Hegarty et al. (1988); H = Hermanrud et al. (1998); HG = Huang and Gradstein (1990); LR = Liu and Roaldset (1994); SC = Sclater and Christie (1980); T = Tingay et al. (2000); V = Velde (1996); MBD = modeled maximum burial depth of the Rhinestreet shale. The majority of calculated PFRDs fall within the depth range estimated from comparison with published compaction trends.
	Figure 6-2. Upper Devonian-Mississippian stratigraphic relations of western New York state (Lake Erie District) and northwest and north-central Pennsylvania. Stratigraphic columns, including thicknesses, are modified from Lindberg (1985). Devonian time scale (including the Devonian-Mississippian boundary) is from Kaufmann (2006); Mississippian time scale is from Gradstein et al. (2004). Note the marked increase in sedimentation rate halfway through the Famennian stage, perhaps glacio-eustatic in origin (e.g., Veevers et al., 1987); that may have been responsible for the onset of disequilibrium in the Upper Devonian Rhinestreet shale.

 

The most obvious measure of gravitational compaction strain sustained by a volume of sediment following accumulation on the seafloor is the change in layer thickness from the concretion into correlative layers of the encapsulating shale. We measured the thickness of bedding or a bedding interval inside concretions (T_i) (T_d in Figure 2-12) (and the thickness of that same interval in the shale (T_o) (T_c in Figure 2-12), a presumed proxy for the original seafloor thickness of the host sediment. Gravitational compaction strain of

the shale outside the strain shadow of the concretion, ε₃, is calculated by the following expression,

                                                                                                           

e₃=(T_i-T_o)/T_i

 

The mean e₃ of the Rhinestreet black shale based on the analysis of 118 concretions and encapsulating shale throughout the unit, expressed as a negative value, is –0.518 or 51.8% (± 4.9%). This value is noteworthy because normally compacted marine shales typically compact more than 65% upon burial to depths comparable to the maximum burial depth of the Rhinestreet shale.

 

Compaction strain of the Rhinestreet shale can be used as a measure of the porosity achieved at the termination of gravitational mechanical compaction if we assume that all volume loss was caused by vertical shortening, a reasonable assumption based on the lack of layer-parallel shortening caused by Alleghanian tectonics in rocks of the Lake Erie District. Compaction strain is converted to paleoporosity, f_p, by the following equation derived by Jacob (1949).

 

f_p =(f_o +100e₃)/(e₃ +1)

 

in which f_p is expressed as volume percent.

 

However, in order to calculate the f_p of the Rhinestreet shale, we first must obtain a reasonable value for the porosity of the sediment at the onset of normal compaction, f_o. Textural evidence described in more detail by Lash and Blood (2004a) relates the formation of Rhinestreet concretions to the passive precipitation of diagenetic carbonate in void spaces of the organic-rich host sediment. Thus, the estimated Rhinestreet sediment porosity at the time of concretion growth, based on the volume percent carbonate cement (e.g., Raiswell, 1976; Gautier, 1982), varied from 74 to 93%. However, some authors have suggested that the porosity of newly deposited clayey sediment decreases from as much as 90% to perhaps 60-65% within a decimeter or so of the seafloor (Weller, 1959; Von Engelhardt, 1977; Magara, 1978; Luo et al., 1993). Moreover, Kawamura and Ogawa (2004) demonstrated an especially rapid reduction in void ratio of pelagic clay equal to a 5% drop in porosity down to a depth of 10 cm below the seafloor. Luo et al. (1993) postulated that such marked losses of porosity within several meters to a few tens of meters of the seafloor should be considered a continuation of the depositional process rather than the initial phase of normal load-induced mechanical compaction. Thus, we interpret the range in CaCO₃ volume in analyzed Rhinestreet concretions (74 – 93%) to reflect the rapid occlusion of porosity as the water-rich carbonaceous clay passed into and through the zone of anaerobic methane oxidation where concretionary growth occurred (Lash and Blood, 2004a). Indeed, the strong tendancy of organic matter to absorb water thereby favoring a very open depositional microfabric results the rapid collapse of the clay grains into a preferred orientation very early (and at very shallow depth) in the diagenetic history of these types of deposits (e.g, Meade, 1966; Keller, 1982). Thus, we arbitrarily set f_o = 70% for our calculation of the Rhinestreet shale f_p. Our calculated Rhinestreet shale f_p using Jacob’s equation is 37.8% (±7.1%), a value markedly higher than that expected for shale normally compacted to the modeled 3.1 km maximum burial depth of the Rhinestreet shale (Lash and Blood, 2006).

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Conclusions 

Strain analysis of overburden-induced differential mechanical compaction of shale around early (and shallow) formed carbonate concretions in the Rhinestreet shale indicates that the host shale was mechanically compacted ~ 58%, less than that expected for shale buried to 3.1 km, the modeled maximum burial depth of the Rhinestreet shale. Using a reasonable assumption regarding the porosity of the organic-rich sediment at the onset of normal compaction, the calculated compaction strain was translated to a paleoporosity of 37.8%, well in excess of the actual porosity of the Rhinestreet shale as determined by mercury capillary injection pressure measurements. Normal compaction of the Rhinestreet shale was halted well before it entered the oil window as a consequence of the pre-catagenic elevation of pore pressure above hydrostatic. The depth at which the Rhinestreet shale was overpressured, the paleo-fluid retention depth, was estimated by (1) comparison of the paleoporosity with published normal compaction curves and (2) use of several empirically derived porosity-depth algorithms describing the normal compaction of shale. The onset of overpressuring of the Rhinestreet shale appears to have taken place between 850 and 1,380 m below the seafloor, not even half way to its maximum burial depth. The most likely explanation for the early and relatively shallow onset of overpressure in the Rhinestreet shale is disequilibrium compaction. The marked increase in sedimentation rate from the Frasnian and early Famennian (30 m Ma^-1) to the latter half of the Famennian (118 m Ma^-1) followed by a sharp decrease in sedimentation rate in the Mississppian suggests that disequilibrium compaction was induced toward the end of the Famennian in response to an acceleration of the rate of progradation of the Catskill Delta Complex perhaps induced by a late Famennian glacio-eustatic event. The presence of ~1.1 km of Devonian strata on the base of the Rhinestreet shale suggests that the PFRD must have been at a depth of ~ 1,100 m, well within the estimated range of the PFRD based on published compaction curves and porosity-depth algorithms.

 

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