B1: THE UPPER DEVONIAN RHINESTREET BLACK SHALE OF WESTERN NEW YORK STATE-EVOLUTION OF A HYDROCARBON SYSTEM
Gary G. Lash
Department of Geosciences
State University College
Fredonia, NY 14063 USA
and
DAVID R. BLOOD
Chesapeake Appalachia L.L.C.
Charleston, WV
INTRODUCTION
Organic-rich, low permeability, sometimes fractured, shale deposits, once ignored by drillers seeking more readily understood plays and faster returns on their investments, are boosting the fortunes of mid-sized producers across the United States and becoming an increasingly important resource base. This burgeoning interest is driven largely by increased natural gas prices, improved completion technologies and a lack of conventional reservoir targets. However, there is no universal model applicable to each and every unconventional or continuous-type reservoir. Indeed, most vary in terms of basic stratigraphic facies distribution, mineralogy (i.e., quartz content and clay type and content), fracture parameters (length, orthogonal spacing, connectivity, anisotropy), porosity and permeability, and rock mechanical properties. There clearly is a need to further our understanding of these economically important deposits. This field trip focuses on the evolution of the heavily fractured Upper Devonian Rhinestreet black shale, which has served not only as a hydrocarbon source rock, but also as its own seal and reservoir, the essence of the unconventional reservoir. Specifically, we address the Rhine street shale in terms of the hydrocarbon system, which comprises three basic elements (Demaison and Huizinga, 1994). The first component, charge, is the hydrocarbon volume available for entrapment and is dependent upon source rock richness and volume. Migration, the second parameter, can be thought of in terms of the lateral movement of hydrocarbons via permeable carrier beds versus vertical migration via faults and, more to the point of the Rhinestreet shale, fractures. The final component of the petroleum system is entrapment, which involves consideration of those rock properties that would favor the arrest of migrating hydrocarbons.
We will consider the Rhinestreet shale from deposition through early diagenesis, including mechanical compaction, followed by the early and relatively shallow generation of abnormally high pore fluid pressures caused by a marked increase in sedimentation rate at the end of the Devonian. Soon after this, at the onset of the Alleghanian orogeny, the Appalachian Basin of western New York state and western Pennsylvania was uplifted resulting in the initial phase of fracturing in the Rhinestreet shale. This was followed by rapid burial of the organic-rich rocks to the oil window and the generation of natural hydraulic fractures (joints) in response to catagenesis. Indeed, the jointing history of these rocks provides a record of the complex overpressure history of the Rhinestreet shale and other black shale units of the western New York state region of the basin.
Results reported herein represent the record of six years of work on the Upper Devonian shale-dominated succession exposed along the Lake Erie shoreline and general vicinity, the Lake Erie District (LED). This work is now being extended into the subsurface of western New York state, northwest Pennsylvania, and northeast Ohio by use of wire line logs. During his involvement in the study of the Rhinestreet shale, DRB was supported by two SUNY Fredonia Summer Undergraduate Research Fellowships. Most recently, his work, which constitutes the better part of his MS thesis at the University of Buffalo, was supported by an AAPG grant-in-aid. We acknowledge Peter Bush and his staff at the University of Buffalo, South Campus Instrumentation Center, School of Dental Medicine, for their unflagging help with the electron microscopy, Ross Giese and Anja Dosen, University of Buffalo, for the XRD analyses, and Taury Smith, Rich Nyahay, and James Sandor of the Reservoir Characterization Group at The New York State Museum, Albany, New York. Finally, we thank Terry Engelder for many fruitful discussions on compaction, jointing, the geology of the Catskill Delta Complex, and more in the field, over the phone, and via E-mail.

B1: THE UPPER DEVONIAN RHINESTREET BLACK SHALE OF WESTERN NEW YORK STATE-EVOLUTION OF A HYDROCARBON SYSTEM

STOP 1. PIKE CREEK/LAKE ERIE SHORELINE
Significance:
I) the preferential generation of early NS-trending joints at the contacts of gray shale and black shale;
II) geographic variations in the magnitude of the pre-Upper Devonian or Taghanic unconformity and the significance of the so-called "Driller's Tully";
III) preferential propagation of NW and ENE joints in the Rhinestreet black shale.
I. Working down creek toward Lake Erie, the first exposure encountered reveals the contact of the West River gray shale and overlying Middlesex black shale (Fig. 40). A prominent NS-trending joint set as well as ENE-trending joints can be observed here (Fig. 41). The NS joints terminate at the contact or a short distance above it, a theme common to other Upper Devonian black shale units of western New York state as described earlier. We interpret the generation of the NS joints to be a response to the passage of a peripheral bulge produced by loading of the Laurentian plate edge at the onset of the Alleghanian orogeny across the western New York state region of the Appalachian Basin. Reduced compressive stress as a consequence of uplift-related thermoelastic loading initiated joints in higher modulus diagenetic carbonate. Propagation of these joints through the host gray shale was aided by moderately overpressured pore fluids at the top of the gray shale, perhaps sealed in by the overlying low permeability black shale.
II. The mouth of Pike Creek exposes the Middle-Upper Devonian interval from the Tichenor limestone (perhaps the top of the Ludlowville shale, depending upon the lake level) to roughly two-thirds the way up the Middlesex black shale (Fig. 40). This exposure provides the opportunity to (1) consider the magnitude of the Taghanic unconformity in the subsurface of western New York state and (2) offer some preliminary thoughts on the subsurface stratigraphy of the Moscow-Tully interval in this region of the basin. De Witt and Colton (1978) placed the unconformity at the contact of the Penn Yan shale of the Geneseo Formation and the North Evans limestone of the underlying Moscow shale, Hamilton Group (Fig. 40). Rocks absent this exposure include, in descending order, an unknown thickness of the Penn Yan shale and the underlying Geneseo black shale, the Tully limestone, and some thickness of the Moscow shale (Windom Member). The hiatal extent or magnitude of the Taghanic unconformity diminishes to the south and east as the Moscow shale increases in thickness and the Geneseo black shale and Tully limestone appear and thicken (Fig. 42). De Witt et al. ( 1993) maintained that the disconformity confined the Middle Devonian Geneseo black shale to the eastern region of the basin in New York state; however, Upper Devonian black shales, including the Dunkirk, Pipe Creek, and Rhinestreet shales were largely restricted to the western part of the basin by the unconformity. However, the distribution of the Middle Devonian Oatka Creek black shale of the Marcellus Formation (Hamilton Group) appears to have been unaffected by the Taghanic unconformity.
We are using wireline logs to define stratigraphic trends in the subsurface of western New York state, northwest Pennsylvania, and northeast Ohio. Figure 40 links the stratigraphy observed at the mouth of Pike Creek (as well as the cliff exposure to the northeast along the beach) to the Eckhardt-Agle 1 gas well (API# 17937) located ∼10 km southeast of this location (Fig. 42). The Middlesex shale and Tichenor limestone are easily recognized on the gamma-ray log. Locating the North Evans-Genundewa interval, and, therefore, the Taghanic unconformity is more difficult. The Genundewa limestone, ∼21 - 30 cm thick on Eighteenmile Creek between the beach exposure and the Eckhardt-Agle 1 well (Fig. 42), is difficult at best to be resolved on a normal gamma-ray log. Thus, our placement of the unconformity is based on extension of the subsurface stratigraphy from the east, where the Tully limestone and overlying Geneseo black shale are present, to the Eckhardt-Agle 1 well. There is a marked increase in the thickness of the West River shale to the south away from the lakeshore (Fig. 40). Cross-section 1 (Fig. 42) carries the interval exposed at the mouth of Pike Creek (i.e., the base of the Tichenor limestone to the base of the Middlesex shale) east-southeast into northeast Alleghany County. The Geneseo black shale first appears in Wyoming County followed in northwest Alleghany County by the Tully limestone. Notice also that the West River/Penn Yan interval displays an almost six-fold increase in thickness to the east. The Tichenor limestone also thickens to the east. In sum, cross-section 1 displays, over a relatively short distance, the westward onlap of progressively younger deposits over the Taghanic unconformity as well as the truncation of underlying units, both typical traits of the unconformity elsewhere in the Appalachian Basin (Hamilton- Smith, 1993).
An especially interesting unit that we are mapping in the subsurface, the so-called "Driller's Tully," is a carbonate interval that is less clean (i.e., 20 API units or more) than the true Tully limestone. There is a good amount of confusion regarding this unit; Oliver et al. (1969) placed the "Driller's Tully" of Erie County, Pennsylvania, and Chautauqua County, New York, well down the Hamilton Group, perhaps at the level of the Tichenor limestone. Similarly, Wright (1973) referred to the "Driller's Tully" as limestone "B" and suggested that the Tichenor limestone is a tongue of limestone " B." This interpretation was accepted by de Wltt et al. (1993) who also echoed Wright's proposition that the Tully limestone is not present in the subsurface of Chautauqua County and northwest Pennsylvania. De Witt et al. (1993) pointed out that conodonts collected from the upper part of the "Driller's Tully" in a core from the Eastern Gas Shales Project well PA3 in Erie County, Pennsylvania, included elements of Polygnathus linguiformis,
We agree with Rickard (1989) that the Tully limestone is present in the subsurface of Chautauqua County and northwest Pennsylvania. Indeed, our analysis of close-spaced wells in far western Chautauqua County indicates that Wright's (1973) Tichenor limestone and limestone "B" is actually the Tully limestone and underlying "Driller's Tully," the latter resting unconformably on the Tichenor limestone. However, we interpret the "Driller's Tully" to be a discrete unit in erosional contact with the Moscow shale, and locally the Tichenor limestone, rather than a stratigraphic equivalent of the Moscow shale and Tichenor limestone as suggested by Rickard (1989). Truncated calcareous intervals of the Moscow shale and locally abrupt changes in thickness of the "Driller's Tully" confirm the erosional contact of this unit with underlying deposits. The "Driller's Tully" is recognized in the subsurface of northern Allegany County, that area of Chautauqua County where the Tully is present (Fig. 43) and parts of Warren and Erie counties, Pennsylvania. The contact of the "Driller's Tully" and overlying Tully limestone is a thin shaley interval (Figs. 42 and 43). However, the local absence of this interval and as well as the occasional absence of the "Driller's Tully" beneath the Tully suggests that the contact of these units is an unconformity. Cross-section 2 extends southwest along the Lake Erie shoreline from the Eckhardt-Agle 1 well to Westfield, New York, northern Chautauqua County (Fig. 43). Noteworthy features in this section include: (1) the thickening of the Tichenor limestone to the west of Erie County; (2) the appearance of the Tully and underlying "Driller's Tully" in the Westfield area; and (3) the thinning of the interval between the base of the Tichenor limestone and the base of the Middlesex shale to the southwest (the Middlesex and Tichenor are separated by less than one meter of gray shale in the Muscarella I well; Fig. 43) followed by its rapid thickening as the Tully reappears near Westfield. This latter point is consistent with localized uplift. That the Middlesex shale and younger deposits show no evidence of this event provides clues to the timing of uplift, perhaps a reflection of localized tectonic activity of the post-Tully basin in advance of the Acadian thrust load and related forebulge (e.g., Tankard, 1986; Beaumont et al., 1988; Hamilton-Smith, 1993; Ver Straeten and Brett, 2000; Filer, 2003). Indeed, Filer (2003) defined a Famennian forebulge in West Virginia based on isopach and sedimentary facies maps that, when traced north, extends through the eastern Chautauqua County-Erie County region. We are just beginning to consider this event in a regional sense based on subsurface information from western New York state, northwest Pennsylvania, and northeast Ohio.
The Taghanic unconformity appears to be erosional throughout western New York state (Johnson, 1970; Wright, 1973; Baird and Brett, 2003). Heckel (1973) maintained that the contact is a slight diastem in central New York state, increasing in stratigraphic magnitude to the west where the disconformity represents an increasing period of non-deposition. However, our ongoing investigation of the subsurface stratigraphy of western New York state reveals a more complex situation made more so by the presence of the "Driller's Tully." Indeed, the initial event may have been that phase of erosion that occurred before deposition of the "Driller's Tully," which locally rests unconformably on the Tichenor limestone. Heckel (1973) pointed out that the Geneseo black shale conformably overlies the Tully limestone east of Cayuga Lake. West of this meridian, however, the contact becomes one of disconformity as progressively younger deposits onlap westward above the unconformity (de Witt et al., 1993) until the Geneseo shale feathers out in easternmost Chautauqua County into eastern Wyoming County (Fig. 42). Rickard (1989) suggested that the post-Tully unconformity was entirely subaqueous. Similarly, Baird and Brett (1991) postulated that the erosional contact of the Geneseo black shale and underlying Tully limestone is a submarine erosional feature (maximum flooding surface) related to the abrupt increase in water depth, likely the result of the sudden sinking of a forebulge in response to tectonic loading to the east (e.g., Hamilton-Smith, 1993).
III. Moving northeast along the beach for perhaps a kilometer enables us to observe in cliff exposure the entire Moscow shale - basal Rhinestreet shale interval (Fig. 40). Note the diagenetic carbonate intervals within the Cashaqua shale. The heavily fractured Rhinestreet black shale at the top of the exposure is easily recognized from the beach. The upper several meters of the Cashaqua and lower few meters of the Rhinestreet shale carry the older NS-trending joints, similar to what we saw on Pike Creek and likely for the same reasons. NW- and younger ENE-trending joints, most densely formed in the basal organic-rich interval of the Rhinestreet shale, formed as a consequence of catagenesis as these source rocks approached and finally reached their modeled maximum burial depth of ∼3.1 km (see Fig. 9). This exposure demonstrates the strong stratigraphic control on jointing in these rocks. The NW joints probably formed soon after the rocks had entered the oil window and at that time that the basin was feeling the same remote stress field responsible for emplacement of the Blue Ridge-Piedmont Megathrust. The younger ENE-trending joints propagated after the Rhinestreet shale had been in the oil window for a protracted period of time, thus their greater density. The stress field under which these joints formed reflected the oblique convergence of Laurentia and Africa and had been responsible for the generation of the early (perhaps Chesterian) ENE-trending coal cleat throughout the Appalachian Basin and similarly oriented joints in the deeply buried Frasnian black shales of the Finger Lakes District (Engelder and Whitaker, 2006).

STOP 2. EIGHTEENMILE CREEK/ROUTE 20 BRIDGE
Significance:
I) approximately the lower half of the Rhinestreet shale can be observed in the east wall of the creek exposure immediately south of the Route 20 bridge over Eighteenmile Creek;
II) the presence of NS joints at the contact of the Cashaqua gray shale and overlying Rhinestreet shale, and the high density of NW- and younger ENE-trending joints at the organic-rich base of the Rhinestreet shale;
III) basin modeling of measured vitrinite reflectance and the possibility of vitrinite suppression;
IV) recognition of the Cashaqua-Rhinestreet contact in the subsurface- gamma-ray vs. bulk density logs. I. A bit less than the lower half of the Rhinestreet shale is visible in the cliff exposure immediately north of the Route 20 bridge. We will have an opportunity to look at the concretion horizon - the middle concretion horizon- visible at the top of the cliff- ∼34 m above the bottom contact (see Fig. 1) - at Stop 4. The Rhinestreet section displayed in the cliff contains two intervals of gray shale and thin-bedded black shale at ∼10.0- 12.5 m and 27.5-29.3 m above the contact (Fig. 1). These horizons, which probably represent eustatic oscillations, can be observed in gamma-ray and bulk density logs (Fig. 44). Moreover, the lower gray shale interval contains a lenticular limestone bed as thick as 20 cm, one of Luther's (1903) scraggy layers and our lower scraggy layer (see Fig. 1 ). The geochemistry of the scraggy layers, discussed above, confirms their early diagenetic origin and suggests that they formed in a manner similar to the concretions. The scraggy layers, which are consistently found associated with organic-lean gray shale intervals (see Fig. 1), are recognized in gamma-ray logs as a sharp deflection to the left (Fig. 44) and may serve as markers for basinwide correlation. The fact that the limestone deflection on the gamma-ray logs cannot always be correlated among neighboring offset wells demonstrates the discontinuous nature of these deposits, which is evident in field exposures.
II. The Cashaqua shale-Rhinestreet shale contact is exposed beneath the bridge. The Cashaqua shale along Eighteenmile Creek comprises ∼11.5 m of silty, organic lean (TOC 0.7%) gray shale and nodular limestone horizons. The overlying Rhinestreet shale is composed of ∼54 m of organic-rich black shale, much lesser gray shale, sparse thin siltstone beds, and several carbonate concretion horizons and nodular limestone intervals, the latter termed "scraggy layers" by Luther (1903; see Fig. 1 for the measured Rhinestreet section along Eighteenmile Creek). TOC content of the Rhinestreet shale at its base is 8.09%, diminishing upward (Fig. 1; Lash and Blood, 2004b; Lash et al., 2004). Vitrinite reflectance of the Rhinestreet shale at its base along Eighteenmile Creek is 0.74%, indicating that these rocks entered the oil window during their burial history (Lash, 2006c).
Joints of the three dominant sets carried by the Rhinestreet shale are represented at this exposure. Several long NS-trending joints, can be seen at the contact. We contend that uplift of the Upper Devonian sedimentary succession in response to passage of a peripheral bulge at the onset of the Alleghanian orogeny, and resultant thermoelastic loading, induced the NS joints in the high modulus carbonate. These joints propagated upward under the influence of moderately overpressured pore fluid at the top of the Cashaqua shale. Elevated pore pressure may have resulted from a combination of disequilibrium compaction and the tight, impermeable nature of the organic-rich Rhinestreet shale. The lack of bioturbation in the finely laminated organic-rich sediment preserved a strong strength anisotropy that precluded easy vertical movement of fluid. The squeezing of ductile organic matter into voids and pore throats further reduced the permeability of the black shale (Lash, 2006b).
NW- and ENE-trendingjoints are densely formed in the black shale immediately above the contact with the Cashaqua shale (station EMC 13 and 16, Fig. 38). A similar relationship of joint density and TOC has been observed in the Dunkirk shale along the Lake Erie shoreline as well as in the Geneseo shale of the Finger Lakes District (Loewy, 1995; Lash et al., 2004). The correlation of joint density with TOC provides compelling evidence that the joints formed as natural hydraulic fractures as a consequence of the transformation of organic matter to hydrocarbons when these rocks were buried to the oil window during the Alleghanian orogeny (Lash et al., 2004; Lash and Engelder, 2005).
III. As noted earlier, we used the EASY%Ro kinetic model of vitrinite reflectance (Sweeney and Burnham, 1990) to model the burial/thermal history of the Rhinestreet black shale. Thermal modeling by use of the EASY%Ro algorithm requires knowledge of (1) the age(s) of the unit(s) of interest (the base of the Rhinestreet shale), (2) at least a partial thickness of the local stratigraphic sequence, and (3) the measured vitrinite reflectance of the unit(s) of interest (vitrinite reflectance of the base of the Rhinestreet shale = 0.74%). Our model for the Rhinestreet shale, which includes passage of the early Alleghanian peripheral bulge (see Fig. 9), requires ∼1.5 km of Permian strata in this area of the basin, an amount close to that modeled Beaumont et al. (1988) (Fig. 45).
Our method of modeling the burial/thermal history of the Rhinestreet shale depends on accurate %Ro values. However, several workers (e.g., Nuccio and Hatch, 1996; Rimmer and Cantrell, 1988; Rimmer et al., 1993; Rowan et al., 2004) have recognized the likely occurrence of vitrinite suppression in Devonian-Mississippian black shales, including those of the Appalachian Bain. Suppression of vitrinite leads to an underestimation of thermal maturity and, therefore, an underestimated burial depth. Suppression is most readily recognized by anomalously low %Ro on maturity profiles of wells or stratigraphic columns (e.g., Price and Barker, 1985). There are several lines of evidence suggesting that Devonian black shales of the Appalachian Basin have been affected to some extent by vitrinite suppression. Rimmer and Cantrell (1988), based on a vitrinite reflectance gradient derived from coal rank trends, claim that measured %Ro values of the Cleveland Member of the Ohio shale in eastern Kentucky are suppressed by as much as 0.3 - 0.5 %Ro. More recently, Rowan et al. (2004), in the course of their burial/thermal modeling of a cross-section from central West Virginia to northeast Ohio, demonstrated that the predicted %Ro of Devonian shale is locally twice the measured %Ro. Rowan et al. (2004) constrained their burial/thermal model with measured %Ro of Pennsylvanian coals and color alteration index values of conodonts recovered from Ordovician and Devonian limestones.
We suggest that Middle and Upper Devonian black shale of the western New York state and northwest Pennsylvania region of the Appalachian Basin was also been affected by suppression. Our data base includes published thermal/maturity values (Weary et al., 2000; Repetski et al., 2002) and%Ro and Rock-Eval data from our work on the Upper Devonian shale succession of the Lake Erie District. We limited our data to the western New York state and northwest Pennsylvania region of the basin in order to assume a common thermal history for these deposits. A plot of Rock-Eva! HI versus %Ro of Appalachian Basin black shale samples, principally from the Marcellus shale, reveals a trend similar to that observed by Nuccio and Hatch (1996) for the Upper Devonian-Mississippian New Albany shale, which has been affected by vitrinite suppression (Fig. 46). The higher HI samples, including samples collected from the Dunkirk and Rhinestreet shales, fall along a relatively narrow %Ro trend; however, at an HI of ∼125 mg/g, measured %Ro increases rapidly from ∼0.75 to 2.5 as HI drops to near zero (Fig. 46). This plot appears to show suppressed values in the lower to mid oil window(%Ro; = ∼0.7 to 1.0), consistent with Lo's (1993) findings regarding vitrinite suppression of hydrogen-rich organic matter. An especially interesting subpopulation of Marcellus data exists for the Finger Lakes District of New York state and adjacent Potter and Tioga counties, Pennsylvania (Fig. 46). These data are defined by %Roranging from 0.5 to 1.0 and HI 55 (Fig. 46). These samples, which appear severely suppressed, were collected from that part of the basin affected by emplacement of Cretaceous ultramafic intrusions (Kay et al., 1983).
Upper Devonian black shales exposed along the Lake Erie shoreline show a good amount of variation in measured %Ro over a relatively small distance (see Fig. 7). Measured %Ro in a well cutting sample collected from the base of the Rhinestreet shale in northern Chautauqua County is 0.51%. The %Ro of the same stratigraphic interval ∼40-45 km to the northeast is 0.73 - 0.77 based on the analysis of four outcrop samples. Finally, a sample collected from the base of the Rhinestreet 7.25 km to the east on Cazenovia Creek has a %Ro of 0.59. We recognize similar erratic variations in the measured %Ro for Dunkirk shale samples. Rowan et al. (2004) suggest that variations in the type of organic matter in marine sediments may account for erratic variations in %Ro (e.g., Ujiie et al., 2004).
Vitrinite suppression has also been attributed to the overpressuring of rocks (McTavish, 1978 1998; Carr, 2000). Pervasive natural hydraulic fractures in the Rhinestreet shale provides evidence that these rocks were overpressured during their burial history, which could have contributed to suppression of vitrinite reflectance. Regardless of its mode of origin, suppression of vitrinite reflectance is a phenomenon that should be taken into account in any exploration and production considerations of Devonian black shales.
IV. The contact of the Cashaqua gray shale and overlying Rhinestreet black shale is readily defined in the field; both color contrast and relative weathering profile provide easy means for making this recognition. The contact is rather obvious in gamma-ray logs, which provide a measure of the intensity of natural gamma-rays (uranium concentration) emitted by a formation (Fig. 44). Still, the initial deflection marking the contact is not what one might expect based on the abrupt contact between the low-TOC (<0.7%) Cashaqua shale and the carbonaceous (TOC>8%) Rhinestreet shale. However, the Cashaqua-Rhinestreet contact is very obvious in bulk density logs (Fig. 44), no doubt a reflection of the low density of organic matter relative to shale minerals (Smith and Young, 1964; Schmoker, 1979). Schmoker (1981) claimed that the bulk density log is a more accurate proxy for organic matter content than the gamma-ray API value. Indeed, the strong inverse relationship of bulk density and TOC in Upper Devonian black shale of western New York state has been confirmed by a proprietary data set we have had the opportunity to study (Lash, 2006c). It is interesting to note that the organic-rich Pipe Creek black shale (TOC = 3.2 - 7.9%) is not easily recognized on the gamma-ray log, but is readily picked on the bulk density log (Fig. 44). On the other hand, the base of the overlying Dunkirk shale is readily recognized on both types of logs (Fig. 44).
Luning and Kolonic (2003) have pointed out that many black shale deposits show a strong correlation of uranium, manifested by gamma-radiation in API units, and TOC. However, they also note those examples where the correlation is weak at best. The basal Rhinestreet shale and the Pipe Creek shale provide examples of the latter phenomenon. Indeed, a spectral gamma-ray log from a well in northeast Chautauqua County reveals the base of the Rhinestreet shale to be marked by an increase in uranium from 6 to 10 ppm; the Pipe Creek shale is defined by an increase of only 2 ppm. Luning and Kolonic (2003) suggest that the precipitation of uranium from seawater onto the surface of organic particles under anoxic conditions is governed by several factors, including carbonate content, sedimentation rate, and primary uranium content of the seawater. Upper Devonian shales of western New York state are defined by very low CaCO3 abundances ruling out the first factor. Luning and Kolonic (2003) point out that the longer organic matter is in contact with seawater, the greater the amount of uranium that can precipitate onto organic grains. Low sedimentation rates favor high uranium/TOC ratios. The anomalously low API values of the most organic-rich interval of the Rhinestreet shales (as deduced by TOC analysis of field samples and bulk density values determined from wireline logs) may indicate a high sedimentation rate of this interval such that lesser amounts of uranium were fixed onto organic particles. The same can be said of the Pipe Creek shale. The organic-rich base of the Dunkirk shale (Lash et al., 2004; Lash and Engelder, 2005), however, defined by the highest API value and lowest bulk density of the unit, would have accumulated relatively slowly.
Finally, it is possible that the introduction of low uranium freshwater into the Devonian Sea, such as might be expected as a consequence of the reduction of polar ice volume, was responsible for some of the variations in gamma-ray response observed in wireline logs of the base of the Rhinestreet shale and especially the Pipe Creek shale, which has been correlated with the Lower Kellwasser Bed recognized in Europe (Over, 1997). The concentration of uranium in seawater is an order of magnitude greater than freshwater (Broecker and Peng, 1982). The presence of the diagnostic uranium signature of the Pipe Creek shale in well logs recovered from northwest Pennsylvania, western New York state, northeast Ohio, and northernmost West Virginia suggests a basinwide cause. The Pipe Creek interval is a particularly interesting horizon. McGhee (1990) argued that this interval is a significant crisis interval similar to the younger Frasnian-Famennian boundary. In fact, he suggested that the Pipe Creek interval was more critical in terms of the maximum number of species extinctions and the extinction rate. It has been suggested that the Kellwasser Beds, including the Pipe Creek shale, resulted from eustatic transgressions perhaps caused by a reduction in polar ice volume (Sandberg et al., 1988; Buggisch, 1991 ; Joachimski and Buggisch, 1993). It may be that the same global mechanism that created the Pipe Creek crisis - rapid climatic warming - also accounts for the unique wire line log signature of the Pipe Creek shale.

STOP 3. STURGEON POINT
Significance:
I) comparison of the degree of jointing in black shale versus gray shale and the preferential generation of NS systematic joints in gray shale at contacts with overlying black shale;
II) carbonate concretions in black shale and analysis of compaction strain;
III) variation in the thickness of the bottom gray shale interval of the Rhinestreet shale.
I. Depart the parking area walking northeast toward the bedrock cliff a few hundred meters distant. We will follow this exposure of gray and black shale of the Rhinestreet shale for perhaps 0.75 km. The 2.75-m-thick gray shale interval exposed at the west end of the exposure is the equivalent of the lower gray shale interval of the Rhinestreet shale on Eighteenmile Creek (see Fig. 1; 9.9-12.5 m). The gray shale includes several horizons of high aspect ratio (lenticular) carbonate concretions and thin grayish-black shale layers. One can readily observe differences in the degree (density) of jointing between black and gray shale in this exposure. Black shale above and below the gray shale carries NW joints and younger ENE-trending systematic joints (Fig. 4 7). NW and ENE joints can be observed in the gray shale, but these deposits also contain the older NS trending systematic joints (Fig. 47). The black shale clearly is more densely jointed than the gray shale, a recurring theme throughout the Appalachian Plateau (Fig. 48; Loewy, 1995; Lash et al., 2004). Moreover, ENE joints are more densely formed (i.e., closer to saturation) than are the older NW joints. This latter observation can also be made of ENE and NW joints in the gray shale (Fig. 48). NS-trending systematic joints in the gray shale are more densely formed than are the younger NW joints. As described earlier, the NS joints are interpreted to have originated in the higher modulus carbonate concretions as a consequence of thermoelastic loading caused by passage of a peripheral bulge across the Appalachian Basin at the onset of the Alleghanian orogeny. Indeed, it is more common for a NS joint to penetrate or cut a higher modulus carbonate concretion than it is for ENE and NW joints to cleave concretions. Note also that very few NS joints penetrate more than a few tens of cm into the overlying black shale; more often than not, they are arrested at the base of the organic-rich deposits. This exposure presents a nice example of the stratigraphic control on jointing in these rocks.
Continue NE along the beach toward a large drainage pipe in the cliff. Careful observations of the gray shale horizon while working along the beach to this location would have revealed a gradual decrease in thickness of this interval. The thickness of the gray shale immediately east of the drainage pipe (determined by digging through beach sand to black shale) is ∼80 cm, markedly reduced from what is was at the start of this trek along the beach. Note also that the gray shale is not as complexly jointed here; indeed, the median spacing of NS joints is 50% greater than at the western end of the exposure (Fig. 48). Finally, the thickness of the gray shale interval at the NE end of the exposure is at least 1.5 m. NW- and NS trendingjoints are carried by the gray shale in this area. Further, NS joints are more densely formed at this location of the exposure than elsewhere along the beach (Fig. 48).
There is a school of thought that holds that all jointing in the exposed Devonian succession of the Appalachian Plateau of western New York state is a consequence of post-Alleghanian (perhaps even post-glacial) uplift of the Appalachian Basin. This view fails to take into account the well-defined chronology of the joints (Engelder and Geiser, 1980; Lash et al., 2004) and the fact that the joints provide information regarding the orientation of the remote or lithospheric (Engelder and Whitaker, 2006) stress field that was responsible for their formation. There are likely subtle differences in rock properties between the black and gray shale that perhaps played some ancillary role in the jointing of these rocks. We suggest that the black shale, principally by virtue of its greater amount of ductile organic matter, was not as stiff (i.e., lower modulus) as the gray shale, a view at odds with the more common thinking regarding heavily fractured black shale. Accordingly, gray shale, as a consequence of its higher porosity and locally greater abundance of calcite cement (e.g., Lash, 2006b) would have had the higher modulus. Joints driven by fluid decompression, natural hydraulic fractures, form at great burial depth and, therefore, under high confining pressure. All else being equal, it would have been more difficult to initiate open mode cracks in higher modulus organic-lean gray shale under these conditions. Moreover, movement of the black shale into the oil window and resultant transformation of kerogen to hydrocarbons would more readily create a negative effective stress necessary for the propagation of natural hydraulic fractures, their growth direction being controlled by the remote stress field. NW-trending joints were produced by an Alleghanian remote stress field that was responsible for emplacement of the Blue Ridge-Piedmont Megathrust sheet; ENE joints, however, reflect a longer-standing lithospheric scale stress field related to continued oblique approach of the African portion of Gondwana against the Laurentian plate boundary (Engelder and Whitaker, 2006).
The remaining question, then, relates to the preferential formation of NS joints in the gray shale as observed at this exposure. Passage of the peripheral bulge across the Appalachian Basin at the onset of the Alleghanian orogeny and related uplift of the Upper Devonian shale-dominated succession of the Lake Erie District subjected these rocks to thermoelastic contraction. The higher modulus rock - the gray shale would have carried a higher tensile stress and, therefore, would have been closer to the negative effective stress necessary to initiate joints. The inferred contrast in modulus between black and gray shale may explain why the NS joints terminate at gray shale-black shale contacts; i.e., the lower modulus black shale carried a lower tensile stress than the gray shale. However, the highest modulus rocks in the Rhinestreet shale were the carbonate concretions. As described earlier, concretions remained in compression during catagenesis so that fluid-driven NW and ENE joints were unable to penetrate them any more than a cm or so. However, during thermoelastic contraction, the concretions carried a high level of tensile stress (higher than that of the gray shale and certainly much higher than that of the black shale) making them more likely to fail under true tension. Initiation of the NS joints in the diagenetic carbonate was followed by their propagation through the host gray shale aided by a modest pore pressure.
In sum, this exposure, like others in this region of the Lake Erie District, provide evidence that systematic joints carried by the Upper Devonian shale succession resulted from two different loading mechanisms before post-Alleghanian uplift of the Appalachian foreland. However, we are investigating the intriguing possibility that an ENE-trending set of joints, some of which cut diagenetic carbonate, formed as a consequence of the uplift of the Appalachian Plateau. These joints appear to have been driven by thermoelastic contraction under a remote stress field perhaps induced by spreading at the Mid-Atlantic Ridge; thus their ENE strike.
II. This exposure also allows for the differentiation of concretionary carbonate found in gray shale from concretions common to black shale (spectacular examples of this latter type will be seen at Stop 4). The former are defined by high aspect ratios (they may better be described as lenticular diagenetic limestone) and lack obvious internal structures, principally layering. The scraggy layers are an example of this type of diagenetic carbonate. Carbonate concretions in the black shale have lower aspect ratios and display nicely preserved layers of the host organic-rich sediment. The above distinctions is readily made at this exposure.
Numerous small, internally laminated concretions can be observed in black shale within a meter or so of the upper contact of the gray shale interval, especially along the eastern stretch of the exposure. These concretions lend themselves to compaction strain analysis as described earlier (Fig. 49). Compaction strain is determined by measuring a stratigraphic interval within the concretion, Ti, and that same interval traced into the host shale but far enough beyond the strain shadow to provide a full measure of compaction strain, To (Fig. 49). Estimated compaction strain of six concretions at this exposure is 60.8%, ∼9% higher than the average for the Rhinestreet shale as discussed earlier. We believe that this reflects concretionary growth in a stratigraphic horizon that had not compacted to the extent that most of the unit did before the onset of normal compaction (see earlier discussion).
III. Moving SW to NE along the beach, we observed the thickness of the gray shale interval diminish from ∼2.75 m to ∼0.8 m; farther along, though the thickness increases to at least 1.5 m. We observed no evidence of faulting that might account for the described change in thickness. Perhaps we are looking at the effects of differential compaction (e.g., Conybeare, 1967; Finely, 1990; Ryder, 1996). Indeed, the local presence of flame-like structures at the top of the gray shale interval suggests that these deposits were rather mobile and perhaps characterized by moderately high pore pressures, as suggested by the NS joints. Indeed, the relatively low density of NS systematic joints in the middle interval of this exposure where the gray shale is thinnest may reflect the effects of differential dewatering and resultant differential compaction. Preferential dewatering of the gray shale could account for (1) the reduced thickness of the gray shale at this location of the exposure and (2) a reduction in pore pressure, which would not have favored the generation of NS-trending joints in these rocks. Clearly, much more work is needed to elucidate this hypothesis.

STOP4. EIGHTEEN MILE CREEK (main branch)
I) the contact of the Rhinestreet shale and overlying Angola gray shale;
II) general characteristics of the carbonate concretions.
I. Follow the creek upstream (to the left) to where the east branch of Eighteenmile Creek enters the main creek. Cross the east branch just upstream from this point and follow a trail through the woods for roughly several hundred meters or until the path approaches the main branch. Cross the creek and walk to where you can view the cliff exposure across the creek. The middle concretion horizon will be visible a meter or so above the waterline. The upper ∼21 m of the Rhinestreet shale and lower few meters of the Angola shale are exposed in the cliff (see Fig. 1). The Rhinestreet-Angola contact is transitional and marked by an increase in the frequency of less resistant (to the effects of weathering) gray shale layers, which can be observed both in the field at this stop and in well logs (Figs. 1 and 50). Luther (1903) suggested that the contact be placed at an easily recognizable discontinuous nodular limestone bed locally as thick as ∼30 cm, the upper scraggy layer (see Fig. 1 ), which can be seen near the top of the exposure. The upper scraggy layer can also be recognized in some gamma-ray logs by a sharp deflection to the left (e.g., Fig. 20).
II. Carbonate concretionary horizons like the one exposed here are conspicuous in Middle and Upper Devonian black shale units of central and western New York state (Dix and Mullins, 1987; Lash and Blood, 2004a,b), southwestern Ontario (Daly, 1900; Coniglio and Cameron, 1990) and Ohio (Clifton, 1957; Criss et al., 1988) suggesting that diagenetic conditions favorable to the formation of these features existed regionally. Concretions are especially useful in the analysis of the early post-depositional history of encapsulating shale, for not only can they preserve the depositional texture of the host sediment (e.g., Woodland, 1984; Duck, 1990), but they carry a record of the evolution of pore fluids carried by the sediment (e.g., Hudson, 1978; Coniglio and Cameron, 1990). Details of our work on the carbonate concretions of the Rhinestreet shale can be found in Lash and Blood (2004a,b) and are presented in an abbreviated format above.
The concretions visible here are among the most spectacular in the world. They are similar to those described from such well known units as the Kimmeridge Clay (Astin and Scotchman, 1988) and the Jet Rock (Hallam, 1962). Close inspection of the exposed Rhinestreet concretions reveals their laminated nature and presence of septaria. Our principal interest in the concretions has been their use as compaction strain indicators. However, before they could be used in this way, we had to understand their mode of origin and depth of growth, which was considered earlier. Most carbonate concretions of the Rhinestreet shale are found in three stratigraphically confined but laterally persistent horizons (see Fig. 1) - a lower horizon- 6 m above the base of the unit, a middle interval slightly more than halfway up the Rhinestreet (Stop 4), and an upper, less well defined, horizon ∼43 m above the base of the unit. Most concretions are oblate ellipsoids with maximum diameters and thicknesses ranging up to 2.7 m and 1.1 m, respectively (Lash and Blood, 2004b); coalesced concretions have also been observed and are locally abundant. We carried out a detailed analysis of the dimensions and general shapes of concretions of the lower and middle horizons (Fig. 51). The dimensional data for both horizons reveals an especially intriguing feature; the maximum vertical dimension of the bulk of the concretions is ∼80 cm (Fig. 51). However, the scatter of data from the middle horizon suggests that some concretions stopped growing in the vertical dimension at ∼80 cm but continued to form laterally (Fig. 51). This suggests that the middle concretion horizon remained in the zone of anaerobic methane oxidation (AMO) longer than the lower concretion horizon.
Crucial to precipitation of diagenetic carbonate by the AMO mechanism discussed earlier is a marked reduction in sedimentation/burial, which would stabilize the sediment in the AMO zone thereby enabling protracted carbonate precipitation in that horizon (Raiswell, 1987, 1988). Two observations considered earlier provide evidence of a marked reduction in burial rate during concretion growth: (1) little or no change in carbonate volume percentage outward from the centers of studied concretions and (2) negligible variations in laminae thickness across concretions (Lash and Blood, 2004b).
The middle and lower concretion horizons differ in a fundamental way that yields indirect information regarding such parameters as the magnitude and duration of the change in sedimentation/burial rate. The middle horizon, 1 to 2.5 m-thick, contains concretions distinctly larger in the horizontal dimension than are concretions of the roughly 2.5-m-thick lower horizon (Fig. 5 1). The thickness of a concretion horizon is interpreted to be proportional to the change in sedimentation/burial rate; i.e., a marked reduction in burial rate yields the minimum vertical range of carbonate precipitation because the zone of AMO in the sediment stabilized (Raiswell, 1987, 1988). The more oblate, occasionally flat-topped, concretions in the middle horizon, then, likely reflect the restriction of carbonate precipitation to a relatively narrow, fixed zone defined by the upper limit of methane and CO32- diffusion and the maximum depth to which seawater sulfate can diffuse (Raiswell, 1988). Thus, the generally larger horizontal dimensions of concretions of the middle horizon indicate that the break in sedimentation persisted longer at this time than during formation of the lower concretionary horizon.
The middle Rhinestreet concretion horizon, by virtue of its limited thickness and the geometry of its concretions, records a reduction in sedimentation/burial rate of greater magnitude than that which led to formation of the thicker lower concretionary interval (Lash and Blood, 2004b). However, the thickness and geometry of the scraggy layers likely reflects an even greater duration of the break in sedimentation/burial and, therefore, a protracted period of time that isotopically light methane and 13C-enriched dissolved carbonate were delivered to the stationary AMO zone (Raiswell, 1987, 1988). That is, the near cessation of sedimentation and, more importantly, subsidence, resulted in confinement of carbonate precipitation to a narrow, fixed zone defined by the upper limit of methane diffusion and the maximum depth to which seawater sulfate could diffuse (Raiswell, 1988). Because the events that contributed to the reduction in sedimentation/burial rate may have been basinwide, concretionary horizons may serve as time lines in establishing regional time-stratigraphic relationships.