A1: FAULTING AND MINERALIZATION IN THE CAMBRO-ORDOVICIAN SECTION OF THE MOHAWK VALLEY
Paul Agle, Robert Jacobi, Charles Mitchell
UB Rock Fracture Group
Department of Geology
University at Buffalo
Buffalo, NY 14260
Rich Nyahay, Brian Slater, Langhorne Smith
Reservoir Characterization Group
New York State Museum
3140 Cultural Education Center
Albany, NY 12230
Sponsored in part by NYSERDA (New York State Energy Research and Development Authority)
We thank STATEMAP and NYSERDA for funding research reported herein.
INTRODUCTION
The Cambro-Ordovician section in the Mohawk Valley region is one of the most well known and longest studied Cambro-Ordovician sections in the world. From Amos Eaton in the 1820s (e.g., 1824) to Vanuxem (1834, 1838, 1843) to Emmons (1842), Schuchert (e.g., Clarke and Schuchert, 1899) to Ruedemann (1897, 1925) to Cushing (1905) to Kay (1937, 1968) to the more recent geologists (e.g., Bird and Dewey, 1970; Fisher, 1977, 1979; Fisher et al., 1970; Landing et al., 1996; Jacobi 1981, 2002; Bradley and Kidd 1991; Bradley and Kusky, 1986; Cisne and Rabe, 1978; Cisne et al., 1982; Mitchell et al., 1994; Goldman et al., 1994; Lehmann et al., 1995; Brett and Baird 2002; Baird and Brett 2002), and many, many more (see review in Jacobi and Mitchell, 2002), the Cambro-Ordovician geology in the Mohawk Valley (Fig. 1) has provided critical data for understanding Cambro-Ordovician stratigraphy (Fig. 2) and the inferences that can be drawn from the stratigraphy and structure. For example, the original application of plate tectonics to land geology utilized the Mohawk Valley geology (Bird and Dewey, 1970; Fig. 3A), as have revisions to the original model (e.g., Jacobi, 1981; Fig. 3B; Bradley and Kidd, 1991; Fig. 3C). More recently, the Mohawk Valley section has functioned as the test case for detailed volcanic ash geochronology (tephrachronology) and graptolite correlations (e.g., Mitchell et al., 1994), and for gauging the relative importance of tectonic versus eustatic sea level signals on the apparent sequence stratigraphy in the basin (e.g., Mitchell et al., 1994; Lehmann, 1994; Joy et al., 2000; Jacobi and Mitchell, 2002; Brett and Baird, 2002; Baird and Brett, 2002).
During the past decade, the Mohawk Valley section has been used as an analogue for subsurface hydrocarbon reservoirs in the Cambro-Ordovician section. The major gas discoveries in the Trenton/Black River in central NYS (and elsewhere) have driven a renewed interest in the exposed Mohawk Valley Cambro-Ordovician section. The present attention involves determining and understanding the factors that controlled the development of the Cambro-Ordovician gas reservoirs.
The present field trip examines a spectacular quarry floor that is a probable analog for the dolomitized Trenton/Black River reservoirs in central NYS. Trenches, cores, GPR, and water blasting of the outcrop led to an integrated, very comprehensive picture of how secondary porosity and dolomitization develop. The field trip also visits faults that may have been the pathways for the fluids that dissolved the limestone, resulting in vuggy porosity, and that precipitated hydrothermal dolomite and other minerals in the void space. The trip includes several optional outcrops that display the stratigraphy and the characteristic porosities in various units. Finally, the trip stops at the well-known NYS Thruway locality where the Thruway Disconformity is exposed, separating ribbon limestones of the Dolgeville Formation from the overlying black shale of the Utica (Indian Castle Member). This last stop is particularly interesting for those who have an interest in the black shales of the Utica. Because of severe time constrains, the trip cannot stop at all the localities, but we have included the optional stops for geologists to visit at a later date. Here we can examine in outcrop the stratigraphy, faults, fluid migration along these faults, and tectonics in the Mohawk Valley.
This field trip is the outgrowth of the work of two research groups, the NYS Museum Reservoir Characterization Group (e.g., Smith and Nyahay, 2005), and the UB Rock Fracture Group (e.g., Jacobi and Mitchell, 2002). Each group has worked for some time in the Mohawk Valley; the NYS Museum Reservoir Characterization Group has investigated a hydrothermal dolomite reservoir analog in the Palatine Bridge Quarry (STOP 1); their integrated technologies yielded a significant understanding of the hydrothermal systems. The Reservoir Characterization Group led a workshop and field trip to this site, (Smith and Nyahay, 2005). The UB Rock Fracture Group and ITAS (Integrated Tectonics and Stratigraphy Group at UB) have been working on stratigraphy and structure farther west in the western part of the Mohawk Valley sequence for 25 years (albeit sporadically; e.g., Ritter, 1983; Mitchell et al., 1994; Jacobi and Mitchell, 2002).
Below we first review the Mohawk Valley Cambro-Ordovician stratigraphy that incorporates the units and structures we will inspect on this field trip. Overall, the Cambro-Ordovician sequence in the Mohawk Valley records a Laurentian passive margin carbonate bank that is overstepped by black shales and later coarser clastics. The overstep resulted primarily from a continent/island arc collision wherein the Laurentian continental passive margin first passed over the peripheral bulge and then dropped down into an east-dipping subduction zone (e.g., Jacobi, 1981; Rowley and Kidd, 1981; Stanley and Ratcliffe, 1985). A eustatic sea level rise may have been superimposed upon the tectonic signal (e.g., Brett and Baird, 2002). We also review the structure of the region, and the concepts involved in the dolomitization and development of the porosity typical of the Cambro-Ordovician carbonate section. A key component of the trip will be the link between faulting, fluid flow and reservoir development.

A1: FAULTING AND MINERALIZATION IN THE CAMBRO-ORDOVICIAN SECTION OF THE MOHAWK VALLEY
P.Agle, R.Jacobi, C.Mitchell, R.Nyahay, B. Slater, L.Smith

STOP 1. Palantine Bridge Quarry
The Palatine Bridge study site is located in an inactive quarry along the Mohawk Valley, approximately 50 miles west of Albany, NY. The outcrop occurs as a highly localized dolomite alteration in the Tribes Hill limestone that makes up the floor of the quarry. Although there are at least three areas of the quarry that have been dolomitized, research has been focused on the one region most accessible. The site consists of two dolomite bodies (Figs 7 &8). The eastern most body (body 1) is a long, linear feature approximately 55 feet long, 5 feet wide, and has a strike of 305°. The second body (body 2) is located immediately west of the first, and is also a linear feature with similar strike, however this body is substantially longer than the first, measuring 110 ft. Body 2 is broken into two parts which are separated by a southerly bend, referred to as the jog. The dolomite is easily distinguished from the surrounding limestone by its distinct orange color and massive texture.
Analog Comparison
Although it is much smaller than the hydrothermal dolomite fields in production today, all other characteristics of the quarry are virtually identical to those of its larger counterparts (Fig. 9). Hydrothermal dolomite fields such as the Rochester field of Ontario, CAN, the Albion-Scipio fields of Michigan, and the numerous gas fields of South Central NY all bear striking resemblance to the Palatine Bridge Quarry outcrop.
Setting
The Palatine Bridge Quarry lies within the Tribes Hill unit of the Upper Beekmantown Group. It is separated from the Precambrian basement by the Little Falls dolostone (lower Beekmantown Group) and the Potsdam sandstone which may or may not be present as it thins to the east. The Tribes Hill is a thin to medium-bedded limestone which was deposited in a shallow, peritidal to subtidal environment. It is underlain by the Late Cambrian Little Falls dolostone which is described as a thick-bedded dolostone and is also believed to have formed by deposition in a shallow, peritidal to subtidal environment. The Little Falls is approximately 400ft. thick in this portion of the Mohawk Valley.
The Tribes Hill Limestone outcrops in Palatine Bridge; however, further south it is unconformably overlain by the Upper Ordovician Trenton- Black River Group and Utica Shale. The Trenton and Black River units are well known for their oil and gas plays along the eastern portion of the US and into southern Ontario. The majority of these plays are contained in hydrothermal dolomite reservoirs.
The Beekmantown Group was deposited during the Late Cambrian and Early Ordovician along a passive margin following the opening of the Iapetus Ocean. However, deposition of the Middle Ordovician Trenton- Black River Group has been demonstrated to have been fault controlled. Active tectonism during the Middle-Late Ordovician is associated with the Taconic Orogeny in which Proto-North America collided with the Taconic Island Arc terrain. Flexure of the North American plate during its attempt to be subducted under the island arc led to extensional faulting and produced a series of north east-south west striking normal faults, some with offsets of over 1000 ft.
Field Relations
There are many characteristics of the outcrop that indicate a fault related hydrothermal origin. The central sag flanked by monoclines is a typical feature of many hydrothermal structures (Fig. 10). Fault breccia and vuggy porosity can be observed in several places (Fig. 11). These areas are commonly located near, but not limited to, the tips of each body. An intense fault and fracture zone surrounds the entire outcrop increasing the width of the affected area to approximately 15 ft. (Fig. 12). Many faults have vertical slickenlines which indicate a stress regime that was predominantly extensional. Groups of faults commonly join to form a relay ramp structure as seen in Figure 13.
Ground Penetrating Radar
In the spring of2004 a shallow ground penetrating radar survey was run over body 2. The purpose of this survey was to aid in the excavation process and to get a profile view of the outcrop. Although the radar was only able to penetrate a couple meters, it showed that the outcrop has geometry very similar to that of the Trenton / Black River gas plays (Fig. 14). Each body consists of a central syncline, or sag, that is flanked by smaller anticlines on either side.
Cores
A series of six cores 40 - 60 ft. deep were taken from strategic points on the outcrop so that cross sections could be constructed both along strike and perpendicular to the bodies. The perpendicular cross section (Fig. 15) demonstrates that the dolomitization is highly localized as Hole Six, less than 20 ft. outside the body, is almost completely undolomitized. The parallel cross section (Fig. 16) shows that the limestone gap between bodies 1 and 2 is only 3 feet thick. The two bodies join at depth.
Trenches
Six trenches measuring 3' wide by 15'long by 18" deep have been cut across the outcrop (Fig. 17) to expose the folds of the body as well as the dip and offset of the fractures (Fig. 18).
Paragenic Sequence
Examination of thin sections made from outcrop and core samples indicate a fairly complicated diagenetic history including multiple episodes of faulting and mineral precipitation (Fig. 19).
Geochemistry
Strontium isotope analyses done on dolomite samples yield values significantly higher than that of seawater during the time of Tribes Hill deposition (Fig. 20 & 21 ). Limestone and calcite samples plot on the curve as expected. The elevated strontium content is interpreted to be caused by interaction of the parent fluid with siliciclastic rocks, such as the Grenville basement, for an extended period of time.
Stable Isotope analyses show that dolostone crystals formed from a fluid much lighter than seawater dolomites (Fig. 22). It is believed that these lighter isotope ratios are an indication of higher temperature fluids traveling upward from deeper, hotter depths.
Fluid inclusion analyses conducted on 40 samples indicate that both dolomite and calcite formed at temperatures greater than 100°C (Fig. 23). Although thermal chronology studies indicate that the Tribes Hill has been buried to a geothermal temperature of over 100°C, core analysis and paragenic sequencing suggest that dolomitization occurred at a shallow depth before significant burial.
P.Agle, R.Jacobi, C.Mitchell, R.Nyahay, B. Slater, L.Smith

STOP 2. Ingham Mills
Ingham Mills is a well known paleontological and stratigraphic locality due to the complete Trenton/Black River section at this location (Fig. 24). The section here is over 23 m thick and is the farthest eastward extent of the Watertown and Napanee Formations. The section is exposed on the spillway beneath the hydro dam that holds back the waters of Keyser Lake. The lowermost outcrop at the base of the spillway is a buff-weathering, fine-grained dolostone. This dolostone has previously been classified as the uppermost surface of the Little Falls Formation (Cameron et al., 1972; Cameron and Kamal, 1977; Mitchell and Marsh, unpublished data 1998). The hummocky nature of the surface does resemble thrombolitic horizons within the Little Falls Formation, though the fine-grained texture of the dolostone is very rare within this formation. However, new observations reveal that this is not a simple sedimentary contact. The dolomite observed at the section is actually an aureole of dolomitization hosted within the Lowville limestone along a fracture trending 290° (Fig. 25). On the north side of this fracture, the limestone layers clearly "onlap" over the dolomite body, but on the south side of the fracture, the dolostone grades rather abruptly into limestone within the same bedding plane. This gradation indicates that this dolostone was formed by migration of a dolomitizing fluid through the limestones along this fracture trend. The geometry of this dolomitization aureole and the surrounding limestones are strikingly similar to those observed at the Palatine Bridge Quarry (STOP 1).
Above the dolomite body another 8.66 m of Lowville Formation is exposed. The rocks here consist of blue-gray to dove-gray, very fine-grained limestone. Many horizons are riddled with vertical burrows of Phytopsis tubulosum and vertical fingers of anthraxolite (filling in voids of dissolved burrows?). One layer in the upper Lowville contains large colonies of Tetradium both over-turned and in life position. Two horizons within the Lowville show distinct tidal channel structures. The lowermost of these horizons show a rubbly lag on the lower surface of the channel. The rocks herein assigned to the Lowville Formation have been previously designated as Gull River by Cameron and others (1972), whereas Cornell (2003) classified the lowermost 3 m of this section as the Pamelia Formation and assigned the remainder to the Lowville Formation. At the top of the Lowville Formation lies 0.35 m of dark gray, rubbly limestone which we assign to the Watertown Formation. These rocks have also been called the Selby Formation by Cornell (2003). Above this horizon is 3.42 m of Napanee Formation (the lowermost formation of the Trenton Group). These rocks are fine-grained dark gray limestones that weather to a distinctive chocolate brown color. The limestones also contain abundant dark gray, calcareous shale interbeds. A prominent deformation feature of enigmatic origin is located on the west side of the outcrop; this feature incorporates an unknown amount of the upper Lowville, the entire thickness of the Watertown Formation, and the lowermost 2 m of the Napanee Formation. This structure is probably the result of karst/dissolution collapse in the Lowville Formation. The overlying 9.3 m of the Kings Falls Formation is differentiated from the Napanee Formation by its coarser-grained texture and large ripples indicating a higher energy depositional environment. A few of the rippled horizons contain large clasts and one bed at 14.2 m contains clasts of some underlying units including gneiss pebbles derived from the Precambrian basement. These clasts had to have been shed from a nearby fault scarp that must have existed nearby during the deposition of these limestones. The inferred position of the Dolgeville Fault lays a mere 400 m to the west of Ingham Mills and may well have been active at this time (Jacobi and Mitchell, 2002). The uppermost 1.2 m of section belong to the Sugar River Formation, which is differentiated from the underlying Kings Falls by a decreased grain size, absence of high-energy ripples, and numerous bedding planes with abundant Prasopora colonies.
P.Agle, R.Jacobi, C.Mitchell, R.Nyahay, B. Slater, L.Smith

STOP 3.
The power dam at Dolgeville on East Canada Creek is the most accessible exposure of the Dolgeville Fault (Fig. 26). The fault plane itself is not visible at this outcrop, though the effects of faulting are clearly and dramatically observed in the downthrown block. The fault disappears beneath thick glacial cover to the north and cannot be traced reliably for any great distance. To the south, the actual fault plane outcrops along the east bank of Kyser Lake (now accessible only by boat) and consists of a well-cemented fault breccia with iron/sulfide mineralization (Fig. 27). Cushing (1905) drew an excellent cross-section of this southern outcrop along East Canada Creek as it was before the dam at Ingham Mills raised the river level to create Kyser Lake (Fig. 28). The fault does not outcrop south of this location, but stratigraphic evidence reveals that it probably crosses the southern part of the lake and continues southward (with diminishing throw), where it passes somewhere between West Crum and East Canada Creek. If the fault crosses the Mohawk River, then the throw is severely reduced. At the Dolgeville Dam, the fault trends roughly N-S and drops down to the west (antithetic to the other major faults). Cushing (1905) estimated the throw of the fault at 300 ft (91.4 m), whereas Fisher (1954) estimated 450 ft (137.2 m). The Dolgeville Fault here exhibits dip slip kinematic indicators. Minor drag folds have nearly horizontal axes, and down-dip slickensides (Fig. 26 inset) occur on minor faults and on bedding around some of the folds (Jacobi and Mitchell, 2002). Farther south Bradley and Kidd (1991) also found down dip slickensides on a minor fault near the Dolgeville Fault (their Figure 8a). Thus, if these kinematic indicators are related to Ordovician motion, then the Dolgeville Fault was primarily a dip slip (normal) fault. We believe this fault originated in response to plate stress as it descended into the trench. The fact that this fault cross-cuts gravity anomalies and that the N-trending fractures (Dolgeville trend) usually post-date the NNE trending fractures (Little Falls trend) indicate that slip on this fault likely initiated relatively late as a result of peripheral bulge tectonics and is younger than the Little Falls Fault. There are a few folds and faults that indicate a later reversal in motion on the Dolgeville fault.
At this site, the up thrown block consists of sub-horizontal Little Falls dolostone that outcrops along Dolge Ave. overlooking the falls. Looking northeast at the spillway outcrop, the down thrown block consists entirely of Indian Castle black shale with rare limestone beds. The beds of the falls are sub-horizontal near the powerhouse, but slowly begin to dip to the west nearing the fault. At the small line of trees, the bedding dip abruptly increases across a minor fault. The beds then continue to gradually increase in dip to angles measured as high as 59°W, although the beds are much steeper on the inaccessible shear cliff-face. Within the steeply dipping beds, the Dolgeville/Indian Castle contact is gradational, with an upsection increase in the thickness of the intervening shales. No time gap is thought to exist at this locality between the Dolgeville and the Indian Castle (Jacobi and Mitchell, 2002). The lack of a significant, observable "Thruway disconformity" (viewed at STOP 5) is consistent with the lack of a significant number of the distinct folds that characterize the NYS Thruway outcrop. No unequivocal slump folds, no time gap, and no locally angular unconformity all suggest that the "Thruway disconformity" is not a regional unconformity rather it is a slide scar, and the sediment slide did not affect this section (Jacobi and Mitchell, 2002). That scenario is consistent with the tectonic position of this outcrop - it is in the base of the Dolgeville graben, whereas the NYS Thruway outcrop is located on the horsts of the Little Falls fault system.

STOP 4: Outcrop at Intersection of Rts. 5 and 169
Park along wide shoulder near base of incline across from intersection with NY 169
Cushing (1905) described several outcrops of the Little Falls Fault in the field area. Today, these outcrops no longer exist or are severely degraded. The only locality that exposes the fault plane is the Thruway cut south of the Mohawk River (STOP 5). To the north of this locality, a strong NNE-trending topographic lineament is easily recognized. In the creek to the east of Moreland Park, a NNE-trending fracture intensification domain (FID) occurs within the Little Falls Formation along with loose blocks of stock-work breccia. The fractures within this FID have an average strike of 038° with a frequency of 20 fractures/m. This lineament appears to offset both the Precambrian-Little Falls contact and the Little Falls-Tribes Hill contact with the western side down-dropped. To the south, this topographic lineament truncates Moss Island on the west side. Across the Mohawk River, this lineament is clearly visible as a small cleft in the "Rollaway". The Precambrian-Little Falls contact appears to be offset down to the east across this cleft, though no other evidence for faulting was observed at that locality. The fact that the westerly block is down-dropped on the north side of the Mohawk River, whereas the easterly block is down-dropped on the south side indicates that this splay is either a scissors fault or there is an unrecognized cross fault between the two localities. At the present location, this lineament is expressed as the small creek that we see at the stop. At this site, we observe another NNE trending (average strike= 040°) FID (fracture frequency = 13.5 fractures/m) and also a fairly dramatic change in lithology in the Precambrian basement. The east side of the creek is well-foliated charnockitic gneiss, whereas the west side is darker, much more mafic, poorly-foliated gneiss. Slickenlines can be observed on both NNE and WNW surfaces at this outcrop (Fig. 29). Fractures of the NNE-trend increase in frequency nearing the fault (Fig. 30). The preferred growth orientations of crystals within the slickenlines indicate that the NNE trending fault experienced right lateral oblique slip with the SE fault block down-dropping (Fig. 29A). This sense of motion is opposite to the stratigraphic offset along this splay of the Little Falls Fault on the north side of the Mohawk River, thus the offset on the Little Falls Fault may be the result of several cumulative slip events with fault motion reversing at intervals throughout its history.
The slickenlines on the WNW surface are sub-horizontal (plunge= 04°) and indicate left-lateral slip with the northern block being slightly down-dropped (Fig. 29B). Leaving the creek and looking at the road cut exposures along Rt. 5, another WNW-trending surface shows well developed slickenlines. The slickenlines at this location record left-lateral oblique slip with a slight down on the south component (Fig. 29C). This sense of offset does not agree with the offset along the inferred WNW transfer fault to the north, but it is consistent with the sense of motion required to drop the Precambrian-Little Falls contact down on the south side of the Mohawk River (on an inferred WNW fault). On the north side of the river, this contact lies at 560 ft (170.7 m), whereas on the south it is at 400 ft (121.9 m). This offset can be partially explained by regional dip on this contact, which is estimated at 03° to the south based on the topographic gradient observed on the north side of the Mohawk River. Projecting this dip across the Mohawk River would account for only 125 ft (38.1 m) of offset. The kinematics of all the slickenlines (Fig. 29D) at this locality are consistent with the expected motions in an eastward directed far-field compressional regime, though these faults probably experienced several stages of slip and reversals in sense of motion.

STOP 5. Thruway cut South of Little Falls
Park in construction zone at northwest corner of Paradise Rd. and Thruway intersection
The NYS Thruway roadcut south of Little Falls provides a 1.2 mile long look at the ribbon limestones of the Dolgeville Formation, the overlying black shale of the Indian Castle Member, and the unique contact between these two units (Fig. 31 ). At the Thruway road cut, the upper few meters of the Dolgeville Formation are intensely folded. Fisher (1979) first studied these folds in 1953 when the roadcut was dug. Fisher (1979) noted the dominant westward vergence of the folds, and suggested that westward slumping away from the foredeep (not toward the basin) had caused these folds. He proposed that westward gravity sliding of the Taconic Giddings Brook "Slice" into the foredeep (Magog, or Snake Hill, Basin), which was located east of the thruway roadcut along an axis between the Hudson River/Lake Champlain and the Green Mountains/Taconics) had depressed the crust below the gravity slides and caused the Adirondack Arch to rise. Since the arch was located east of the roadcut, the rising arch would have reversed the original eastward slope to a westward slope, and thus the slumps traveled west, not east, off the carbonate ramp. The overlying Indian Castle Member lies largely undisturbed above the local unconformity known as the Thruway Discontinuity (after Baird and others, 1992).
Fisher's (1979) fold vergence was supported in general by studies by two of Jacobi's students who analyzed the outcrop in the 1980s (Ritter, 1983; Koslosky, 1986). Jacobi and Mitchell (2002) further analyzed the folds and paleoflow indicators at this site. Most of the folds are disposed in "trains" and affect only a relatively few beds; i.e., they are intrastratal (or intraformational), disharmonic folds. As described in Jacobi and Mitchell (2002):
Only kink band folds (box folds) affect more than a few beds. At the unconformity, eroded crestlines and troughs filled with breccia from the ribbon limestones, demonstrate that at least here the folds were exposed at the seafloor to erosive processes. These folds are not the result, therefore, of relatively deeply buried bedding-parallel slip. Both Fisher (1979) and Koslosky (1986) suggested that the folds were "slump folds" related to downslope slump processes, based on fold form and character. Highly contorted crestlines of some folds indicate that these folds underwent variations in strain along their axes as the sediments were transported downslope during the sliding event(s).
At this stop (near milepost 213), the asymmetric folds have fairly straight crestlines and west vergence. Application of the arc separation method (Fig. 32) yielded a down-paleoslope direction of 234.5° at the time of folding (Fig. 33). At a western outcrop (at milepost 214.8), the asymmetric folds exhibit a southwest vergence and the arc separation method yielded a down-paleoslope direction of 229° at the time of folding. These values are in accord with the detailed analyses of 698 folds by Koslosky (1986). Other features (Fig. 34) in the eastern outcrop that suggest a southwesterly-directed paleoslope at the time of sliding include the following observations:
1. small west-directed thrusts that affect single limestone layers in the Dolgeville Formation,
2. westward-directed intrastratal micro-fold nappe in the Dolgeville Formation,
3. imbricate, east-dipping clasts in breccias in the troughs of folds at the unconformity that suggest a westerly current flow immediately after the folding (sliding event[s]), and
4. a small slide scar that faces west.
Paleo flow data from the Dolgeville Formation at this outcrop imply a flow toward 207°; this direction is only 27° away from the paleoslope determined from the asymmetric folds. Graptolites and parting lineations above the unconformity show that the local paleoslope reversed directions after the time of slumping.
The major splay of the NNE-striking Little Falls Fault ("C") cuts the units in this stop (Fig. 35). The fact that bedding dip in the down thrown Utica (on the east side of the fault) is not perpendicular to the fault plane is evidence that this fault probably experienced oblique slip at this location (Jacobi and Mitchell, 2002). The newly recognized oblique-slip slickenlines along faults in the Precambrian basement at Little Falls (STOP 4 along Little Falls Fault "B") are consistent with the sense of rotation on the block inferred from the anomalous dip. In the past, no kinematic indicators were observed in the fault surface at the Thruway, but with the recent NYS Thruway slope abatement project, we may find kinematic indicators. Minor splays of the Little Falls Fault occupy valleys to the east and are recognized by minor changes in dip across the valleys. It is our contention that motion on these faults reversed the local paleoslope, causing the slumping activity recorded as the slump folds and the Thruway disconformity (a slide scar). This motion was related to the plate descending into the trench. (It should be noted that others maintain that the paleoslope did not reverse; Brett and Baird, 2002).

STOP 6. (Optional) Manheim Power Station
At the Manheim Power Station, the Manheim Fault is exposed along both sides of East Canada Creek (Fig. 36). Access from the Power Station driveway is on the west bank and permission is needed as this is private property. From this side of the creek, the rocks are easily accessed even at high water. The rocks along this bank superficially resemble the interbedded ribbon limestones and shales of the Dolgeville Formation and have been designated as such by previous workers (Megathlin, 1938; Fisher, 1954; Bradley and Kidd, 1991). However, faunal evidence indicates that the rocks observed here are the "Allen Road beds" of the Flat Creek Member (Mitchell, 2006 personal communication). Numerous fractures and veins dissect the outcrop at this locality. The fractures are of NE, WNW, and NNW orientations. It was expected that the NE-trending (fault-parallel) fractures would increase in frequency nearing the fault plane, but actually the NNW (fault-perpendicular) fractures show the most marked increase (Cross, 2004; Cross et al., 2004; Fig. 37). At the northern extent of the outcrop on this side of the creek is a fault breccia. This breccia contains lithologies similar to those of the adjacent Flat Creek Member consisting of large, rounded clasts of limestone that are well-cemented in a dark, calcareous mud matrix. Across the creek on the eastern side, one can see beds of the Flat Creek Member drag-folded upward on the eastern block to angles approaching 60°SE juxtaposed against gently dipping (07°SE) beds of the Little Falls Formation on the western block (Bradley and Kidd, 1991). Here the fault strikes roughly 50° (it is drawn as roughly N-trending in Fisher, 1980) and drops down to the east (Cross, 2004). According to early workers (Conrad, 1837; Vanuxem, 1838) there is a one inch thick calcite vein with associations of galena and pyrite present, which were mined for a brief period in the area of the fault. In addition, the fault zone hosts a peridotite dike. Similar dikes in the area roughly parallel the fault trend and have been dated using Rb-Sr as 130±10 million years old (Fisher, 1980). Estimates of the offset on this fault vary greatly with Darton (1895) claiming 152 ft (46.3 m), Megathlin (1938) 60 ft (18.3 m), Fisher (1954) 400 ft (121.9 m), and Bradley and Kidd (1991) 131 ft (39.9 m).

STOP 7. (Optional) Miner Rd. Outcrop
In the course of widening Miner Rd., an excellent outcrop of the lowermost Tribes Hill Formation was exposed. At this locality, the rocks are a light gray, medium-bedded (11 cm average), coarse-grained dolostone. The bedding at this locality trends 002/03W. Several beds show excellent herring-bone cross-stratification typical of a tidal environment. Four distinct fracture sets are present at this outcrop (N, NE, WNW, and NW). At this outcrop, the NE trending fracture set (average strike = 53°) is master (oldest) and parallel the inferred orientation of the Little Falls Fault just to the east. Reddish staining along the NW set is interpreted as sulfide and/or iron-rich vein mineralization (Fig. 38). The outcrop also hosts many vugs within the dolostone matrix. These vugs are variably filled with white calcite (Fig. 39) and occasional buff-colored dolomite. The presence of the veining and vug development are indicative that this horizon was favorable to the migration of fluids from the nearby fault. Heading south along Miner Rd. from this outcrop, there are excellent exposures of the upper Little Falls Formation in the woods on the western side of the road. These exposures contain stromatolite colonies and also host vugs partially filled with white calcite.

STOP 8. (Optional) Route 167 Overlook
The scenic overlook of Rt. 167 just before entering the city of Little Falls, NY provides an excellent vantage point to appreciate the magnitude of the fault offsets in the region (Fig. 40). Though the Little Falls Fault is not directly exposed at this locality, the steep linear scarp must be a close approximation of the fault plane. If one parks in the designated area and walks to the fence, a grand overview of the Mohawk River Valley opens up before you. In contrast to the narrows at Little Falls, NY, in the resistant Precambrian basement, the valley opens broadly to the east in the much less resistant shales of the Utica Group. If the gate is open, it is possible to walk out onto a small outcrop of Precambrian basement at the edge of the scarp. The foliation here is well defined and trends 284/26NE. This foliation is fairly uniform throughout the Little Falls area. Fractures in this outcrop trend NNE and WNW and are mutually abutting. The NNE striking fractures (average strike = 037°) of this outcrop parallel the topographic lineament and the inferred trend of the major splay ("C") of the Little Falls Fault in this area. While standing on this outcrop of Grenvillian basement, one can look down into the valley approximately 150 feet below and see the Thruway Interchange and its impressive exposures of the Upper Ordovician Indian Castle Member. Cushing (1905) estimates the throw on this fault at this location at 650-750 ft (198.1-228.6 m), whereas Fisher (1954) estimates 900 ft (274.3 m). The fault extends northward into the crystalline rocks of the Adirondack Dome and southward where it dies out in the Silurian shales. Across Rt. 167 from the scenic overlook is an old quarry. This quarry contains a 16 m section of the lowermost Little Falls Formation, though the lower contact with the Precambrian basement is not exposed here. This section of Little Falls dolostone is very coarse-grained and contains large amounts of quartz and some large detrital feldspar grains. Many beds display cross-bedding and a few isolated horizons host vuggy porosity.

STOP 9. (Optional) Little Falls Bike Path Area
The bike path on the south side of the Mohawk River now occupies an old railroad bed. Walking eastward from the designated parking area, large exposures of Precambrian basement can be observed. Approximately halfway through the cut about 20 ft from ground level on the south side, the contact between the Precambrian basement and the Little Falls Formation is exposed (Fig. 41). The contact is inaccessible without climbing gear, but clearly visible. Walking back to the parking lot and heading south, there is a footpath that winds its way up above the bike path. Along this path, there are excellent exposures of Little Falls dolostone. These outcrops are buff-weathering, thickly-bedded, coarse-grained dolostone that display crossbedding, stromatolites, and numerous vugs. These vugs are filled with saddle dolomite, quartz, and anthraxolite (Fig. 42). Dolomite usually lines the vugs, followed by anthraxolite, though a few vugs contain anthraxolite films that pre-date the dolomite. Where quartz is present, it is always the last mineral to precipitate, although anthraxolite commonly occurs as solid inclusions within the crystals. The fact that the age relationships of dolomite and anthraxolite are inconsistent, suggests that these minerals may have significant overlaps in their time of precipitation. Dunn and Fisher (1954) analyzed the anthraxolite from three similar vugs in the area and obtained an average composition of 90.39% carbon, 7.03% water, and minor components of nitrogen, sulfur, and ash. The temperature of precipitation of these vug-filling minerals is poorly studied. Dunn and Fisher (1954) cite Dr. F.G. Smith as having obtained a temperature of 51 °±2°C for a quartz crystal from the formation. O'Reilly and Parnell (1991), based on limited analysis of bitumen reflectance in the anthraxolite and fluid inclusions in calcite and quartz, developed a four-stage model in which the first stage (70°C) deposited calcite cements, the second stage precipitated calcite veins and vug fills (70 to 130°C), a third stage of anthraxolite (and presumably dolomite) and quartz (120°C), and a fourth very high temperature stage 200±10°C. Further fluid inclusion analyses of vein fill in the study area is currently underway.

STOP 10. (Optional) Nowadaga Creek
The exposures along Nowadaga Creek at the River Rd. bridge offer an opportunity to study the Dolgeville Formation and the contact with the overlying Indian Castle Member. The creek bed downstream of the bridge exposes the upper surface of the Dolgeville Formation and is readily observable at low water levels. The upper beds of the Dolgeville Formation are a complexly interference-folded surface (Fig. 43) and the contact with the overlying Indian Castle Member is very abrupt. For a detailed study of the folds and paleoflow directions at this locality see Jacobi and Mitchell (2002). Downstream from the contact is exposed a nearly complete section of the Dolgeville Formation. Very small (1O cm wavelength) asymmetric folds with irregularly-bearing axes are the result of mini-blind thrusts involving single limestone beds (Marsh, 2000). These thrusts suggest that the entire Dolgeville section was deposited during times of instability. The fractures present within the Dolgeville trend NNE, WNW, and NW. There are also veins present in the Dolgeville Formation at this locality. These veins parallel the NNE fracture trend and range in thickness from 1-3 mm of white calcite. Upstream of the bridge, the creek is floored by the Indian Castle Formation. This abrupt transition from contorted Dolgeville ribbon limestones and shales to undisturbed calcareous black shales represents a significant time gap, but not as much as that at the Thruway Discontinuity (Jacobi and Mitchell, 2002). In an outcrop on the northern bank, the lower Indian Castle Member is seen to contain a few thin carbonate beds, but overall the lithology is dominated by black calcareous shale. The fractures hosted within these shales show three prominent orientations: NNE, WNW, and NW (Fig. 44).