undergraduate research opportunity....

 

ANALYSIS OF FRACTURED DEVONIAN STRATA IN NORTHERN CHAUTAUQUA AND ERIE COUNTIES, NEW YORK

Gary Lash

Department of Geosciences

State University College

Fredonia, NY 14063

The distribution and origin of fractures exposed in Paleozoic strata of the Appalachian Plateau have long been the subject of geologic debate. More recently, renewed effort has been devoted toward gaining an improved understanding of fracture arrays in Devonian rocks of New York State.  Much of this work stems from petroleum exploration and concerns over the influence of urban expansion on groundwater systems.    

Current research interests in northern Chautauqua and Erie counties focus on  1) the respective roles of thermal maturation and organic carbon content on fracturing of organic-rich Devonian strata; 2) potential mechanisms of overpressuring of Devonian strata in the western New York Appalachian Plateau; 3) determination of the paleodepth of overpressuring by use of calcite concretions; 4) demonstration of vitrinite reflectance suppression as a consequence of the development of hard overpressures in the Upper Devonian organic-rich shale sequence; 5) the formation of gas capillary seals early in the structural history of the black shale deposits; and 6) chronology of fracturing in the Upper Devonian shales.  
Students  Pat Dwyer (right) and Anthony Soricelli (center), along with Dr. Ann Deakin, prepare to run a scanline along Canadaway Creek in October 2000

Of especial interest is the preferential jointing of the Dunkirk Shale and older black shale units in the western New York Appalachian Plateau.  We have gone through an evolution of thought regarding these structures.  Individual joints are very continuous and show little deviation in orientation along their strike. Further, the average strike of this set is within a few degrees of the N72oE direction of the maximum principal horizontal stress (SH) determined from a borehole elongation measurement made in southern Chautauqua County (Plumb and Cox, 1987, Journal of Geophysical Research).    

Moreover, locally ENE joints are orthogonal to post-glacial pop-ups.  We had considered that perhaps we were seeing evidence of joints produced by thermoelastic effects related to unloading within the contemporary stress field.  As such, these fractures were initially considered to be neotectonic in origin and produced during the unroofing event that formed the Lake Erie Escarpment. 
ENE-trending joints in the Dunkirk Shale along Lake Erie
More recently, though, we have uncovered compelling evidence to suggest that the ENE joints formed under conditions of high (tectonically-driven?) fluid pressure.  Their planarity is quite impressive; individual joints continue for tens of meters.  Also, they are vertically extensive, extending for four or more meters up section.  Further, where we see the ends of two ENE joints overlap, the traces remain straight rather than curving toward each other (hook-shaped geometry).  This latter observation suggests that the joints formed under high differential stress, something that would not be expected for shallow-formed neotectonic joint systems.  
very planar ENE fractures in the Dunkirk; note planar overlap of joint segments
vertically extensive ENE fracture (parallel to view)
One of the fascinating aspects of the ENE fractures is the fact that they do not propagate out of the Dunkirk into the underlying Hanover shale.  Indeed, what we find in the Hanover is a highly planar roughly N5oW joint set that extends only several meters up into the Dunkirk.  Moreover, excellent exposures on Walnut and Eighteenmile creeks show similar relations; i.e., NNW joints, which are best seen in the upper 10 meters of the Hanover do not propagate very far (if at all) into the Dunkirk.  The significance of this observation is that perhaps the Dunkirk Shale was acting as a pressure seal during generation of the NNW joints.  



ENE joints (extending roughly right to left) in a black shale bed at the top of the Hanover.  Note that the joints do not extend into the underlying gray-green shale (Hanover).  The large fracture that extends from the bottom to the top of the view is a NNW joint.
Contact of Dunkirk Shale and Hanover Shale (at notch).  Note that the dominant ENE fractures in the Dunkirk do not extend into the Hanover.

In the few exposures (that have been studied as of this time) where ENE joints can be seen interacting with concretions, the joints tend to propagate a small distance into the concretions.  Moreover, some smaller concretions are completely cut by ENE joints.

Concretion in black shale of the Rhinestreet Shale. Small concretion (slightly right of center) cut by an ENE fracture; Rhinestreet Shale.

 

Finite element and numerical modeling of McConaughy and Engelder (1999, Journal of Structural Geology) suggests that these observations are consistent with a fluid-driven joint loading configuration as opposed to jointing driven by thermoelastic loading under neotectonic conditions.  Such an interpretation is also consistent with the great vertical extent (>>width of a concretion) of the joints.
ENE fracture (on right) penetrating a concretion (directly above the compass).

One of the problems we are faced with concerns the age of the ENE joints relative to NW (roughly N50oW)- and NNW (see picture above) -trending systematic joints.   In many cases, abutting relations are ambiguous.  However, results of the past (2001) summer's work on Walnut and Eighteenmile creeks suggest that the ENE joints are younger than either the NNW or NW sets.  

Rose diagram of joint orientations of the Hanover Shale.  The NNW set is dominant followed by the ENE and NW sets.

Student Participation

Student participation in the ongoing study of the ENE joints and other problems mentioned above is crucial to this work.  Indeed, this program affords students the opportunity to become involved in original research.  Students are involved in the collection of such data as fracture orientation, fracture spacing along scanlines, evaluation of the vertical and horizontal extent of fractures, abutting relations of fractures of different orientations and relative ages, and the description of such associated structures as pop-ups and faults.  Students are also involved in the analysis of collected data by such means as the plotting and analysis of rose and standard histograms, frequency plots of fracture spacing values and fractal analysis of joint spacing values, and other geostatistical treatments.  Also, they have the opportunity to evaluate data from a GIS point of view with Dr. Ann Deakin, Department of Geosciences.  At the end of a student's involvement in this project, he or she will be expected to present results of the fieldwork and analysis at an undergraduate research exposition. 

Fredonia State offers motivated students the opportunity to apply for research funds to the tune of as much as $500.  Not only do students gain invaluable experience at writing grants and organizing research projects, the stipend covers many of the basic costs of carrying out the field work.  Moreover, faculty grants from such agencies as the Petroleum Research Fund (American Chemical Society) for basic undergraduate research includes a summer stipend.
Undergraduate students Pat Dwyer (left) and Anthony Soricelli.  Note that they are not really pointing to any specific location on the map.  But seriously, these guys spent a very productive fall (2000) in the field. 

Pat Case, recipient of an Undergraduate Research Fellowship for the summer of 2001, has been working with me since May.  He has spent much of his time walking Walnut Creek and Eighteenmile Creek. Over the fall and winter, he will be working with me on analysis of vitrinite reflectance and Rock-Eval pyrolysis data.  

Randy Blood, recipient of an Undergraduate Research Fellowship for the summer of 2002, stands within a zone of spectacular carbonate concretions within the Rhinestreet shale.  Randy is working on an analysis of the differential compaction of the black shale around the concretions as well as a study of the degree of development of joints in the Rhinestreet.  The goal here is to assess the possible role of total organic carbon content in the jointing of these rocks.  

Interested students must be either Geology or Earth Science majors who have completed a Structural Geology course.  The student participant must be academically strong, as demonstrated by a strong grade point average, and, perhaps most importantly, must be well motivated to carry out basic field geologic research.  Interested students should contact Gary Lash.

 

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