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Skempton's Poroelastic Relaxation Datashare 129 Skempton’s poroelastic relaxation: The mechanism that accounts for the distribution of pore pressure and exhumation-related fractures in black shale of the Appalachian Basin Terry Engelder and Rose-Anna Behr AAPG Bulletin, v. 105, no. 4 (April 2021), pp. 669–694 Copyright ©2021. The American Association of Petroleum Geologists. All rights reserved. EXPLANATIONS FOR TABLE 2: In an expansion of work by Sheldon (1912) on COMPARATIVE TERMINOLOGIES FROM the Appalachian Plateau, east-northeast fractures were SELECTED JOINT STUDIES IN THE mapped across a region large enough to sample rocks APPALACHIAN BASIN through a 20° swing of the Appalachian oroclinal bend (Parker, 1942). Parker (1942) relabeled the strike fi The rst depictions of Appalachian Basin fractures joints in Sheldon (1912) as set III (accompanying paper were in wood block prints by Sarah Hall as they appear Table 2 and in this Datashare). This was necessary ’ in her husband s paper from the geological survey of because the azimuth of these set III joints (i.e., the east- the fourth district of New York (Hall, 1843). A print northeast set) did not follow around the oroclinal of Taughannock Falls cascading over the Geneseo bend as defined by fold axes. Parker (1942, p. 406) black shale a few kilometers north of Ithaca, New York, writes, “Their different character and constant strike shows two major joint sets that are not quite orthogonal. over the whole region point definitely to an indepen- The artist even captured a hint of lithological control dent origin, perhaps simultaneous or perhaps much of the black shale on joint development. Among other later.” He was uncertain about the time of propagation attributes, these joints crosscut in a streambed of black by adding, “Possibly they formed early during the shale along Taughannock Falls gorge. The two joint Acadian disturbance or perhaps much later during the sets depicted in Hall’s block prints (Hall, 1843) were post-Triassic Palisade movements” (Parker, 1942, p. identified by their geometric relationship with gentle 406). For more than 70 yr, geologists have known that folds of the Appalachian Plateau as strike and dip joints east-northeast fractures did not form as a consequence (Sheldon, 1912). According to Sheldon (1912, p. 68), of major Alleghanian folding and faulting, per se. The the lithological control was abundantly evident as set III joints in Parker (1942) are J1 if they propagated “the strike joints (east-northeast) are better developed before Alleghanian folding and J3 if they propagated in the shales and the dip joints (J2) in the sandy beds.” afterward. Sheldon (1912, p. 67) also states that the strike joints are “best developed in homogeneous shales, especially ” the Hamilton beds. The Marcellus gas shale is found J1 JOINTS at the base of the Hamilton beds. Sheldon (1912) was mapping mainly in the Geneseo black shale At the Bear Valley strip mine in the anthracite dis- and the overlying Ithaca Formation, but her ob- trict of Pennsylvania, the sequence of brittle fractures servations apply to other units constituting the starts with the propagation of a pre-Alleghanian mechanical stratigraphy of the Devonian Catskill set of systematic joints in coal that strike between Delta Complex (accompanying paper Figure 4). In 050° (N50°E) and 070° (N70°E) (Nickelsen, 1979). sum, the effect of black shale on the development Nickelsen (1979, p. 230) correlates the pre- of joint patterns in the Appalachian Basin was Alleghanian joint set with other joints in the Ap- recognized as far back as the geological survey of palachian Basin by stating, “Joints in coal are not the fourth district of New York State by Hall (1843) symmetrical with any of the later structural stages in the late 1830s. recognized in the mine, but they do parallel the Table 2. Comparative Terminologies within Selected Joint Studies in the Appalachian Basin Joint Sets (Chronology Implied Only at the Level of J1, J2, J3, Neotectonic, and Late Erosion) Bed-Parallel East-Northeast Cross-Fold Cross-Fold Cross-Fold Cross-Fold Cross-Fold Cross-Fold Fold- Related Neotectonic Neotectonic Late Erosion (J1) (J2) (J2) (J2) (J2) (J2) (J2) (J3) Approximate Orientation Range Author (Chronological) 40°–80° 280°–300° 295°–315° 310°–330° 325°–345° 340°–0° 355°–15° Strike Joints Northeast to East-Northeast Horizontal Cross Joints Hall (1843) ————— —Wood block print ——Wood block print —— Sheldon (1912) ————— —Dip {GS and ST} (D) —— Strike {BS} (D) —— Parker (1942) ————— —Set I {SS and ST} (D) Set I — Set III (D) — Set II (D) {SS/ST} (D) Ver Steeg (1942) ——Dip {SS} Dip {SS} —— — ——— —— Nickelsen and Hough (1967) — Coal joints Set C Set B Set A Set A Set D Set E ———— Secor (1969) ————— —Episodic joints —— — —— Nickelsen (1979) — Coal joints —— — — — — — — — — Engelder and Geiser (1980) ————Set Ib Set Ib Set Ia Set Ia ———Set II Engelder (1982) ————— — — ——{BS} (neotectonic) —— Bahat and Engelder (1984) ————— —C-rhythmic —— — —— Hancock and Engelder (1989) ————— — — —— {Lst} (O) —— Srivastava and Engelder (1990) Veins (O) ———— —Veins (O) — Veins (O) ——— Engelder and Gross (1993) ————— — — ——Curvy cross joints —— Kulander and Dean (1993) — Domain 6 joints —— — — — — — — — — M. A. Evans (1994) ——Stage III Stage II Stage II — Stage 1 — Stage IV Stage V —— CFJV3a CFJV2b CFJV2a CFJV1 SJV CFJ4 Zhao and Jacobi (1997) ————Northwest-striking ————— —— Ruf et al. (1998) ————Partially filled ———Joints (D) ——Cross joints Younes and Engelder (1999) — J1 {GS} (D) —— — — J2 J2 ———— G. G. Lash and Engelder Bitumen (D) ———— — — —— — —— (2005) Engelder and Whitaker (2006) — (D-M-P) —— — — — — — — — — Engelder et al. (2009) — J1 {BS} — J2 J2 J2 J2 J2 — J3 —— M. A. Evans (2010) — Pre-North ——North Mtn. Stg. — Pre-North Mtn. —— — —— Mtn. Wilkins et al. (2014) —————CSV {BS} (D) CSV {BS} (D) —— — —— M. A. Evans et al. (2014) Veins — Veins — Veins ———Veins ——— Tan et al. (2014) ————— J1 J1 —— — —— Lacazette and Morris (2015) —————TFI {BS} (D) TFI {BS} (D) —— — —— Hooker et al. (2017) ————— Veins ——— Veins —— Engelder and Gross (2018) ————— — — —— — Pancake — Present paper — J1 —— — — J2 J2 — J3 —— Shown are joint and vein sets of the Appalachian Basin sorted by orientation from 29 different studies. Each column contains the names that likely apply to the same fracture set. The letter in parentheses is the geological period of the host rock (e.g., D is Devonian, etc.). The letter in brackets is the rock type (e.g., BS is black shale). A more detailed explanation of this table is found in the supplementary material available as AAPG Datashare 129 at www.aapg.org/datashare. Abbreviations: — = not applicable; BS = black shale; CFJ4 = cross-fold joints; CFJV1 = cross-fold joints and veins; CFJV2a = cross-fold joints and veins; CFJV2b = cross-fold joints and veins; CFJV3a = cross-fold joints and veins; C-rhythmic = cross-fold joints; CSV = cross strike vein; D = Devonian; GS = gray shale; J1 = east-northeast joints (pre-cross-fold); J2 = cross-fold joints; J3 = east-northeast joints (post-cross-fold); M = Mississippian; Mtn. = Mountain; O = Ordovician; P = Pennsylvanian; SS = sandstone; ST = siltstone; Stg. = stage; SJV = strike joints and veins; TFI = tomographic fracture image. Figure S1. Joint development in the Devonian Brallier Formation along the Allegheny structural front, Pendleton County, West Virginia. The east-northeast joints (pre-cross-fold) (J1), cross-fold joints (J2), and strike joints (SJ) well developed in turbidite beds (Engelder, 2004). systematic joints at the eastern end of the arc of set I of less than 4100 m, which was an overburden thick- [not set I by Parker, 1942] joints in coal described on ness involving just the Devonian–Carboniferous the Appalachian Plateau, 60 km to the northwest section, leaving open the possibility that east-northeast (Nickelsen and Hough, 1967, plate 3, p. 619).” The fractures in the region are Acadian rather than Alle- Bear Valley paper was the first to unequivocally ghanian (Evans et al., 2012). One outcrop of frac- conclude that a fracture set in the east-northeast tures in the Brallier Formation of the Virginia–West orientation was pre-Alleghanian and thus a bona Virginia region illustrates why both Engelder fide early (J1) joint set. Evidence for timing was (2004) and Evans (2010) interpret east-northeast based on a geometric argument (Nickelsen, 1979). It joints as prefolding (Figure S1). Here, the east- seemed intuitive to Nickelsen (1979) that bed-normal northeast joints remain normal to bedding while til- joints propagated vertically and were later reoriented ted at an odd angle relative to the dip of bedding and with folding. In fact, Parker (1942) used this same the axes of local folding. Early jointing is common in reasoning to date his set II joints as prefolding. The same other mountain belts as well (Silliphant et al., 2002). argument applies as well to other places in the Appa- Tilted, bed-normal east-northeast joints appear in sev- lachian Basin (Kulander and Dean, 1993; Engelder and eral locations along the Allegheny structural front from Whitaker, 2006). Pennsylvania to Virginia, further reinforcing the per- The Allegheny structural front near the Virginia– ception that they predate Alleghanian folding (Engelder West Virginia border is one part of the central Ap- and Whitaker, 2006). However, it is puzzling that early palachian Mountains where the nonorthogonality of joints show no evidence of either vein development east-northeast fractures and Alleghanian structures or later slip despite the likelihood that they were
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