Abstract. Two components of magnetization were isolated in the Upper Ordovician Juniata Formation sampled in the area of the Pennsylvania salient.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. 82, PAGES 1843-1849, FEBRUARY 10, 1989
PALEOMAGNETISM OF THE UPPER ORDOVICIAN JUNIATA FORMATION OF THE CENTRAL APPALACIDANS REVISITED AGAIN John D. Miller1 and Dennis V. Kent Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York
Abstract. Two components of magnetization were isolated in the Upper Ordovician Juniata Formation sampled in the area of the Pennsylvania salient. The thermally distributed, reversed polarity B component was most likely acquired during Alleghenian deformation, and although it is poorly grouped, it is similar to other Appalachian synfolding magnetizations. The pre-Alleghenian age C magnetization is entirely of normal polarity and shows a difference in declinations between the mean magnetizations isolated on the northern and southern limbs of the salient of 24° ± 23°. This anomaly is consistent with the sense and magnitude of declination anomalies observed in pre-Alleghenian magnetizations isolated in other throughgoing Appalachian red beds of Silurian, Devonian, and early Carboniferous age. The mean inclination of -44.7° suggests a paleolatitude of about 26°S for the central Appalachians in the Late Ordovician. This paleolatitude fits a trend of southward motion of North America from the Ordovician to the Early Devonian, followed by northward drift through the remainder of the Paleozoic. Introduction The Paleozoic sedimentary sequence of the North American Appalachians contains several clastic wedges within which red beds occur, principally the Upper Ordovician Juniata, Middle Silurian Rose Hill, Upper Silurian Bloomsburg, Upper Devonian Catskill, lower Carboniferous Mauch Chunk, and the upper Carboniferous/Lower Permian Dunkard formations [Thomas, 1977]. This sediment package was deformed in the Permo-Carboniferous Alleghenian orogeny [Dennison, 1982]. The relatively simple structure of the Valley and Ridge Province and the easily measured, high stability magnetizations of the red beds made these rocks prime candidates for paleomagnetic study. Indeed, these units were some of the first rocks to be sampled for paleomagnetic study in North America, with one of the first positive fold tests having been recorded in samples taken from the Rose Hill by Graham [1949]. The Juniata red beds were first sampled for paleomagnetic study some 30 years ago [Collinson and Runcorn, 1960]. Data from this early study were used to help bolster the first-order observation of continental drift between North America and Europe. These original data represented the total natural remanent magnetization (NRM) of the samples as no demagnetization was done on the collection. In the middle to late 1960s and in the 1970s, recognition of primary and secondary magnetizations in Appalachian rocks such as the Bloomsburg [Irving and Opdyke, 1965; Roy et al., 1967] clearly showed the value of employing thermal
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Copyright 1989 by the American Geophysical Union Paper number 88JB03637 0148-0227 /89/88JB-3637$05.00
demagnetization in the study of red beds. During this time period all of the major Appalachian red beds were studied or restudied using modern paleomagnetic techniques. The revised results from the Juniata were reported by Van der V oo and French [1977], and were incorporated into the analysis of Schwartz and Vander Voo [1983], which concluded that there was no oroclinal rotation involved in the formation of the Pennsylvania salient, a major structural feature of the central Appalachians. Recent controversy regarding the Paleozoic reference poles for North America and their tectonic implications [Kent and Opdyke, 1978; Vander Voo et al., 1979; Irving, 1979; Roy and Morris, 1983; Irving and Strong, 1984] sparked another round of study into the paleomagnetism of the Appalachian red beds. Although late Paleozoic remagnetization of Appalachian red beds has long been documented [Roy et al., 1967], the new studies brought to light the previously unsuspected complication that the Kiaman remagnetization was synchronous with the Alleghenian deformation. Thus although the previous results from the Mauch Chunk [Knowles and Opdyke, 1968] and Catskill [Van derVoo et al., 1979] had reported dual polarities and positive fold tests, they were in fact seriously contaminated by the synfolding remagnetization [Kent and Opdyke, 1985; Miller and Kent, 1986a, b]. The revised results from the Mauch Chunk and Catskill, as well as new data from the Bloomsburg Formation [Kent, 1988], reveal a declination anomaly in the pre-Alleghenian age magnetizations between the northern and southern limbs of the Pennsylvania salient which could indicate oroclinal rotation. In addition, the recent results from the Lower Devonian Andreas red beds [Miller and Kent, 1988a] and from the Bloomsburg [Kent, 1988] both show a best grouping of the highest stability component at less than full tilt correction, perhaps indicative of some remagnetization and folding associated with the Devonian Acadian orogeny even though structural evidence for Acadian deformation in the central Appalachians is ambiguous. The major Paleozoic Appalachian red beds which have not been restudied since the recognition of synfolding Kiaman remagnetizations are the Juniata, Rose Hill, and Dunkard formations. The purpose of the current study is to report new results for the Juniata. In the study of Vander Voo and French [1977], the Juniata was sampled primarily in the southern limb of the Pennsylvania salient. Here sampling was obtained from both the southern and northern limbs so as to allow for better documentation of evidence for oroclinal rotation. The Juniata and underlying Bald Eagle formations of the central Appalachians lack a distinctive fossil assemblage but are constrained to be Late Ordovician since they are underlaid by the Ashgillian Reedsville shale and overlain by the Llandoverian Tuscarora sandstone [Thompson, 1970a, b]. Samples were collected and data analyzed using standard paleomagnetic techniques [see Miller and Kent, 1986a]. Oriented samples from a total of21 sites were drilled in the Juniata Formation with eight sites in the northern limb of the salient, three sites in the hinge zone, and 10 sites in the southern limb. We reoccupied the same location as site 13 of Vander Voo and French [1977] as sites E,F,G and their site 14 as sites L,M,N. Site locations are given in Table 1 and Figure 1.
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1844
Miller and Kent: Upper Ordovician Juniata Formation TABLE 1. Juniata Formation Site Data and Paleomagnetic Directions. C Component
B Component S L s STK/DP
LAT/LON
n
AN 5 256m B N 6 255/90 C N 5 250/54 D N 5 259/54 E S 5 213/96 F S 4 215/100 G S 5 205/94 HS 5 26/45 I s 5 26/45 J s 6 193/42 K S 7 192/43 L S 5 14/52 MS 5 12/58 NS 5 12/57 0 H 8 236/90 P H 5 53136 QH 5 53136 RN7 96/12 S N 7 250/37 TN 6 74/80 UN6 251135
4o.3on7.13 4o.3om.t3 40.z8m.z7 4o.z8m.z7 40.01n8.48 40.01n8.48 40.01n8.48 39.88ns.5o 39.88n8.50 40.17n8.50 40.17n8.50 40.16n8.37 40.16n8.37 40.16n8.37 40.95m.75 4t.oon7.5o 4t.oom.5o 40.92m.5o 40.92m.5o 40.85m.t7 40.85m.t7
2* 1* 0* 2* 4 4 5 5 4 1* 5 5 5 5 2* 5 5 7 4 6 3
k
a95, deg
GD, !leg
GI, !leg
-
286.7 122.6
14.6 72.3
155.9 206.4 210.4 199.0 145.5 167.4 209.8 225.5 160.5 156.2 159.5 242.5 167.8 154.7 179.1 194.3 152.4 194.3
-26.6 -19.8 -20.1 -35.4 37.3 37.0 -18.2 16.9 6.9 .7 3.9 -52.6 -25.1 -37.9 -8.9 1.7 38.8 -35.5
181.2 180.0 181.1 181.0 174.4 176.1 161.7 181.6
-1.3 -5.0 -9.4 3.4 -5.9 -7.2 -31.7 -64.0
19 10 51 5 4
21.5 31.4 10.8 36.3 57.0
7 32 46 48
31.7 13.8 11.5 11.2
17 12 32 4 14 6
22.8 10.9 57.8 18.7 53.7
-
19.1
n
k
a95, deg
GD, deg
GI, deg
16 17 20 4 7
19.5 19.3 17.7 77.1 50.2
340.2 341.4 347.4 4.1 337.4 24.5
38.7 37.5 8.7 -1.9 53.7 48.8
13
36.0
21.3
-60.7
21 11 17
27.4 24.0 31.1
337.7 319.7 86.9
-22.4 -3.9 -64.5
38 13
12.5 35.2
27.5 344.4 67.9 10.3
-4.0 53.3 -67.9 -65.4
6
30.2
3.4
-13.7
6
35.1
359.7
-13.0
6/8
10
22
353.6
9.3
5/10
44 2
10.2 73
357.4 346.9
-47.8 -27.6
19
18.1
333.0 37.5 351.6
-43.0 -69.3 -44.0
5 5 5 3 3 1* 0* 3 0* 3 5 3 0* 1* 2* 5 3 0* 6 0* 5
Means: O%TC 50%TC 100%TC O%TC 80%TC 100%TC O%TC 100%TC
North Limb South Limb Hinge
4/8 5 4/8 12 4/8 5 9/10 5 9/10 8 9/10 8 2/3 2/3
44.0 28.3 43.5 26.3 19.2 19.9
2/3
S is letter designation for site; L denotes salient limb site was located on (N,H,S = north, hinge, south); s is number of samples taken at site; STK/DP are strike (90° counterclockwise from direction of dip) and dip of bedding; LAT/LON are latitude (0 N) and longitude (OE) of site; n is the number of samples used in site calculations, or for means, the ratio of sites used in mean calculation to total sites (not used with *); k is Fisher's precision parameter; a95 is semi-axis of radius of confidence; GD and GI are declination and inclination in geographic coordinates; TC is tilt correction.
Demagnetization Behavior Thermal demagnetization behavior of the samples was much as described by Vander Voo and French [1977]. Samples rarely contain a low unblocking temperature (
