w N N

SCALE

0 10 20 30 40 SOMI

0 20 40 60 BOKM

EXPLANATION

D D D D D D -9,800-10,000 10,000-10,600 10,600-10,800 10,800-11 ,000 11 ,000-11 ,200 11,200-11,400 11,400-11,600 11 ,600-11,800 11,800-12,000

Total intensity magnetic field of the earth Values are in gammas relative to an arbitrary datum.

Figure 24-1. Aeromagnetic map of Pennsylvania (modified from Zietz and others, 1980). Contour interval is 200 gammas; inter­mediate 50-gamma contours are omitted in the southeast. A, New Bloomfield high; B and C, subsurface nappes; D and E, anoma­lies over largely subsurface Proterozoic rocks; F, inferred basement fault .

-12,000-12,200

Part IV. Regional Geophysics

CHAPTER24 AERO MAGNETICS

ELIZABETH R. KING U.S. Geological Survey National Center, MS 927 Reston, VA 20192

INTRODUCTION Aeromagnetic mapping began in Pennsylvania in

1944 with a survey over Boyertown using a flux-gate magnetometer. This was the first test of this method in the western hemisphere (Hawkes and others, 1953). The area was selected because of the effectiveness of ground magnetic investigations of Cornwall-type magnetite deposits by the U.S. Geological Survey and the U.S. Bureau of Mines in the early to mid-1940's. In 1948, a major new "blind" iron orebody was located at Morgantown by a private company using similar equipment (Jensen, 1951). That ore­body was subsequently developed as the Grace mine by the Bethlehem Mines Corporation. In the follow­ing years, Pennsylvania was completely covered by surveys by or under the auspices of the U.S. Geologi­cal Survey, most of which were in cooperation with the Pennsylvania Geological Survey (Joesting and others, 1949; Henderson and others, 1963; Popenoe and others, 1964; Bromery and Griscom, 1967; U.S. Geological Survey, 1969, 197 4a- g) . A more recent survey covering the whole state at a flight-line spac­ing of 3 to 6 miles was made by the U.S. Department of Energy under the National Uranium Resource Evaluation (NURE) Program (see Chapter 25).

A composite magnetic map based on these sur­veys (Figure 24- 1) shows a profound contrast be­tween southeastern Pennsylvania, where the exposed rocks have highly complex magnetic anomaly patterns, and the Appalachian basin, where broad, smooth magnetic anomalies reflect the deeply buried rocks of the crystalline basement. Nearly all the compo­nent surveys were flown north-south, but flight-line spacing varied widely, from 0.25 mile to as much as 6 miles in one area. Flight altitudes also varied, from 500 feet aboveground to a barometric altitude of 3,000 feet above sea level (Figure 24- 2; Table 24- 1). For areas where the distance from the sources of the anomalies is great, as it is in most of the state, these differences in flight level are tolerable.

SOUTHEASTERN PENNSYLVANIA Most of southeastern Pennsylvania is covered

by a survey made by the U.S. Geological Survey in

323

324 E. R . KING

Figure 24-2. Index map of aeromagnetic surveys used to produce the aeromagnetic map of Pennsylvania shown in Figure 24-1. Para­meters of each survey are given in Table 24-1.

The diabase anomalies terminate abruptly along the northern edge of the basins, where the Mesozoic rocks have been downfaulted against Proterozoic and Paleozoic rocks . Near the southern margin of the Newark basin, the clastic rocks thin, and the under­lying basement rocks give rise to a couple of broad northeast-trending highs (Socolow, 1974, p. 55- 57, 69). A prominent magnetic anomaly over Buckingham Mountain (see Figure 24- 3) is produced by a fault-bound sliver of Proterozoic and Paleozoic rock that forms a northeast-trending ridge, dividing the Newark basin into two parts . The faults (the Furlong and others; see Chapter 21) cut the Mesozoic rocks and bound small diabase sheets on the north just as the larger sheets are bounded along the northern boundary fault. The Buckingham magnetic high in­

Table 24-1. Flight-Line Direction, Altitude, Spacing, and Source of Data for the Aeromagnetic Map of Pennsylvania

dicates a large subsurface ridge of magnetic Proterozoic rocks extending 15 miles south­west from the state line. Models calculated from the magnetic data (Zietz and Gray, 1960) indicate a Proterozoic basement slop­ing to the south that is 7, 000 feet deep 3 miles south of the exposed Proterozoic rocks.

Flight-line Altitude 1 Spacing Area2 direction (feet) (miles) Source of data

A N-S 500 Popenoe and others (1964) B N-S 33,000 2 U.S. Geological Survey (1969) c N-S 500 'A Bromery and Griscom (1967) Magnetic rocks occur just north of the

Mesozoic basins in the Reading Prong and Blue Ridge (Figure 24- 3). The intricate, closely spaced pattern of magnetic anoma­lies over the Reading Prong is produced by a complex of magnetite-rich, gneissic Pro­terozoic rocks at the surface (Bromery and Griscom, 1967). Work by Drake (1970) in­dicates that these rocks are allochthonous and form the core of a huge nappe system overthrust from the southeast. North of the

D N-S 1,000 'h Henderson and others (1963) E N-S 31,000 I U.S. Geological Survey (1974g) F N-S 1,000 4 U.S. Geological Survey (1974a-f) G N-S 1,000 'h Joesting and others (1949) H E-W 500 'h U.S . Geological Survey, unpub-

lished data E-W 500 6 High Life Helicopters and QEB

(1982)

1 Aboveground unless indicated otherwise. 2Shown on index map in Figure 24-2. 3Barometric altitude.

1956 and 1957 with the primary goal of locating Cornwall-type iron ore deposits. The resulting series of 77 maps at 1:24,000 scale was used to locate numerous concentrations of iron ore, which were drilled by private industry. Socolow (1974) made brief interpretations of each map, describing cor­relations and noncorrelations of the magnetic anoma­lies with surface geology, and Bromery and Griscom (1967) prepared a composite magnetic map of the entire area on a geologic base with a detailed inter­pretation of the magnetic patterns on the map .

The predominantly clastic rocks of the Newark and Gettysburg basins (Figure 24- 3) do not have a significant magnetic susceptibility . They have been intruded by a number of highly magnetic saucer­shaped bodies of diabase that are delineated by mag­netic anomalies with oval or loop like map patterns.

exposed Proterozoic rocks of the Reading Prong, there is a northeast-trending magnetic high (C in Figure 24- 3) with a dip or saddle near Allen­town. Bromery and Griscom ( 1967) concluded that it was caused by similar Proterozoic rocks at depth be­neath the Paleozoic sedimentary rocks of the Great Valley . Drake ( 1978) calculated from the magnetic data that these Proterozoic rocks form the core of the Lyons Station-Paulins Kill nappe at a depth of 1.6 km (I mi) . A broader magnetic high to the northeast (Bin Figure 24-3) may indicate an even deeper nappe or thrust sheet. Recent drilling and a Vibroseis pro­file near the Pennsylvania-New Jersey border (Rat­cliffe and others, 1986) show that the fault boundary between the Newark basin and the Proterozoic rocks dips gently southward. These data also indicate that there is a series of imbricate thrust slices in the Pro­terozoic rocks of the footwall block (Ratcliffe and

PRONG

SCALE

0 w ~ ~~

0 10 20 30 40 KM

Figure 24-3. Detailed aeromagnetic map of southeastern Pennsylvania showing the location of the Gettysburg and Newark basins, Reading Prong, Blue Ridge, and Martie Line. Contour interval is 100 gammas; intermediate 10-gamma contours are shown in the northwest. A, New Bloomfield high; B and C, inferred subsurface nappes; D and E, anomalies over largely subsurface Proterozoic rocks; HBU, Honey Brook Upland. Map is from a portion of the Aeromagnetic Map of Pennsylvania (U.S. Geological Survey, 1978).

(') :I: > ::':l tTl :>::1 N

""'" I > tTl :>::1 0 3: > 0 z ~ n "' (..,) N lit

326 E. R. KING

others, 1986), which is compatible with the magnetic data.

The smoother pattern over the Catoctin metavolc canic rocks of the Blue Ridge anticlinorium (Figure 24- 3) indicates that these rocks are only moderately magnetic. The Blue Ridge anticlinorium south of Pennsylvania is typically marked by a pair of large, linear magnetic highs . The Catoctin is mostly meta­basalt to the south, but in Pennsylvania, less mag­netic metarhyolite predominates (Espenshade, 1970).

The region south of the Mesozoic basins is di­vided by a pronounced northeast-trending magnetic lineament (Martie Line in Figure 24-3) into an in­tensely magnetic southern province and a less mag­netic, more heterogeneous region to the north. The eastern part of this lineament coincides with the Mar­tic Line (Stose and Jonas, 1935), but the western part is slightly south of the traditional Martie Line, which Wise (1970) distinguished from the "magnetic Martie Line." South of this lineament, the rocks consist of a thrust stack of highly magnetic "eugeosynclinal" rocks, including ophiolitic fragments. To the north, much less magnetic carbonate and clastic rocks pre­dominate. Socolow (1974, p. 73- 75) suggested that the magnetic line may be a function of magnetic suscepti­bility controlled by the preexisting mineralogy (miner­alization?) of the metasediments rather than structure.

North of the Martie Line, most of the magnetic anomalies are related to Proterozoic basement rocks . An area of numerous small anomalies east of Lan­caster (D in Figure 24- 3) coincides with a domal area of Middle Proterozoic gneisses exposed in the Mine Ridge anticline and related structures. The mag­netic data indicate similar rocks at shallow depths both to the west toward Lancaster and to the east of the Honey Brook Upland (HBU in Figure 24- 3) under the Triassic basin. A southwest-trending mag­netic high west of Lancaster (E in Figure 24- 3), having peaks at either end over exposed mafic Pro­terozoic rocks, indicates a shallow basement ridge or thrust sheet.

South of the Martie Line, Middle Proterozoic basement gneisses are exposed in several domelike structures that are marked by characteristic magnetic lows. These lows have an open pattern that is in sharp contrast to the pattern of the surrounding highly magnetic units of the overlying Glenarm Supergroup. These so-called domes include the Woodville, Avon­dale, and West Chester, and the smaller Mill Creek dome east of Philadelphia, which was first identified from magnetic data (Higgins and others, 1973). Mc­Kinstry (1961) concluded that these were domal up­lifts, but Bailey and Mackin (1937), Mackin (1962), and later Fisher and others ( 1979) concluded that

these were refolded nappes . Bromery (1968), in a study of similar domes north of Baltimore, was able to trace the magnetic expression of the surrounding rocks under the gneisses. More recent geological and gravity studies by Muller and Chapin (1984) have shown that these structures are part of a refolded nappe system.

THE APPALACHIAN BASIN

The broad, low-gradient magnetic anomalies typical of the Appalachian basin are produced by magnetic rock units in the Precambrian crystalline basement underlying the thick sedimentary sequence, which is essentially nonmagnetic . Only a few drill holes have penetrated basement, all in the western part of the basin (Saylor, 1968). Vacquier and others (1951) developed and tested mathematical techniques for determining the depth and shape of a magnetic body using several of the early surveys made by the U.S. Geological Survey, including a survey in an area northwest of State College in central Pennsylvania (area Gin Figure 24- 2). Depth calculations indicated that the basement surface is 19,000 to 22,000 feet be­low the surface in the western part of this area and slopes to much greater depths to the southeast (Joest­ing and others, 1949). Prior to the availability of aero­magnetic data, there was much uncertainty about the degree of basement involvement in the Appalachian deformation. In the controversy between advocates of so-called thick -skinned deformation versus those fa­voring thin-skinned tectonics (Rodgers, 1949), mag­netic mapping provided independent evidence that the crystalline basement was not involved in the folds mapped at the surface. The major basement trends de­termined from the magnetic surveys are independent of the surface trends of the mapped Appalachian folds in Pennsylvania.

Other aeromagnetic surveys have added infor­mation on the basement. Analysis of an aeromagnetic survey of a large area in western Pennsylvania (area A in Figure 24- 2) showed that the basement slopes from about 8,000 feet below sea level at the north­west comer of the area to over 20,000 feet below sea level in the southeast, near 79° west longitude (Beck and Mattick, 1964). In 1974, a drill hole (#1 Leonard Svetz) penetrated the Upper Cambrian at 18,980 feet below sea level in Somerset County, close to the location of the magnetic calculation. Data from a regional survey across southwestern Pennsylvania (Zietz and others, 1966), which indicated a pro­found deepening of the basement to the east toward the Blue Ridge where exposed Proterozoic rocks have been overthrust toward the basin, were used by Gwinn

(1970) to back up his arguments for thin-skinned deformation in the central Appalachian fold belt. The same survey also revealed the presence of a large wedge-shaped block of strongly magnetic rocks in southwest Pennsylvania and adjacent West Virginia. On the northeast, the block has a sharp, linear bound­ary (F in Figure 24-1) that is 100 miles long and appears to be fault controlled. Lavin and others (1982) have proposed that this magnetic boundary, along with other geophysical and geologic data, delineates a much longer feature, the Pittsburgh-Washington lineament, that is one of several cross-structural lineaments in­volving basement rocks in the northeastern United States (Parrish and Lavin, 1982; Rodgers and Ander­son, 1984; also see Chapter 22) .

As soon as magnetic coverage became available for the entire Appalachian basin (King and Zietz, 1978), a major magnetic lineament was apparent along its entire length. The New York-Alabama lin­eament divides the basement into two magnetically distinct areas (Figure 24- 1). To the southeast, the few anomalies present are very broad and have gen­tle gradients consistent with the profound basement depths of the region adjacent to the Blue Ridge. To the northwest, numerous anomalies indicate a base­ment composed of large units of rocks with strongly contrasting magnetic properties. Several anomalies have well-defmed northeast trends that are discordant with the more easterly trends of the fold axes at the surface, and other anomalies trend north or north­west.

The New York-Alabama lineament may mark the edge of a stable craton of older Precambrian rocks that limited the strong Appalachian deformation on the east, as arcuate salients of the fold belt in both Tennessee and Pennsylvania are tangential to it. The linearity of this feature suggests that it may be a strike-slip fault analogous to the strike-slip faults of the Tibetian Plateau that are associated with the col­lision of India with the Asian continent (King and Zietz, 1978) .

Southeast of the New York-Alabama lineament and west of Harrisburg, there is a large triangular anomaly (A in Figure 24-1) that has an associated positive gravity anomaly . Fleming (1975) modeled both magnetic and gravity data for this feature, which he called the New Bloomfield high. He concluded that it is caused by a large block of basaltic material in the Proterozoic basement, possibly related to Catoc­tin metabasalts at the surface and emplaced in late Precambrian time during continental rifting and the opening of the proto-Atlantic ocean.

CHAPTER 24-AEROMAGNETICS 327

PROBLEMS AND FUTURE RESEARCH

Although Pennsylvania has been in the fore­front in the development and application of aero­magnetic techniques, the available data are uneven in quality. The older surveys, such as the one for southeastern Pennsylvania, were recorded in ana­logue form with a fluxgate magnetometer. Much of the Appalachian basin is covered by data spaced 3 or 4 miles apart. Simultaneous acquisition of aero­magnetic data by modem proton magnetometers and gamma-ray data where gamma-ray coverage is cur­rently inadequate (see Chapter 25) would be very worthwhile .

An updated database would aid in resolving many of the questions about the Precambrian base­ment. Perhaps the most important problem concerns the degree of participation of the basement in the deformation of the overlying sedimentary rocks in the plateau area, where basement control of some of the surface lineaments and small kimberlite intru­sions has been postulated by Parrish and Lavin (1982) (see Chapters 13 and 20). The location and configu­ration of the Rome trough in western Pennsylvania is poorly known and might be delineated by high­precision aeromagnetic data.

RECOMMENDED FOR FURTHER READING Bromery, R. W., and others (1959- 61 ), Aeromagnetic maps of

southeastern Pennsylvania, U.S. Geological Survey Geophysi­cal Investigations Maps GP-200-210, 213-245, and 254- 287, scale 1:24,000.

Fisher, G. W. , Higgins, M. W., and Zietz, Isidore (1979), Geo­logical interpretmions of aeromagnetic maps of the crystalline rocks in the Appalachians, northern Virginia to New Jersey , Maryland Geological Survey Report of Investigations 32, 43 p.

Fisher, G. W., Pettijohn, F. J. , Reed, J . C., Jr., and Weaver, K. N., eds . (1970), Studies of Appalachian geology: central and southern , New York, Interscience Publishers, 460 p.

Nettleton, L. L. (1971), Elemenrary gravity and magnetics for geologists and seismologists, Society of Exploration Geophysi­cists Monograph Series 1, p. 73-121.

Socolow, A. A. (1974), Geologic interpretation of aeromagnetic maps of southeastern Pennsylvania, Pennsylvania Geological Survey, 4th ser. , Information Circular 77, 85 p.

Telford, W. M., Geldart, L. P., Sheriff, R. E., and Keys, D. A. (1976), Applied geophysics , Cambridge, England, Cambridge University Press, p. 105-217.