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An abstract and introduction to a geologic report on gravity anomalies in New Jersey. The report includes a revised gravity anomaly map of New Jersey and its surrounding areas, a brief explanation of gravity anomalies and their interpretation, observed gravity values, data reductions, and the significance of gravity anomalies. The document also mentions the contributions of various researchers and institutions to the study of gravity anomalies in New Jersey.
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B O UGU E RG RA V ITY _ : _OM A L Y MA P OF NEWJERSEY By W illi om E. Bonin i Assoc i ate P rofessor Deportm ent o f G eologica l Engineering P ri n ceton Un i versi ty A B STRACT A Bougu e r G r avity A nomaly Map of N e w J e r s ey (s c al e 1: 25 0,000) is p r e se n te d. I t is based on over 4 000 gravity observations, half of which were previously unpublished. Con- tour interval i s 5 milligals and the map is translucent , so that it ca n be us ed a s an overlay on the State Geologic Map. A cross-section from Philiipsburg, New J er s ey, to Baruegat Bay is given with Boaguerand isostati c anomalies show n. The tra n s-New Jersey g r avity high is a marked positive Boug u er and isostatic anomaly and is r elat e d to basement or sub-ba s ement de n sity excess. The trans-New J e r sey high i s part of a series of gravity highs run n i n g fro m Georgia to Vermont and generally a s sociated with the Pied m ont Provin c e south of New Jers e y•
INTRODUCTION
S c ope and P urpos e Over 4000 gravity observations have been made in New Jersey and in nearby areas of New York and Pennsylvania. G. P. Woollard (1943a) published a regional gravity map based on 1000 stations of the middle Atlantic states which covered New Jersey, as well as Delaware, Connecticut, southern New York, and eastern Pennsylvania. He further reports the results of a detailed gravity sur v ey consisting of 700 gravity observations in a 5-mile wide section between Barnegat Bay and Phillipsburg, New Jersey. Steenland and Wooll u rd (1952) report the results of a gravity study of the Courtlandt Complex near Peekskill, New York, based on 185 stations. J. B. ttersey (1944) has made a gravity map of an a / 'ea west of Phillipsburg_ New Jersey, based on 700 stations. Faculty and students at Princeton University in unpublished detailed studies have add e d over 2000 new gravity observations to this published data. About 1700 of these new measurements have been reported in six thes e s in the Department of Geological Engineering (Bonini, 1949; Meier, 1949; Johnson, 1956; Stewart, 1962; Vreeland, 1965; and Wofford, 1962).
The purpose of th i s geologic report is t o present the r evised gravity anomaly map of New Jersey and sur r ounding area compiled from both the published and unpublished data. For those not familiar with g r avity anomalies, there is a b r ief section on anomalies and their inter- pretation in New Je r sey.
ACKNOW L EDGE M ENTS Geo r ge P. W oo llard, D i r e cto r _ I n stitute of G eo physics, Uni v ersity o f H awa i i, kindly supplied a copy of his revised 1943 data. John E. Hardaway and John H. Vreeland, while students at Princet o n University, provided c o nsiderable assistance both in the field and in preparation of the gravity map. Princeton University supplied the vehicle and gravity instrument for the field work. The Higgins Fund of Princet o n University and the National Science Foundation provided partial support for the field work and office compilation of the data. The writer is grateful t o Kemble Widmer, New Jersey State Ge o logist, for his encouragement with respect to preparation of the map and fo r making possible its publication as a translucent overlay for the State Geologic Map. GRAVITYDATA This section ref er s only to the Princeton Unive r sity data, since He r sey (1944), Woolla r d (1943a), and Steenland and Woollard (1952), have covered the following items with respect to their observations. Observed Gravity Values Values for observed gravity are based on the abs o lute value (980.1776 gals) 1 in the cen- ter of R o om 142 , Guyot Hall, P r i n c e ton University, and all field stations a r e tied to this v alue. The Guyot Hall value has been established by measurements between it and the fundamental gravity sta- tion in Washington, D_._ C., by Bonini and Woollard (1957).
SI G N I F ICANCE O F GRAVITY ANO M A L I E S General The gravityanomaly at a given locationis the differencebetween the value observed at that point and the predictedtheoreticalvalue. In the calculationof a theoreticalgravityvalue two thingsmust be considered: (1)the latitudeof the stationand (i_)itselevationwith respect to sea level. The theoreticalvalue of gravityat sea level at a given latitudeis determined according to the 1930 InternationalGravity Formula (U. S. Coast an d Geodetic Survey, 1942),which considers thenon-spherical shape of the earth and the effectsof the centrifugalforce. The "latitudeeffect" in New Jersey is approximately1.2 regal mile,increasingnorthward. The s e cond theoreticalconsiderationconcerns the elevationof a stationwith respect to sea level,since the InternationalGravity Formula predictsgravityat sea level. Ifa stationis 100 feet above sea l e vel_ it is 100 feet further from the earth's center and the theoretical value o f gravity there will be less than that given by the Formula. If one considers only the change in elevation between the station and sea level (as would be the case in a tall building), this effect is 0.094 mgal / ft decreasing upward from sea level. This is called the free air effect. On the other "
hand, for field observations when one is at 1 0 0 ft elevation, t h e r e is mass (rock) between the station and sea level, and this mass in t h eory would increase the gravitational attraction by an amount determined by the elevation and the density of the material. This effect is called the Bouguer, or mass , effect. Using a density between the station and sea level of 2.67 gm / ce, t his effect is 0.034 mgal / ft, increasing with increase in elevation. This assumes that there is
no topography near the station, i.e., that the station is in this case on a p l ain 100 feet above sea level. For detailed work where there is considerable t o pography, terrain corrections must be made. Normally , the f r ee air effect and the Bouguer, or mass, effect a r e combined as follows for elevation above sea level: Free air effect -0.094'mgal / ft Bouguer (mass) effect Combined effec t -0.060 mgal / ft The theoretical result is a decrease in g r avity with an inc r ease in elevation. Therefore a correction has to be made for eleva t ion. When these elevation effects are taken into consideration, we can predict a theoretical valile for gravity for any given location. The difference between the theoretical and observed gravity va l ues is the anomaly. I f both the free air and Bouguer (mass) corrections are applied, the resulting anomaly is the "Bouguer anomaly." This is the anomaly of interest to geologists, since the Bouguer anomaly is rela t ed more directly to subsurface mass distribution. If the subsurface mass distribution were uniform everywhere, the anomaly would be uniform. In general, a positive Bouguer anomaly indicates an excess of mass beneat , h the station and a negative Bouguer anomaly, a deflclen:cy. An isostatic anomaly is derived by applying one further theoretical correction. Simply stated, the theory of isostasy says that the major topographic features of the earth are associated with mass irreguh:rities, i.e., mountainous areas have roots of lower mass , and mountains t end to "float" in equilibrium in the ear t h's crust, much as an iceberg does in water. The isostatic correction takes into account the lower mass of the root and therefore lower observed g r avi t y to be expected if the area is in perfect isostatic equilibrium. There are several ways of correcting
0 5 vD^ SCAL_^ Ih t^ MILES $ 5^ ZO 25 VERTICAL^ E X_G G E R A T R3N - v _ B
FIGU R E i. CR A N OOMALIE SS - SE CTION S. GEOLO F R GY OM PHIS ILL I PMO S BU D E I FG IE (^) D FR TOM O (^) BA SEC R NETION GAT (^) C - BD A Y (^) OF SH OWINT HE G (^) N E W BO U JEGUE R SE R Y (^) AGE N D OLO (^) G ICI S O S TATI C M A P.
e x plain r esidual anomalies on the order of 1 or 2 mgals o n the crest of the hig h , but that mos t of the high t rend itself was caused by high density basemen t or sub-basement rocks.
Rocky Hill Diabase Stewar t (1962) further outlined t h e residual gravity high associated with the R ocky Hill diabase, confirming the earlier work of Woollard (1943a) on this body. R ound Valley Wofford (1962) inves t igated the diabase of Cushetunk Mountain, site of the Round Valley Reservoir. This data, in addi t ion t o later work by the au t hor and his studen t s, confirm the ori- ginal es t imate that the diabase dips up to 80 ° outwardly from R ound Valley on the northeast , llmb, and more gently (around 30 °) on the southwest limb (see Widmer, 1960). Magnetic anomalies also confi r m this estimate. Sanford (1963) indicates that the Cushetunk material is a flow and structural l y an anti- cline. Based on detailed geological s t udies in this area by the New Jersey Geological Survey, W idmer (personal communication) indicates that t he petrologic classification of the Cushetunk materia l is diabase and the attitude of the surrounding Brunswick shale indi c ates that the diabase is intrusive and probably in the form of a cone sheet.
RE G IONAL FEAT U RES I n ge n eral, the g r avity anomal y tre nd s are pa r a l lel to sub - pa r al l el to the struct ur al trends of the Piedmont and Appalachians. The most prominent feature in New Je r sey is the trans-New Jersey gravi t y high, on which there is a peak Bouguer value of 42 regals. It crosses the state in a northeast direction, from near Wilmington, Delaware, through Skyesville, New Jersey, just east of Staten Island and into Manhattan and Queens, New York (see map and cross-section). Woollard (1943a) shows a continuation into Connecticut and Massachusetts on the northeast and southward less pro- minently through Washington, D. C.* A lesser group of highs is located near Cedar Grove and Millville, New Jersey, and the trend is nearly parallel to the trans-New Jersey high. In gross aspect, the trans-New Jersey high is part of a series of gravity highs running from Georgia to
Hersey,Amer., J. B.,v. 55,1944, p. Gravity417-444 investigations of central-eastern Pennsylvania: Bull. Geol. Soc. Johnson,B.S.E. G. M.,thesis, 1956, Princeton Gravity Universityand magnetic study of the Fort Dix area, New Jersey: unpublished Longwell,Hudson C. (^) ValleyR., 1943, region: Geologic Bull. interpretationGeol. Soc. (^) Amer.,of gravityv. 54, anomaliesp. 555-590 in the southe r n New England-
Meier,published D. R., 19 M.S.E. 4 9, Geophysical thesiS, Princetoninvestigation University in the Trenton-Old Bridge area, New Jersey: un-
Sanders,Sc., J.v. E., 2 61_ p. 1963, 501-524 Late Triassic tectonic history of northeastern United States: Amer. Jour. Steenland, N. C. and Wootlard, G. P., 1952, Gravity and Magnetic Investigation of the Structure of the Cortlandt Complex, New York: Bull. Geol. Soc. Amer., v. 63, p. 1075- Stewart,published D. D., B.S.E.1962, Gravitythesis, Pstudy r inceton of theUniversity Princeton-New Brunswick area, New Jersey: un- U. S. Coast and Geodetic Survey, 1942, Theoretical gravity at sea level for each minute of latitude by the International Formula: Publ. G-53, 9 p. Vreeland_ J. H., 1965, Gravity anomalies and geology of the Jenny Jump Mountain area, New Jersey: unpublished M.S.E. thesis, Princeton University Widmer,Set (^). K.,II, 1960,v. 22 , Geologicalp. 22 3 -2 3 (^) 2 problems in the construction of dams: Trans., N. Y. Acad. Sci., Wofford,unpublished G. T ., 1962,B.S.E. Gravity thesis, studyPrinceton of Cushetunk University. Mountain and surrounding area, New Jersey: Woollard, G. P. , 1941, Geophysical methods of exploration and their application to geologic pro- blems in New Jersey: Dept. of Cons. and Dev. of N. J., Bulletin 54, 89 p. 1943a,vicinity: GeologicBull. Geol.correlation See. (^) Amer.,of areal v. gravitational54, p. 791-818 and magnetic studies in New Jersey and 1943b, Transcontinental gravitational and magnetic profile of North America and its relation to geologic structure: Bull. Geol. Soc. A mer., v. 54, p. 747-