HARRY J. HANSEN Geological Survey, Baltimore, Maryland 21218

A Geometric Method to Subdivide the

Patapsco Formation of Southern Maryland into

Informal Mapping Units for Hydrogeologic Use

Abstract: The of southern Maryland is a thick, heterogeneous sequence of unconsolidated rocks occurring in the upper part of the Potomac Group. Experience suggests that it functions as a multi-aquifer unit of hydraulic complexity. In the absence of definitive geologic criteria an adaptation of Hakes' (1963) technique of perspective correlation is used to subdivide the Patapsco Formation into consistently denned mapping units. These units are useful for delineating vertical and horizontal changes in such parameters as sand percentage and coefficient of transmissibility.

Introduction remian), the Arundel Clay (Aptian), and the Patapsco Formation (Albian). In the Baltimore- Purpose of investigation. One of the more Washington, D.C., area, subdivision of the onerous problems of hydrogeology is the sub- Potomac Group into three formations was division of nonmarine, fluvio-deltaic sediments made possible by the Arundel Clay, which into hydrostratigraphic units (Maxey, 1964). separates two predominantly arenaceous se- In Maryland and vicinity this problem is quences. Recent investigators have generally brought into sharp focus by the Patapsco For- corroborated these findings (Bennett and mation of Lower Cretaceous age, a thick Meyer, 1952; Otton, 1955; Glaser, 1966). lithologic unit of disconcerting heterogeneity. Soon after its establishment, the Potomac Before quantitative modeling of this for- tripartition was prematurely extrapolated into mation is possible, an interim hydrogeologic adjoining areas, such as , without framework showing trends in such parameters regard to the lithologic complexities of fluvio- as sand thickness and permeability is pre- deltaic sedimentation (Mathews, 1933). Later requisite. To accomplish this, a set of con- workers recognized this, and, as a consequence, sistently denned mapping units must be es- some have reverted to McGee's initial con- tablished. Ideally, the boundaries of these units ception of an undifferentiated "Potomac For- should coincide with hydrogeologic boundaries; mation" (Groot, 1955; Richards and others, initially, however, such refinement is often 1957; Southwick and Owens, 1968). impossible. Under these circumstances ex- Recent hydrogeologic studies have demon- perience suggests that in the beginning phases strated, however, that Potomac sediments can of investigation, indirect geometric or statistical be subdivided into informal rock stratigraphic methods of subdivision are preferable to a units using subsurface data, chiefly geophys- single, undifferentiated formation of lithologic ical logs. (Sundstrom and others, 1967; Slaugh- heterogeneity. ter and Otton, 1968). To recognize hydrogeo- Historical review. The name, "Potomac logic trends within these thick, heterogeneous Formation," was assigned by McGee (1886) to units, consistently denned "mapping slices" a lensoidal sequence of unconsolidated ocherous must be established. The procedure described to drab sands and clays outcropping along the herein is a geometric method for accomplishing Fall Zone in Maryland and adjacent Virginia. this. Later work by Clark and Bibbins (1897) re- sulted in a tripartite subdivision of the Potomac Method sediments into the Patuxent Formation (Bar- Introduction. To show the distribution of

Geological Society of America Bulletin, v. 80, p. 329-336, 5 figs., February 1969 329

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Potomac sediments in southern Maryland, a may not necessarily coincide with formations cross-section well network was prepared. Cor- defined by paleontological data. They do, relations were established after study of geo- however, form consistent intervals for calcula- physical (electric and gamma ray) logs, tions such as sand percentage and may repre- lithologic samples, and available palynological sent discrete hydraulic systems. data (Hansen, 1968). The network consists of Because the Arundel Clay exhibits areal 12 sections subparallel to the dip of the Potomac continuity beneath much of southern Mary- Group and 4 sections subparallel to the strike; land, the traditional Potomac tripartition was it contains 58 wells. Figures 1 and 2 are modi- used in the cross-section network. The sand fied examples of two of these cross-sections. "suites" characterizing both the Patuxent The cross-section network demonstrates the Formation (lower arenaceous unit) and the occurrence of widespread "suites" of multi- Patapsco Formation (upper arenaceous unit) story sands (Potter, 1963) broadly correlative constitute thick multi-zoned aquifers of hy- in the sense implied by Visher (1965a, 1965b). draulic complexity. To minimize misinterpretation, it should be Ideally, any hydrostratigraphic zonation of understood that these "suites" are defined these units should be based on demonstrated primarily by their electrical characteristics and occurrences of hydraulic continuity. Locally this can be done in areas where observation wells are available for prolonged monitoring (for example Otton, 1955; Slaughter and Otton, 1968). In regions the size of southern Maryland, however, an observation well net- work capable of monitoring an aquifer system as complex as the Patapsco Formation is rarely existent, at least initially. Therefore, indirect methods of subdivision using statistical or geometrical techniques are helpful in providing an interim framework. Perspective correlation. An adaptation of Haites' (1963) perspective correlation method was used to expedite an interim subdivision of the Patapsco Formation. Basically, this method assumes that correlation lines con- necting two relatively undisturbed sections in the same geological province have a distant intersection that approaches a geometric perspectivity. Sedimentary prisms defined by this method are bounded by uniformly dipping horizons which, according to the cross-ratio laws of perspective geometry, result in a down- dip increase of thickness (prism 1 of Fig. 3). As would be expected, geologic anomalies, such as unconformities, result in major divergences from true geometric perspectivity (prism 2 of Fig. 3); for example, the Patuxent Formation which overlies deeply weathered Piedmont rocks exhibits such a divergence. Acceptance of sedimentary perspectivity in marine sediments is generally based on the hinge-line concept of basinal sedimentation. To be applicable to fluvio-deltaic sediments, a Figure 1. Geologic cross-section (A-A1) extend- somewhat different rationale is required. ing from northwest to south central Anne Arundel As discussed by Leopold and others (1964, County, Maryland. The resistivity curves from p. 258-266), the sedimentary prism formed by single-point electric logs are used to show lithology an aggrading fluvio-deltaic system exhibits a (after Hansen, 1968, Plate 7). knickpoint at or greater than the altitude of

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Figure 2. Geologic cross-section (Y-Y1) extending from Baltimore, Maryland, to Washington, D.C. The resistivity curves from single-point electric logs are used to show lithology (after Hansen, 1968, Plate 16). prevailing baselevel. Figure 3 depicts the long could be measured without changing sign, an profiles of two aggradational prisms having arbitrary datum of +400 feet mean sea level knickpoints analogous to geometrical per- was selected. spectivities. The well data used to construct the sedi- Patafsco Formation. An arithmetical plot of mentary prism illustrated in Figure 4 are con- thickness versus altitude of top and bottom of tained in the aforementioned cross-section Formation was made to determine whether the network of southern Maryland (Hansen, 1968). Patapsco Formation in southern Maryland Only wells penetrating the entire formation functions as a sedimentary prism amenable to were used to delineate the sedimentary prism. subdivision using geometric cross-ratio By the method of least squares, best-fit lines methods, (Fig. 4). To insure that altitudes were drawn through the two sets of points

LONG PROFILE OF FLUVIAL SYSTEM SHOWING ACTUAL THICKNESS GREATER DEPOSITIONAL PRISMS © THAN DEPOSITIONAL PRISM ACTUAL THICKNESS IESS 0 THAN DEPOSITIONAL PRISM SCHEMATIC: NO SCALE Figure 3. Schematic diagram showing sedimentary prisms. A sedimentary prism bounded by uncon- formable contacts (prism 2) may diverge from geometrical perspectivity.

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+400 (0) SEDIMENTARY PRISM PATAPSCO FORMATION +150 (-250) SOUTHERN MARYLAND AREA

-100' (-500)

-350 (-750) PERSPECTIVE

600 (-1000)

•850 - (•1250)

-1100' (-1500)

•1350' (1750) o TOP OF FORMATION A BOTTOM OF FORMATION 1-2000)6 100 200 300 400 500 600 700 800 900 1000 THICKNESS, IN FEET |Y) Figure 4. Altitude-thickness plot of the Patapsco Formation in southern Maryland.

(dashed lines) and then adjusted to produce a of each block should be hydrogeologically con- structurally meaningful relationship between trolled by changes in one or several key altitude and thickness (solid lines). In this case, parameters, such as transmissibility or sand the fit is reasonably good; the sedimentary thickness. However, in areas where sparsity of prism increases in thickness down-dip, the data initially precludes this approach, an bottom contact having a greater slope than the arbitrary division into equivalent blocks is top. useful. To create smaller mapping units, the Figure 5 shows average trends in sand per- Patapsco Formation, as defined in Figure 4, can centage and transmissibility for each of the be subdivided graphically into any convenient three mapping units. Sand percentages gen- number of intervals, using the cross-ratios of erally decrease from north (blocks one and perspective geometry, in this case, into a lower, two) to south (blocks four and five). The middle, and upper unit. These units are con- paleoenvironmental significance of these and sistently denned and allow one to delineate other lithologic trends have been discussed vertical and horizontal changes in such hydro- elsewhere by the writer (Hansen, 1968; in geologic parameters as sand percentage or press). Suffice it to say that the nonmarine transmissibility. Patapsco Formation contains lithofacies char- Figure 5 demonstrates a way in which this acteristic of a fluvio-deltaic system having its method of zonation can be used. In this ex- axial drainage in the vicinity of Baltimore City. ample, the Patapsco Formation of southern As an initial step toward establishing a Maryland is divided into five blocks of equal quantitative basis for ground-water resources width (13.5 miles); using the procedure dis- management, it is useful to have an estimate of cussed above, each is zoned into a lower, the maximum daily amount of water trans- middle, and upper unit. Optimally, the width mitted by recharge through an aquifer. A con-

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venient method for deriving these data is the Insofar as this perspective point is the modified Darcy's equation (Ferris, and others, geometric analogue of a structurally stable 1962): knickpoint separating erosional and aggrada- Q = TIL tional wedges, it defines the edge of fluvio- where: deltaic deposition during Patapsco time. An assumption of stability is tenuous, however, Q = discharge, in gallons per day because the accumulation of overlying sedi- T = coefficient of transmissibility, in gallons per day per foot ments suggests downwarping has occurred. I = hydraulic gradient, in feet per mile Vertical warping in which the basin edge func- L = length of cross-section of block through which tioned as a fulcrum would not invalidate the flow occurs, in miles. perspective-"fall line" interpretation. Slight angularities in the strike of overlying units In Figure 5 this method is applied to the suggest, however, that rotational tilting has oc- three mapping units comprising each block. curred; therefore, while the perspective point Cited transmissibility values are based, in part, may have at a past time defined the Patapsco on those published by Bennett and Meyer "fall line," projection of present dips probably (1952), Mack (1962, 1966), and Slaughter and do not. Otton (1968). To maximize hydraulic gradient, it is assumed in each case to be coincident with Summary the top of a mapping unit. Each block is 13.5 In southern Maryland, the Patapsco For- miles in length. mation can be subdivided into consistently de- The simplified conduit analysis, described fined mapping units, using an adaptation of the above, should serve only as a frame of reference. perspective correlation technique described by To solve specific time-yield-drawdown prob- Haites (1963). Mapping units, defined in this lems, each mapping unit should be studied way, are useful for delineating trends in such using the mathematical models described by hydrogeologic parameters as transmissibility Ferris, and others, (1962) and Walton (1962). and sand percentage. By establishing an interim hydrogeologic framework, the method Geologic Implication of the Perspective Point can provide a basis for later field studies de- The perspective point of the sedimentary signed to establish hydraulic continuity in a prism defining the Patapsco Formation in multi-aquifier system. southern Maryland occurs at an altitude of The method can be used to subdivide most approximately +1000 feet (Fig. 4); at this basinward thickening rock-stratigraphic units. point the thickness-altitude plot reaches an In unfaulted homoclinal terranes, a scattered hypothetical feather edge. Projection from a altitude-thickness plot generally represents a datum well (AA-Cc86, Fig. 2), using average unit having sharply disconformable boundaries. slopes for top and bottom of formation, sug- Divergences from otherwise satisfactory plots gests that the feather edge occurs approxi- often represent miscorrelations resulting from mately 25 to 30 miles updip from the out- unrecognized changes in lithofacies. cropping top of formation. (See Figure 5 on following page.)

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Figure 5 continued Q = discharge (in gal/day), T = coefficient of transmissibility (in gal/day/ft), I = hydraulic gradient (in ft/mile), and L = length of block (in miles).

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References Cited Bennett, R. R., and Meyer, R. R., 1952, Geology and ground-water resources of the Baltimore area: Maryland Dept. Geology, Mines and Water Resources1 Bull. 4, 573 p. Clark, W. B., and Bibbins, A. B., 1897, The stratigraphy of the Potomac Group in Maryland: Jour. Geol., v. 5, p. 479-506. Ferris, J. G., Knowles, D. B., Brown, R. H., and Stallman, R. W., 1962, Theory of aquifer tests: U.S. Geol. Survey Water-Supply Paper 1536-E, 171 p. Glaser, J. D., 1966, Nonmarine Cretaceous sediments of the middle Atlantic Coastal Plain: Unpub. Ph.D. Dissert., Johns Hopkins Univ., 359 p. Groot, J. J., 1955, Sedimentary petrology of the Cretaceous sediments of northern Delaware in relation to paleogeographic problems: Delaware Geol. Survey Bull. 5, 157 p. Haites, T. B., 1963, Perspective correlation: Amer. Assoc. Petroleum Geol. Bull., vol. 47, no. 4, p. 553- 574. Hansen, H. J., 1968, Geophysical log cross-section network of the Cretaceous sediments of southern Maryland: Maryland Geol. Survey Rept. Invest. No. 7, 46 p. in press, Depositional environments of the subsurface Potomac Group in southern Maryland: Amer. Assoc. Petroleum Geol. Bull. Leopold, L. B., Wolman, M. G., and Miller, J. P., 1964, Fluvial processes in geomorphology: W. H. Freeman and Co., San Francisco, 512 p. Mack, F. K., 1962, Ground-water supplies for industrial and urban development in Anne Arundel County: Maryland Dept. Geology, Mines and Water Resources' Bull. 26, 90 p. 1966, Ground water in Prince Georges County: Maryland Geological Survey Bull. 29, 101 p. Mathews, E. B., 1933, Map of Maryland showing geological formations: Maryland Geological Survey, Baltimore, Md. Maxey, G. B., 1964, Hydrostratigraphic units: Jour. Hydrology, v. 2, no. 2, p. 124-129. McGee, W. J., 1886, Geological formations underlying Washington and vicinity: Am. Jour. Sci., 3rd ser., v. 31, p. 473-474. Otton, E. G., 1955, Ground-water resources of the Southern Maryland coastal plain: Maryland Dept. Geology, Mines and Water Resources1 Bull. 15, 347 p. Potter, E. P., 1963, Late Paleozoic sandstones of the Illinois Basin: Illinois Geol. Survey Rept. R.I. 217, 92 p. Richards, H. G., Groot, J. J., and Germeroth, R. M., 1957, Cretaceous and Tertiary geology of , Delaware, and Maryland: p. 183-230 in Dorf, Erling, Editor, Guidebook for Field Trips, Atlantic City Meeting: Geol. Soc. Amer., New York. Slaughter, T. H., and Otton, E. G., 1968, Availability of ground water in Charles County: Maryland Geol. Survey Bull. 30, 100 p. Southwick, D. L., and Owens, J. P., 1968, Geologic map of Harford County: Maryland Geol. Survey, Baltimore, Md. Sundstrom, R. W., Foley, F. C., Guyton, W. F., and Walton, W. C., 1967, The availability of ground water from the Potomac Formation in the Chesapeake and Delaware Canal area: Water Resources Center, Univ. Delaware, Newark, Del., 95 p. Visher, G. S., 1965a, Use of vertical profile in environmental reconstruction: Amer. Assoc. Petroleum Geol. Bull., v. 49, no. 1, p. 41-61. 1965b, Fluvial processes as interpreted from ancient and recent fluvial deposits: p. 116-132, in Mid- dleton, G. V., Editor, Primary sedimentary structures and their hydrodynamic interpretation: Soc. Econ. Paleon. Mineral, Sp. Pub. No. 12, Tulsa, Okla. Walton, W. C., 1962, Selected analytical methods for well and aquifer evaluation: Illinois Water Survey Bull. 49, 81 p.

MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 27, 1968

1 The Maryland Department of Geology, Mines and Water Resources was renamed the Maryland Geological Survey in July 1964.

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