HARRY J. HANSEN Maryland Geological Survey, Baltimore, Maryland 21218
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HARRY J. HANSEN Maryland 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 Patapsco Formation 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 Delaware, 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/2/329/3428126/i0016-7606-80-2-329.pdf by guest on 23 September 2021 330 H. J. HANSEN—INFORMAL SUBDIVISION OF PATAPSCO FORMATION, MARYLAND 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/2/329/3428126/i0016-7606-80-2-329.pdf by guest on 23 September 2021 NOTES AND DISCUSSIONS 331 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/2/329/3428126/i0016-7606-80-2-329.pdf by guest on 23 September 2021 332 H. J. HANSEN—INFORMAL SUBDIVISION OF PATAPSCO FORMATION, MARYLAND +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