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Mineralogy and Chemistry of the Patapsco Formation, Maryland, Related to the Ground-Water Geochemistry and Flow System: A Contribution to the Origin of Red Beds PAUL R. SCHLUGER Amerada Minerals Corporation of Canada, Ltd., 540 Sth Ave. S.W., Calgary, Alberta, TIP OM3, Canada HERMAN E. ROBERSON Department of Geological Sciences, State University of New York at Binghamton, Binghamton, New York 13901 ABSTRACT PREVIOUS STUDIES Red and drab-colored sediments of the Patapsco Formation were Mineralogy and Iron Content of Red Beds studied to determine if color changes, clay mineralogy, and iron content of the sediments are related to the present ground-water Analytical data compiled in most studies of red beds include the geochemistry (Back and Barnes, 1965) and the present ground- mineralogy of iron-bearing oxides and silicates and the iron con- water flow system (Mack, 1962). The red color of the sediments tent of the rocks. Iron oxides have been the focus of much attention becomes darker in the direction of the ground-water flow, particu- because one or more iron oxide is usually present in significant larly in local discharge areas. This color change is related to the amounts in red or red and drab interbedded rocks. amount and kind of iron oxides in the sediments. Hematite and The most commonly reported crystalline iron oxides in red beds goethite coexist in most of the red and mottled samples; hematite is are goethite (Fe203'H20) and hematite (Fe203). Berner (1969) more abundant than goethite in red sediments but is rarely found in concluded that hematite is the more stable of the two at tempera- drab sediments. Large amounts of amorphous or poorly ordered tures as low as 40°C; if goethite is present in crystal sizes on the iron oxyhydroxides in these sediments indicate that much of the order of hundreds of angstroms, hematite may be stable at even iron has been introduced by diagenetic processes. Lower iron oxide lower temperatures. This agrees with the experimental results of values prevail toward the center of the outcrop belt, and higher Langmuir (1970) and is in accord with the fact that goethite is values prevail in the recharge area. Detrital kaolinite and illite are rarely reported in ancient red-drab sequences (Fischer, 1963). the most abundant clay minerals. Vermiculite and mixed-layer However, the relative and absolute stabilities of hematite and illite-smectite are almost always found in red colored sediments goethite are strongly dependent on particle size (Langmuir and and are probably products of post-depositional diagenesis. Whittemore, 1971). Because particle size measurement of mixtures The mineralogic and chemical variations correlate regionally of iron oxide particles in the range of 100 to 1,000 A is extremely with the observed ground-water flow pattern and with observed difficult, if at all possible, generalizations concerning the relative changes in Eh and dissolved iron content of the ground water. stabilities of these minerals should be made with caution. These results suggest that fluctuations in the ground-water flow Walker (1967) demonstrated that iron-bearing detrital grains system in conjunction with Eh and pH conditions caused precipita- can be altered by intrastratal solutions to produce authigenic hema- tion of iron hydroxides that, on aging, have crystallized as goethite tite. Walker and others (1967) presented evidence that hornblende and (or) hematite. Key words: geochemistry-geohydrology, Cre- in Pliocene deposits in Baja California, Mexico, was altered by in- taceous, red beds. trastratal solutions to smectite clay with a loss of iron, alkalis, al- kaline earths, and probably silica. Friend (1966) reported that the INTRODUCTION clay fraction of the Devonian Catskill rocks was unaltered. Rober- son and Eichenlaub (1971) concluded from their studies of drab The origin of red beds has been recently discussed at length in the and red interbedded Devonian rocks, however, that postdeposi- geological literature. Krynine (1950) has been a major proponent tional alteration of the Fe-chlorite had taken place. A study of the of the hypothesis that hematite pigment in a large number of an- iron and clay content of tropical savannah alluvium in northern cient red-bed deposits is derived primarily from the erosion of red Colombia led Van Houten (1972) to conclude that although some soils developed in an area with a warm, moist climate. Walker ferric oxide might have been contributed by alteration of iron- (1967) suggested that hematite in red-colored sediments and bearing silicates, enough soil-derived brown ferric oxide was pres- sedimentary rocks may be postdepositional in origin and can form ent to produce the hematite pigment of a red bed by postdeposi- as a result of in situ alteration of iron-bearing detrital minerals in tional dehydration and aging. hot, arid, or semiarid climates. A number of investigators (Friend, Determining ferrous and ferric contents of whole-rock samples 1966; Walker, 1967; Van Houten, 1968; Thompson, 1970) are in can provide insight into geochemical processes affecting the dis- agreement that hematite in red beds is diagenetic, but a wide range tribution of iron in sediments. The ferrous and ferric iron contents of opinions exists concerning the mechanisms involved in the can be useful in making inferences about the effects of the oxida- diagenetic reactions and the possible use of red beds as paleo- tion potential of the pore solutions and the possible structural site climatological indicators. Other authors (Horowitz, 1971; Berner, of the iron, that is, whether all the iron is in the form of free oxides 1969) have suggested that coloration patterns in some red se- or some or all of the iron is in silicate minerals. quences may be related to ground-water conditions. To our knowl- edge, no one has published a systematic study of the relations be- Patapsco Formation tween the ground-water flow system and the mineralogy and chemistry of red sediments. In this study, we have attempted to re- The Patapsco Formation of the Potomac Group (Lower Cretace- late the present ground-water chemistry of the Patapsco Formation ous) in southern Maryland has been described by Glaser (1968, in southern Maryland — studied by Barnes and Back (1964) and 1969, 1971) and Hansen (1969). Glaser studied the stratigraphy Back and Barnes (1965) and the present ground-water flow system and sedimentology of the Potomac Group. studied by Mack (1962) in the same area — to regional variations The Potomac Group consists of the Patuxent, Arundel, Patapsco, in color, clay mineralogy, and iron oxide content of the bulk and Raritan Formations (Fig. 1). The Potomac Group overlies Cre- samples. taceous and Holocene sediments of southern Maryland and con- Geological Society of America Bulletin, v. 86, p. 153-158, 9 figs., February 1975, Doc. no. 50202. 153 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/153/3428907/i0016-7606-86-2-153.pdf by guest on 23 September 2021 154 SCHLUGER AND ROBERSON sists mainly of unconsolidated sand, silt, clay, and gravel, most of NW SE which are continental fluvial or shallow marine deposits. These sed- iments have a regional dip of about 1° ESE. This study concentrates on the Patapsco Formation, which crops out between Washington, D.C.; Baltimore, Maryland; and An- napolis, Maryland (see Fig. 2 for sample locations). This area was chosen for study because considerable ground-water and geochem- ical data were available. Another important consideration was that the present ground-water flow system is thought to be similar to that which existed at the time of sediment deposition (F. W. Mack, 1972, oral commun.). Because no large-scale tectonic activity has been recorded within the Potomac Group, and the present dip of around 1° ESE. must approximate the original dip of the sediments, then the flow direc- tion of the present system must be similar to the original flow direc- tion. METHODS Figure 1. Stratigraphie cross section of the study area (modified from Back and Barnes, 1965). Arrows, generalized ground-water flow direction; Data from 87 outcrop samples are presented in Table 1. Stan- vertical lines, well locations. dard x-ray diffractometer and powder camera techniques were used to determine the phases present (Brindley, 1961). Each sample was hand-ground to a fine powder, packed in an aluminum holder, to kaolinite) downdip (to the southeast) and approximate unifor- tamped until smooth, and x-ray diffractometer patterns were ob- mity in ratio along strike (to the northeast). tained using filtered Cu Ka radiation. Each clay mineral sample The 7A/10A peak height ratios of dark-red (DR) and light-red was suspended in distilled water and centrifuged to remove the (LR) clays (Fig. 4) are contoured as solid lines in Figure 4. The. >2fi fraction. The <2/x fraction was pipetted onto a glass slide to figure shows that the 7A/10A peak-height ratio distribution is have oriented clay aggregates for the x-ray diffraction study. more complex than that indicated by the trend of the drab sedi- Glycolation and heating of clay minerals to 250°C and to 550°C ments. The ratio, however, tends to decrease downdip. In this re- were additional techniques used to identify the clay minerals. To spect, both the drab and red clay 7A/10A peak height distribution identify heavy minerals, particularly the crystalline iron oxides, the (Fig. 4) are similar to the color distribution of the Patapsco (Fig. 3). <0.5-mm size fraction of each sample was magnetically separated The 7A/10A peak height ratio for the red clay ranges downdip using a Frantz Isodynamic Magnetic Separator. The magnetic frac- from a maximum of 15 to less than 2. Two centers, 7.0 (approxi- tion was hand-ground in an agate mortar, and x-ray diffraction mate center of map, Fig.

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