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

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 sediments 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 geochemical 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 direction of the present system must be similar to the original flow direction.

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. 3) and 8.7 (southwest sector of map, Fig. data were obtained using a Debye-Scherrer powder camera with 3) are roughly superimposed over the DR isochrome. By excluding filtered cobalt radiation. Twenty samples were selected for deter- the northernmost contour representing the 7A/1 OA = 2, however, mination of the iron content. Atomic absorption spec- the trend is similar to that of the drab clays. That is, they decrease trophotometry was used to determine total iron, and ferrous iron downdip, which indicates that the relative amount of kaolinite is was determined by a volumetric dichromate procedure. greatest to the north and decreases to the southeast. In addition to illite and kaolinite, varying amounts of montmoril- RESULTS lonite, vermiculite, and mixed clays are observed in the samples (Fig. 5). No obvious geographic distribution trend is apparent for Color

Figure 3 shows the color distribution based on samples collected from the Patapsco Formation. Contoured lines are lines of approx- imately equal color. We propose calling these isochromes. Using the Goddard (1951) Rock Color Chart, the isochromes are based on color (in this case, red or yellow) and value (light or dark). When the colors of the field samples are plotted and contoured, three major isochromes become apparent. The YR isochromes indicate that yellowish-red sediments are distributed predominantly along the margins of the Patapsco and in one centrally located area. The LR isochromes indicate that the distribution of the light-red color is more centrally located than the YR. Finally, two DR isochromes show the darkest red samples to be centrally located. The general color trend in the Patapsco Formation indicates a distinct darkening of color toward the central outcrop area.

Clay Minerals

The commonest clay minerals in the Patapsco Formation are kaolinite and illite (see Table 1). All but two samples contain both these clays. The geographic distribution of relative abundances of illite and kaolinite are plotted (Fig. 4) using the 7A/ 10A peak height ratios. The 7A/10A peak height ratios in drab sediments (gray or Figure 2. Outcrop map of the Patapsco Formation, southern Maryland, greenish; Fig. 4) indicate a distinct trend of increasing illite (relative showing sample locations. Numbers, sample locations (Table 1).

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 MINERALOGY AND CHEMISTRY OF THE PATAPSCO FORMATION, MARYLAND 155

(Fe203 H20). Table 1 and Figure 6 show the distribution of these materials. All samples contain either hematite or goethite or both. Only three samples contain lepidocrocite. These three samples are all in areas where the 7A/10A peak height ratio is <5. Two of these samples, no. 6 and no. 49, are within the LR isochrome, while the third, no. 22, is within a DR isochrome. The distribution of hematite and goethite (Fig. 6) appears to be random throughout that part of the Patapsco Formation examined. However, the relative amounts of goethite and hematite were noted by plotting the 4.18A (goethite)/2.69A (hematite) peak height ratios. Hematite is more abundant than goethite in the general areas of the DR isochromes and in the northernmost outcrop areas. Goethite was found in 44.6 percent of all red clays, 42.0 percent of all mottled clays, and 75.0 percent of all drab clays. Hematite was found in 21.5 percent of all red clays, 7.0 percent of all mottled clays, and 5.0 percent of all drab clays. Both hematite and goethite occurred in 33.8 percent of all red clays, 50.0 percent of all mottled clays, and 20.0 percent of all drab clays. Lepidocrocite occurred in 3.0 percent of the red samples and 5.0 percent of the drab clays. The amounts ofextractable free iron (Mehra and Jackson, 1960) in 15 samples (Table 1) show that a significant proportion of the iron in these samples is in the form of amorphous or poorly ordered tribution in the Patapsco Formation. YR = yellow-red; LR = light-red; DR oxyhydroxides. = dark-red. the occurrence of either montmorillonite or mixed-layer clays. Both Iron Analyses clays are distributed in apparent random fashion throughout the Patapsco. Vermiculite, however, is distributed exclusively through- Table 1 shows the amounts of Fe+++ and Fe++ expressed as out the northeastern portion of the map area. Fe203 and FeO, respectively, in the Patapsco Formation. Figure 7 Only 26.6 percent of all red clays contain vermiculite, none is shows a contour map of Fe203 values. Figure 8 shows a contour +++ present in mottled samples, and only 6.0 percent of the drab sam- map of the FeO values. Fe expressed as Fe203 ranged from 1.56 ples contain vermiculite. percent to 11.73 percent, and Fe++ expressed as FeO show a sig- Mixed-layer clays are found in 11.6 percent of the red clays and nificant decrease toward the center of the outcrop area and remain are absent from the drab clays. Montmorillonite is present in 3.0 relatively constant in value along the strike of the sediments. In this percent of the red clays, in 6.0 percent of the drab clays, and in 1.0 respect, these values parallel other trends (color, clay mineralogy) percent of the mottled clays. A total of 30.6 percent of the entire described above, which suggest that they are more than coinciden- sample population contains vermiculite, montmorillonite, or tally related. mixed-layer clays. GROUND WATER Iron Oxides The Patapsco Formation in southern Maryland lies in the humid Three crystalline iron oxides are found in the Patapsco Forma- temperate climatic belt of the eastern United States. The mean tion: hematite (Fe203), goethite (Fe203-H20), and lepidocrocite annual precipitation is around 111.7 cm (44 in.). The mean temperatures (Mack, 1962) range between 2°C (35° Fahrenheit) and 25°C (77° Fahrenheit) and average 15°C (56° Fahrenheit). Ground-water availability in the Patapsco area is dependent on the amount of precipitation and the local geology. The amount of water entering the formation from stream flow is negligible. Mack (1962) suggested that because the area is in the humid climatic belt, the aquifers are full and water is continually being discharged into streams. Extensive well pumping and natural seepage are the chief sources of water discharge from the Patapsco Formation. Poten- tiometric depressions due to extensive pumping occur locally (Fig. 9). Recharge for the Patapsco Formation occurs primarily in the outcrop area. It is difficult to assess how much water from overlying and underlying stratigraphic units flows into the Patapsco. The total recharge of the exposed area mapped by Mack (1962) is ap- proximately 280 Ml/d (70 Mgal/d). The potentiometric surface (hydraulic head; Fig. 9) in the area of the Patapsco Formation examined by Mack (1962) is highest in the western outcrop area, (Mack, 1962, did not map the area constitut- ing the southwestern exposure area of this study; hence, the lines of equal head extend only to the central map area.) Water levels were reported as much as ~45m (140 ft) above sea level at the potentiometric high and decrease to sea level in the southeast. The lowest hydraulic head, 13 m ( — 40 ft), occurs around Baltimore, Mary- dashed lines, drab clays (see text for description). land, and is due to industrial pumping (Mack, 1962).

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 156 SCHLUGER AND ROBERSON

TABLE 1. SAMPLE COMPOSITION, PATAPSCO FORMATION

Sample no. Clay Col or Iron FeO Fe20, Extractable (Fig- i) minerals oxides (% wt) (ï wt) free iron (X wt)

• Vermiculite. i I ,K 5YR5/6 G,H A MMitmorillonite. 2 I,K,Mx 5YR5/6 G.H 0.08 5.37 3.36 3 I,K,Mo 10R5/4 G 0.48 2.39 * Mixed • layer cloys 3 I ,K 10Y6/2 G 3 I ,K 10R4/6 G 4 I ,K 10R4/6 G,H 5 I ,K 10R7/4 G 10Y8/2 6 I,K N7 6 I ,K 5R6/2 6 I ,K 5R5/4 G,H 6 I ,K,L 5YR5/6 G,L 3.93 7 I ,K 5YR5/6 G 8 I ,K 5YR5/6 G 8 I,K 5YR8/1 G 9 I,K 10R6/6 G 10 I ,K 5YR5/6 H 0.24 7.98 6.06 10YR8/2 11 I.K 10YR8/2 G,H Tr. 2.32 11 I.K 10YR6/6 H ANNAPOLIS, MD. 12 I.K 10YR6/6 H 13 I.K 10R5/4 H 0.28 5.90 14 I.K 10R5/4 G.H 15 I.K 10YR6/6 G,H 0.20 3.67 16 I.K 5YR5/6 G 16 I.K 10R4/6 H 17 K 10YR7/4 G 0.06 1.82 18 I.K 10R4/6 H 19 I.K 10R4/6 G 20 I.K 10R4/6 G,H Figure 5. Distribution of montmorillonite, vermiculite, and mixed-layer 21 I.K 5R4/6 H 0.20 7.69 clays in samples from study area. 22 I.K 5R5/4 G.L 23 I.K 10R5/4 H 0.10 5.93 3.54 24 I.K.V 10R5/4 G The coefficient of transmissibility ranges from around 5,200 1/d 25 I,K,Mo 5YR5/2 G,H (1,300 gal/d) near Washington, D.C., to around 20,800 1/d (5,200 26 I.K 10R4/6 G,H 0.24 . 5.70 27 I ,K,V 10YR6/6 G gal/d) at Annapolis, Maryland. Mack (1962) estimated that a 28 I,K,Mo 5YR5/6 . G 0.20 11.73 6.58 29 I ,K,V 10R4/6 G,H theoretical 25 Mgal/d could be transmitted downdip from the re- 30 I.K 10R6/6 G charge area. 31 I,K,V 5YR5/6 G,H 0.24 4.72 3.40 32 I ,K,V 5YR5/6 H 33 I,K,Mx 10YR5/4 G,H 34 I.K.V 10YR8/2 G 10.36 1.56 RELATION OF EH AND 35 I,K,V 10R6/6 G IRON CONTENT TO 36 I.K 10R6/6 G,H 37 I.K 10R6/6 G,H THE HYDROLOGY 5Y7/2 38 I.K 10R6/6 G 5Y7/2 Back and Barnes (1965) determined the oxidation potentials and 39 K,V 10R6/6 H 40 I.K 10R5/4 G 0.12 5.26 2.98 iron content of ground water in the Potomac Group sediments. 41 I.K 10R8/2 G,H They determined that within the Patapsco Formation, the oxida- 42 I.K 10R4/6 G 0.32 5.33 10Y8/2 tion potential ranged from 145 to 700 mV. The general trend 43 I ,K,V 5YR6/4 G,H 44 I.K 5Y7/2 G showed a decrease in oxidation potential in the direction of water 45 I.K 10R6/6 G 0.24 6.27 5.52 movement (southeastward). Back and Barnes (1965) observed that 46 I,K,V,Mx 5YR5/6 G 0.20 2.90 47 I.K 5YR6/4 G within the same aquifer, the oxidation potential is higher in areas 48 I.K.V 10YR8/6 G of recharge than it is in areas of discharge. They also showed that.a 49 I.K 10R6/6 G,H 49 I.K N9 G,H,L 0.24 8.26 correlation exists between the oxidation potential and concentra- 50 I.K 10R6/6 G ++ 51 I.K 5Y7/2 G tion of iron in the ground water. Fe ranged from 0.00 ppm to 51 I.K 5R5/4 G,H 0.24 3.59 +++ 20.0 ppm, and Fe ranged from 0.00 ppm to 1.60 ppm. The low- 52 I.K 10Y8/2 G 53 I.K 10R6/6 G,H est oxidation potentials and highest iron concentrations were mea- 54 I.K I0R6/6 G,H 54 I.K 5Y8/4 G sured down gradient (southeastward) in water associated with gray 55 I.K.V 5YR5/6 G and green sediments. 55 I ,K,V 5YR5/6 G,H 56 I.K.Mx 10YR6/6 G,H 57 I ,K,Mx 10Y8/2 G 58 I.K 10R6/6 H DISCUSSION 58 I.K 10YR8/2 G 58 I.K 10YR8/2 G,H 5YR4/4 Mineralogical study of the clay fraction yielded new information 59 I,K,V,Mx 10R4/6 60 I.K 10R4/6 G that appears to have a bearing on the origin of iron oxide pigment 61 I.K 10R8/2 G in the Patapsco Formation. There are three interesting aspects to 61 I.K 5YR8/4 G 62 I.K 10YR8/2 G the vermiculite distribution in the Patapsco Formation. Vermiculite 62 I.K.V 5YR5/6 G 63 I.K.V.Mx 10R6/6 G is (1) concentrated in the northeast, (2) restricted almost solely to 64 I.K 5YR5/6 G,H sediments that are red in color, and (3) rarely associated with any 65 I.K 5YR5/6 G,H 65 I.K N6 G 10 A minerals. We suggest that this vermiculite formed postdeposi- 66 I.K 5R3/4 G 0.28 3.60 67 I.K 5R4/6 H tionally, mainly because detrital vermiculite generally does not per- 68 K 10YR8/2 G,H sist in ancient sediments. However, the fact that vermiculite is al- 5YR4/4 69 I.K 10R6/6 G most always found in Patapsco sediments that are deficient in dis- 70 I.K 10R6/6 G,H 5YR4/4 crete 10 A minerals prompts the suggestion that much of the ver- illlts, " kaollnite, V " vermiculite, Mo ' nontmorillonite, Kx = mixed-layer, miculite formed as a result of postdepositional alteration of biotite G <= goethlte, < hematite, L " lepidocrocite. and (or) illite. We suggest that biotite was differentially distributed

Figure 8. Map of percent by weight FeO of whole-rock samples. by fluvial processes with a resultant higher concentration of biotite quently show that kaolinite becomes less abundant going from con- in the northeastern part of the area studied. This would account for tinental and nearshore environments into open-marine environ- the occurrence of vermiculite only in this area. Also, because ver- ments. This has been interpreted as primarily reflecting greater miculite is almost always found in red sediments in the Patapsco settling and flocculation rates for detrital kaolinite compared to Formation, the alteration of biotite and (or) iron-rich illite to ver- other detrital clay minerals (Weaver, 1959). miculite would have occurred in an oxidizing environment where Crystalline iron oxides in the Patapsco sediments include hema- the leached iron was immediately precipitated as an iron oxyhy- tite, goethite, and lepidocrocite. Hematite is the dominant oxide in droxide. Samples that contain mixed-layer illite-smectite also were the red-colored sediments. However, many of the red-colored sed- found to have small amounts of illite. This suggests that the ex- iments and the majority of the drab-colored sediments contain pandable mixed-layer clay minerals formed from the alteration of goethite. Heavy mineral separates contain only very fine particles illite. of goethite (most is <2 /a). Because fine-grained detrital goethite Within the area of the Patapsco studied, kaolinite decreases and would not be expected to persist for extremely long periods of time, illite increases southeastward. We believe that this is a reflection of the goethite now present is interpreted to be postdepositional in kaolinite-rich sediment deposition in a fluvial environment in the origin. Moreover, the occurrence of lepidocrocite, which is ex- northern part of the area and kaolinite-poor sediment deposition in tremely unstable at surface temperatures and pressures and trans- a shallow-water marine environment to the southeast. Studies of intertonguing facies of continental and marine sediments fre-

Figure 9. Potentiometrie surface of the Patapsco Formation in feet. Adapted from Mack (1962).

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 158 SCHLUGER AND ROBERSON

forms to goethite rapidly (Schwertmann and Taylor, 1972), is REFERENCES CITED further evidence that much of the iron oxide content can be considered as authigenic. Back, William, and Barnes, Ivan, 1965, Relations of electrochemical poten- Large amounts of extractable free iron in the samples studied tials and iron content to ground water flow patterns: U.S. Geol. Survey (Table 1) show that a significant proportion of the iron in these sed- Prof. Paper 498C, 16 p. iments is in the form of amorphous or poorly ordered oxyhy- Barnes, I., and Back, W., 1964, Geochemistry of iron rich ground water of droxides, which are thermodynamically unstable. They will crystal- southern Maryland: Jour. Geology, v. 72, p. 435-447. Berner, R. A., 1969, Geothite stability and the origin of red beds: Geochim. lize as one of the crystalline iron oxyhydroxides on aging. It is et Cosmochim. Acta, v. 33, p. 267-273. difficult to conceive that these amorphous iron oxides could have Brindley, G. W., 1961, Experimental methods in x-ray identification and persisted more than 100 m.y., which would be called for if the crystal structures of clay minerals, in Brown, G., ed.: Mineralog. Soc. amorphous iron oxides represent detrital material. The possibility London, p. 1-50. that the amorphous iron oxides might be a product of outcrop Fischer, A. G., 1963, Essay review of descriptive paleoclimatology: Am. weathering or some other relatively recent alteration of the mate- Jour. Sci., v. 261, p. 281-293. rial exposed in surface exposures was considered. This does not, Friend, P. F:, 1966, Clay fractions and colors of some Devonian red beds in however, appear to be the case, because we have analyzed a small the Catskill Mountains, U.S.: Geol. Soc. London Quart. Jour., v. 122, number of core samples from Patapsco sediments and found com- p. 273-288. parable amounts of extractable iron in these samples. Glaser, J. D., 1968, Coastal plain geology of southern Maryland: Maryland Geol. Survey Guidebook No. 4, 56 p. We have no evidence to suggest that all of the iron oxide now in -1969, Petrology and origin of the Potomac and Magothy (Cretaceous) the Patapsco sediments was derived directly from diagenetic altera- sediments, Middle Atlantic Coastal Plain: Maryland Geol. Survey tion of detrital minerals within the Patapsco Formation. Additional Rept. Inv. No. 11, 102 p. possible sources of iron exist in glauconite and other iron silicates 1971, Geology and mineral resources of southern Maryland: Mary- and oxides in overlying and underlying stratigraphic units. How- land Geol. Survey Rept. Inv. No. 15, 85 p. ever, the geochemical and hydrological flow system is such that it is Goddard, E. N., 1951, Rock color chart: Geol. Soc. America. easy to visualize the dissolution and migration of iron downdip Hansen, H. J., 1969, Depositional environments of sub-surface Potomac (southeastward). When the ground waters carrying the dissolved Group in southern Maryland: Am. Assoc. Petroleum Geologists Bull., iron are brought into the Eh-pH stability field of Fe (OH) (Hem v. 53, p. 1923-1937. 3 Hem, J. D., and Cropper, W. H., 1959, Survey of ferrous-ferric chemical and Cropper, 1959), it would be expected that iron hydroxides equilibria and redox potentials: U.S. Geol. Survey Water-Supply Paper would precipitate and, on aging, convert to some form of crystal- 1459-A, 31 p. line oxide (lepidocrocite, goethite, or hematite). Horowitz, D. H., 1971, Diagenetic significance of color boundary between The data we have presented along with the data obtained by Juniata and Bald Eagle Formations, Pennsylvania: Jour. Sed. Petrolo- Back and Barnes (1965) and Mack (1962) can be summarized and gy, v. 41, p. 1134-1136. interpreted as follows: Krynine, P. D., 1950, Petrology, stratigraphy and origin of the Triassic 1. The ground-water system flows southeastward. sedimentary rocks of Connecticut: Connecticut Geol. and Nat. His- 2. The iron content in the ground water increases in the direction tory Survey Bull. 73, 239 p. Langmuir, D., 1970, The effect of particle size on the reactions: hematite + of the ground-water flow. water-goethite: Geol. Soc. America, Abs. with Programs (Ann. Mtg.), 3. Eh decreases in the direction of the ground-water flow. v. 2, no. 7, p. 601-602. 4. Color of the sediments tends to darken (become more red) in Langmuir, D., and Whittemore, D. O., 1971, Variations in the stability of the direction of ground-water flow. Also, color variations may precipitated ferric oxyhydroxides: Nonequilibrium systems in natural reflect local topography. water chemistry: Advances in Chemistry Ser. 106, p. 209-234. 5. Both vermiculite, which is found in samples only in the north- Mack, F. K., 1962, Ground-water supplies for industrial and urban de- eastern part of the outcrop area, and mixed-layer clay minerals are velopment in Anne Arundel County, with a section on the chemical found almost exclusively in red-colored sediments and are proba- character of the water by Claire A. Richarson: Maryland Dept. Geol- bly postdepositional in origin. ogy, Mineral and Water Resources Bull. 26, 90 p. Mehra, O. P., and Jackson, M. L., 1960, Iron oxide removal from soils by a 6. Fine-grained goethite, much of which is thought to be au- dithionite-citrate system buffered by sodium bicarbonate: Proc. 7th thigenic, is abundant throughout these sediments. Lepidocrocite, Natl. Conf., Clays and Clay Minerals, 1958: London, Pergamon which must be authigenic because it is quite unstable at surface Press, p. 317-327. temperatures and pressures, appears to be located near local Roberson, H. E., and Eichenlaub, K., 1971, Origin of coloration in upper ground-water discharge areas. Devonian Catskill facies [abs.]: New York, AAPG-SEPM Program. 7. Amorphous or poorly ordered iron oxyhydroxides are Schwertman, U., and Taylor, F. M., 1972, The transformation of lepido- ubiquitous and abundant in these sediments. The presence of rela- crocite to goethite: Clays and Clay Minerals, v. 20, p. 151-158. tively large quantities of this material shows that a high proportion Thompson, A. M., 1970, Geochemistry of color genesis in red-bed se- of the iron oxyhydroxides is definitely authigenic. quence, Juniata and Bald Eagle Formations Pennsylvania: Jour. Sed. Petrology, v. 40, p. 599-615. 8. Total iron content of the samples shows decreasing values to- Van Houten, F. B., 1968, Iron oxides in red beds: Geol. Soc. America Bull., ward the center of the outcrop area and is highest in the north. v. 79, p. 399-416. These contoured values parallel the ground-water flow patterns 1972, Iron and clay in tropical savanna alluvium, Northern Colombia: and the Eh regional trends of the ground water. A contribution to the origin of red beds: Geol. Soc. America Bull., v. 83, p. 2716-2772. ACKNOWLEDGMENTS Walker, T. R., 1967, Geochemistry of hornblende alteration in Pliocene red beds, Baja California, Mexico: Geol. Soc. America Bull., v. 78, p. Fred Mack of the U.S. Geological Survey supplied core samples. 1055-1060. Max Budd and Daniel Li assisted in making iron determinations. Walker, T. R., Ribbe, P. H., and Honea, R. M., 1967, A formation of red beds in modern and ancient deserts: Geol. Soc. America Bull., v. 78, p. William Back of the U.S. Geological Survey and T. R. Walker of the 353-368. Department of Geological Sciences, University of Colorado, read a Weaver, C. E., 1959, The clay petrology of sediments: Clays and Clay Min- draft of this paper and offered comments. Schluger gratefully erals, v. 6, p. 154-187. acknowledges the postdoctoral award, by State University of New York at Binghamton, without which this study would not have MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 22, 1973 been possible. REVISED MANUSCRIPT RECEIVED JUNE 9, 1974 Printed in U.S.A.

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