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A Further Revision of the Stratigraphic Nomenclature of the Wissahickon Formation in Maryland

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A Further Revision of the Stratigraphic Nomenclature of the Wissahickon Formation in Maryland MICHAEL W. HIGGINS U.S. Geological Survey, Beltsville, Maryland 20705 GEORGE W. FISHER Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 A Further Revision of the Stratigraphic Nomenclature of the Wissahickon Formation in Maryland ABSTRACT flexible and objective system for subdividing these rocks, Southwick and Fisher (1967) The Wissahickon Formation, the thickest divided nearly all the rocks above the Cockeys- and most extensive unit of the Glenarm ville into five lithofacies (Table 1) and included Series, was divided into lithofacies several these lithofacies in a single formation, the years ago. We suggest revision of two of Wissahickon. these lithofacies and addition of another. We Recent mapping by members of the U.S. also suggest that the term lithofacies be Geological Survey, the Maryland Geological shortened to facies. The added facies, the Survey, and The Johns Hopkins University quartzite facies, is distinguished by metamor- has indicated the need for revision of two of phosed orthoquartzites and protoquartzites. the lithofacies and for the addition of another; It corresponds in part to the former Peters these are discussed in this note. The term Creek quartzite. lithofacies is changed to fades, in the interests of brevity (Table 1). INTRODUCTION In the Maryland Piedmont, a 1100 m.y.-old PELITIC SCHIST FACIES basement complex, the Baltimore Gneiss, is Pelitic schists make up most of the Wissa- unconformably overlain by the Glenarm hickon Formation; they have been described Series, a thick, variably metamorphosed se- in detail by Hopson (1964) and Fisher (1970). quence of sedimentary and volcanic rocks. For many years, the pelitic schists surround- From the base upward, this sequence includes ing the gneiss domes at Baltimore were dis- three formations: (l) the Setters Formation, tinguished on the basis of metamorphic grade composed of muscovite quartzite and sub- from those west of the belt of diamictite ordinate mica gneiss and mica schist; (2) the facies, in which Sykesville, Maryland, is Cockeysville Marble, composed of siliceous located (Fig. l). The schists east of the marble and a variety of calc-silicate rocks; Sykesville belt, near the gneiss domes, were and (3) Wissahickon Formation, a thick se- called the oligoclase-mica schist facies of the quence of mica schist and phyllite, meta- Wissahickon. Those west of the Sykesville gray wacke, massive conglomeratic paragneiss, belt were called the albite-chlorite schist facies. and subordinate micaceous quartzite (Fig. l). Hopson (1964, p. 70) renamed these the Metamorphosed volcanic, volcaniclastic, and "eastern and western sequences of the volcanic-epiclastic rocks are also part of the Wissahickon" because metamorphic grade series, but are not discussed in this note. does not always distinguish the two. Fisher The Setters Formation and the Cockeysville (1963), and Hopson (1964) interpreted the Marble are distinctive, easily mapped units, eastern sequence as older than the western. and their nomenclature has varied only slightly Southwick and Fisher (1967) accepted this since they were originally defined by Williams interpretation and renamed the schist in the (1892). The rocks above the Cockeysville two belts the lower and upper pelitic schist Marble, however, have proved difficult to sub- lithofacies of the Wissahickon Formation. divide, and their nomenclature has changed There are several objections to the use of many times to conform to different structural the designation upper and lower: (l) Most of interpretations (see Southwick and Fisher, the rocks of the two geographic belts are 1967). In an attempt to provide a more lithologically indistinguishable in outcrop. Geological Society of America Bulletin, v. 82, p. 769-774, 2 figs., March 1971 769 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/769/3432956/i0016-7606-82-3-769.pdf by guest on 24 September 2021 770 HIGGINS AND FISHER-WISSAHICKON FM., MARYLAND ---------------------- -- Figure 1. Generalized geologic map of the -------- - --- - -------- ------- --- - ----- - --- - -------- eastern Maryland Piedmont (modified from 1960). (2) The terms upper and lower imply a better northeast of Baltimore, which do not connect knowledge of Piedmont stratigraphy than is with either the lower or upper pelitic schist currently justified. The schists around some facies and so cannot be assigned to either. of the Baltimore Gneiss domes must be Therefore, we recommend that: (l) the older than schists just west of the Sykesville terms upper and lower be discontinued and all belt of diamictite facies rocks on the west the pelitic schists in the Wissahickon be flank of the Baltimore-Washington anticli- lumped into a single pelitic schist facies; and norium (Fig. l); however, it does not neces- (2) the several belts of pelitic schist be dis- sarily hold that all the western schists are tinguished by informal geographic names younger than all the eastern schists. (3) Pelitic where needed for clarity. For example; the schists crop out in Harford and Cecil Counties, schists near the gneiss domes may be called Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/769/3432956/i0016-7606-82-3-769.pdf by guest on 24 September 2021 DIAMICTITE FACIES 771 TABLE 1. NOMENCLATURE OF THE GLENARM SERIES Southwick and Fisher (1967) This paper Wissahickon Formation Wissahickon Formation - • Upper pelitic schist lithofacies Quartzite facies '_) Q Metagraywacke lithofacies Metagraywacke facies Metaconglomerate lithofacies Metaconglomerate facies ui --H Q Boulder gneiss lithofacies Diamictite facies Lower pelitic schist lithofacies Pelitic schist facies S.S-J Cockeysville Marble Cockeysville Marble Setters Formation Setters Formation the Silver Spring belt of pelitic schists, those clinotium. Similar rocks not connected with west of the Sykesville belt of diamictite facies, those of the Sykesville belt occur: (l) inter- the Rockville belt, and those in Cecil County, stratified with Wissahickon pelitic schist west the Blueball belt. of the Sykesville belt; (2) in Harford County, Maryland (Southwick and Owens, 1968; Southwick, 1969); (3) in Cecil County, DIAMICTITE FACIES Maryland (Higgins, unpub. data); and (4) in Lenses of massive strikingly uniform para- nottnern Virginia (J. C. Reed, Jr., D. L. gneiss and granofels containing scattered Southwick, R. B. Mixon, and V. M. Seiders, pebble- to boulder-sized detrital rock frag- unpub. data). ments are a prominent part of the Wissahickon Gross similarities in lithology, internal Formation in Maryland and northern Virginia. structure, three-dimensional form, and occur- The fragments include: (l) rounded cobbles rence, led Southwick and Fishet (1967) to and pebbles of quartz and feldspar; (2) flat- lump all these paragneisses into a single tened chips of mica schist, chiefly pebbles lithofacies, which they named the boulder and cobbles, but including some boulders as gneiss. Further work has shown that this term long as 15 ft; (3) angular cobbles and some was inappropriate, because many of the rocks boulders of biotite-quartz gneiss, amphibo- to which it was applied to not contain boul- lite, and calc-silicate schist; (4) contorted ders. In the rocks that have boulder-bearing slabs of metagraywacke and schist mixture lenses, granule gneiss and granofels predomi- (Fisher, 1963, p. 42; Hopson, 1964, p. 103- nate when the mass is considered as a whole. 108) or metagraywacke alone (Higgins, un- For example, the predominant rock type at pub. data). In Howard and Montgomery Sykesville is granule gneiss with scattered Counties, the matrix is virtually devoid of pebbles and rare cobbles, and in Harford and potassium feldspar, but the matrix of similar Cecil Counties, Maryland, and in northern rocks in Cecil County and locally in northern Virginia, fine-grained granule gneiss and Virginia contains as much as 5 percent potas- granofels greatly predominate over boulder- sium feldspar. bearing gneiss. Locally, metavolcanic clasts These rocks were originally interpreted as and a matrix derived partly from volcanic magmatic granite, but Cloos (Cloos and sources give the tock an appearance reminis- Cooke, 1953) recognized that the rocks ex- cent of some volcanic breccias. tending south from Sykesville, Maryland, Flint, Sanders, and Rodgers (I960a, 1960b) are metasedimentary and changed their name have proposed the name diamictite (from the from the Sykesville Granite to the Sykesville Greek Diamignymi, to mingle thoroughly) Formation. Hopson (1964, p. 108-112) con- for precisely the range of rock types presently cluded that the mass was a huge submarine being mapped under the tetm boulder gneiss. slide deposit, similar to the argille scagliosi of We propose substituting the term diamictite the Appennines (Maxwell, 1959). Fisher facies for the term boulder gneiss lithofacies (1963, 1970) agreed with this intetpretation (Southwick and Fisher, 1967, p. 12-13). and showed that the rocks in the Sykesville Where necessary for clarity, individual bodies belt nearly connect with similar rocks in a of the diamictite facies may be referred to by belt extending southwest from Laurel, Mary- informal geographic names, such as the land, so that the two belts together outline Sykesville belt, or the Laurel belt. Similarly, the limbs of the Baltimore-Washington anti- the terms boulder gneiss, cobble gneiss, and pebble Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/769/3432956/i0016-7606-82-3-769.pdf
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