Adapted from Fleming and others (1993) and Brown and Laudick (2003)
Glacial deposits of pre-Wisconsin age are abundant in the subsurface of Marion County, and locally reach thicknesses in excess of 250 ft (76 m). In Marion County, at least seven, and possibly more, distinct depositional sequences of pre-Wisconsin age can be documented. In general, each sequence is characterized by one or more sheets of glacial till that are variously associated with lacustrine sediments, other diamicton units, and one or more sand and gravel bodies. These deposits have had a long and complicated history, involving the repeated truncation of one or more older sequences by younger ones during successive glacial episodes, as well as the development and subsequent burial of several interglacial landscapes that cut across multiple sequences. Consequently, the stratigraphy of the pre-Wisconsin deposits is rarely complete in any one location, and sequences of radically different ages are commonly in juxtaposition in different parts of the county. Not surprisingly, this history has obscured many of the details of the sequences, although the overall pattern of events and the salient characteristics of the sequences can be reasonably well documented.
The precise ages of these sequences cannot be reliably determined, but they can generally be assigned to two broad periods: (1) the Illinoian age, which is believed to encompass the period of time between about 130,000 and 200,000 yr B.P.; and (2) everything before the Illinoian, now referred to loosely as the " pre-Illinoian ." The latter used to be known as the "Kansan" and "Nebraskan" ages, but uncertainties associated with the pre-Illinoian glacial record have led many workers to abandon those terms. In Indiana, all pre-Wisconsin deposits have been referred to as the "Jessup Formation" (Wayne, 1963; Hartke and others, 1980), and only two of the individual till units observed in Marion County have been accorded formal rank.
The pre-Wisconsin sequences in Marion County, and elsewhere in the midwestern United States, are commonly separated from one another by weathering profiles and (or) nonglacial sediments, indicating the passage of lengthy ice-free periods in between most or all of the glacial episodes. In particular, the boundary between Illinoian and pre-Illinoian deposits is marked by a major weathering profile that is as much as 20 ft (6 m) or more thick in places where it is well preserved. This weathering profile is widely recognized in many places in the lower Midwest and plains states, and the period of time during which it formed is known as the " Yarmouth interglacial." Other weathering profiles are also recognized within the Illinoian deposits, as well as between some pre-Illinoian sequences. In places where they were protected from subsequent erosion by ice and meltwater, any of these weathering profiles may locally be marked by a well-preserved buried soil, known as a "paleosol." Well-developed buried soil profiles, each representing a part of a former landscape, have been observed in as many as four different horizons in a single section of the pre-Wisconsin sediments in Marion County. There is a strong possibility that as many as five or six distinct periods of soil formation are represented within these deposits.
Pre-Wisconsin deposits are exposed in only a few, small, widely scattered outcrops, consequently identification of individual sequences is based entirely on borehole characteristics, mainly gamma-ray log signatures and physical properties of sediment samples (fig. 1). It is virtually impossible to trace individual sequences in the absence of closely spaced borehole data, thus no attempt was made to systematically map individual pre-Wisconsin sequences across the entire county. Moreover, the presence and continuity of individual sequences appear to show considerable variation among the different parts of the county, and only rarely are more than four or five of these units present in any one area. This variation is due in part to the irregular degree of erosion of older deposits by each succeeding ice advance, and to the large amount of relief on the underlying bedrock surface. Despite these complications, several observations can be made concerning the physical properties, regional distributions, and hydrogeologic importance of the pre-Wisconsin sequences.
Selected downhole gamma-ray logs and sample descriptions that illustrate contrasting sequences of Wisconsin and pre-Wisconsin age in eastern Marion County. (click each image to enlarge)
- a.) Downhole gamma-ray log with sample description illustrating contrasting sequences of Wisconsin and pre-Wisconsin age in eastern Marion County from test hole drilled near the intersection of South Post Road and Rawles Road, Indianapolis East quadrangle. The log shows a thick sequence of at least four Illinoian tills overlying pre-Illinoian sediments. The hole is notable for the thick West Lebanon lake sediments at the bottom and the four paleosols . A fifth paleosol may also be present atop the West Lebanon lake sediments. All three late Wisconsin sequences are present, although thin.
- b.) Downhole gamma-ray log with sample description illustrating contrasting sequences of Wisconsin and pre-Wisconsin age in eastern Marion County from domestic water well just northwest of Fall Creek, opposite Fort Benjamin Harrison, Fishers quadrangle. The hole illustrates well the thick complex of basal Illinoian and pre-Illinoian sand and gravel that is characteristic of this part of the county. The break is marked by the massive weathering horizon atop the pre-Illinoian section. The sand and gravel are capped by a relatively thin and monotonous sequence of Illinoian tills. The late Wisconsin section is exceptionally thin, but all three of the depositional sequences appear to be represented.
- c.) Downhole gamma-ray log with sample description illustrating contrasting sequences of Wisconsin and pre-Wisconsin age in eastern Marion County from a test well at the Marina Wellfield along Fall Creek, McCordsville quadrangle. The log shows relatively thin late Wisconsin outwash directly overlying the latest pre-Illinoian till, which in turn caps the massive sand and gravel complex. The capping till unit is absent from several other holes in this area, resulting in direct superposition of the two outwash units of differing ages. The thin pink till unit at about 80 ft (24.4 m) is probably the so-called "Hillery Till Member".
The pre-Wisconsin deposits are thickest in the eastern third of Marion County, and individual sequences appear to have the greatest continuity in that area (fig. 2). The total pre-Wisconsin section also appears to be the most complete in the east, consequently it is there that the stratigraphy of these deposits is most clear. The oldest known glacial unit in the state is the West Lebanon Member, a distinctive, pinkish to red, clay-rich till with abundant reddish laminated lacustrine mud. The unit is extensively present in bedrock valleys and other low areas in the northeastern and north-central parts of the county, and consists chiefly of red, laminated silt and clay, and much lesser till. It is more than 100 ft (30.5 m) thick in the large bedrock valley near Oaklandon. The unit is believed to be older than 780,000 yr B.P. because, in western Indiana, it overlies lacustrine sediments that are magnetically reversed (Bleuer, 1991).
Gamma-ray log cross section from southeastern Marion County illustrating the character and downhole stratigraphy of late Wisconsin and pre-Wisconsin sequences. Several of the individual sequences appear to exhibit considerable continuity in this particular area and are readily identified on the different gamma-ray logs. The labels "I1," "I2," "I3," and "I4" are used strictly for purposes of example and do not represent formal or informal stratigraphic designations.
At least two other units of probable pre-Illinoian age are also present sporadically in Marion County. The lowermost of these consists chiefly of reddish or pinkish-brown till of silt loam to loam texture and eastern source mineralogy, probably the Hillery Till Member (Johnson and others, 1972; Bleuer, 1991). Small- to medium-sized lenses and sheets of sand and gravel are locally associated with the till. The Hillery is typically the first glacial unit above the bedrock in the south-central and northwestern parts of the county.
The top of the pre-Illinoian section in many places consists of a thick sequence of sand and gravel that is locally capped or interbedded with thin, deeply weathered, greenish-gray loam or sandy loam till. The sand and gravel is commonly quite coarse and is the primary aquifer over much of the northeastern part of the county. In that area, the pre-Illinoian sand and gravel is a mappable unit that is locally more than 80 ft (24.4 m) thick and extends over tens of square miles.
This sequence is commonly capped by a massive paleosol as much as 20 ft (6 m) thick, and the gravel contains abundant dolomite pebbles with prominent dark red coatings of iron oxide 1 to 2 mm thick. The paleosol is variably truncated by sand and gravel units at the base of the overlying Illinoian section, resulting in a massive complex of sand and gravel that is locally of both Illinoian and pre-Illinoian age. This complex is informally termed the "Fall Creek aquifer complex" because it is exhumed within the valley of Fall Creek, and forms the bulk of the so-called "outwash" shown on previous maps of that area (Harrison, 1963; Gray and others, 1979; Hartke and others, 1980). The distribution, thickness, elevation, and other characteristics of the Fall Creek aquifer complex are shown on separate maps for parts of the Fishers, McCordsville, Cumberland, and Indianapolis East quadrangles. The complex also corresponds to unit "pIx" on the pre-Wisconsin surface maps of those quadrangles.
Sand and gravel units occur along the pre-Illinoian surface elsewhere in the county, and can usually be identified by the prominent green paleosol that separates them from the overlying Illinoian deposits. Many of the smaller sand and gravel units are composed chiefly of chert and shale, are interbedded with lenses of silt and fine sand of probable alluvial origin, and contain evidence of multiple periods of soil formation. These characteristics suggest that some of these deposits formed in a fluvial environment during the long, ice-free period of the Yarmouth interglacial. The paleosol associated with the "pre-Illinoian surface" is the best developed of any weathering horizon in the county, and is readily identified in widely scattered localities east of the White River. In most places, the elevation of this surface is typically less than 700 ft (213.4 m), except near the Hamilton County line in northeastern Marion County, where it rises to elevations as great as 770 ft (235 m).
The Illinoian deposits in most places comprise a thick sequence of diamictons, locally separated by small- to medium-sized bodies of sand and gravel. The majority of the diamictons are interpreted to be tills. The tills are distinguished only with difficulty, as most are gray-brown, overconsolidated, pebbly loams. At least four distinct Illinoian sequences (till and associated sand and gravel units) are suggested, however, by gamma-ray signatures in eastern Marion County and, to a lesser extent, by sedimentologic properties. Downhole samples and color changes reported in water well records suggest that weathering zones are present between all these sequences, although evidence for weathering at all horizons is rarely, if ever, seen in any single hole. Analyses of downhole samples suggest that at least two of these tills are of eastern source ( Huron-Erie Lobe ), whereas one may be of northeastern source ( Saginaw Lobe ) or northern source ( Lake Michigan Lobe ).
Thick and laterally extensive sand and gravel bodies are present within the Illinoian section in several localities. The most common of these are locally continuous tabular bodies that occur along the pre-Illinoian surface, which suggests that they originated as outwash aprons or fans deposited during the initial Illinoian ice advance. Lenses and sheetlike sand and gravel bodies are also common between the different Illinoian diamicton units, but only rarely do these appear to have much continuity.
A markedly different situation occurs in the Oaklandon area, where the entire Illinoian section consists of one or more large channel-like units that have coalesced with the underlying pre-Illinoian sand and gravel, forming the massive Fall Creek aquifer complex, which locally approaches 120 ft (36.6 m) in thickness. The stratigraphy in that area is further complicated by the large amount of bedrock relief; above some elevated areas on the bedrock surface, the normally thick Illinoian section is severely attenuated, and in a few places, late Wisconsin deposits directly overlie the pre-Illinoian sequences.
The most extensive known Illinoian sand and gravel body occurs adjacent to the White River valley in the southwestern quarter of the county, and is of sufficient size to map separately on parts of the Maywood, Beech Grove, Indianapolis East, Indianapolis West, Bridgeport, and Clermont quadrangles. This body is very persistent and forms a large, tabular-shaped complex that appears to lie in the lower or middle part of the Illinoian section in that area. It is consistently in the range of 15 to 30 ft (4.6 to 9.1 m) thick, and is capped by a well-developed paleosol in the south and west part of the mapped area. To the north and east of the mapped area, the paleosol rises up onto an Illinoian till unit that overlies the sand and gravel. The top of the sand and gravel body slopes gently toward a large, south-trending, buried bedrock valley that underlies parts of the modern river valley.
The overall configuration of the body suggests that it represents a former outwash plain or apron that was graded to an ancestral White River during middle Illinoian time. South of Marion County, the bedrock valley is filled with pre-Wisconsin sand and gravel that may be related to this unit, although a direct connection between the valley fill south of the county and the inferred Illinoian outwash unit cannot be demonstrated at this time. In any event, this unit is the primary source of groundwater for domestic wells over much of the area in which it is mapped, outside of the immediate confines of the White River valley. Because the precise age of the body is uncertain, it is simply referred to as the "southwestern aquifer complex." The complex is also denoted as unit "mI" on the pre-Wisconsin surface maps of the corresponding quadrangles.
Bleuer, N. K., 1991, The Lafayette Bedrock Valley System of Indiana; Concept, form, and fill stratigraphy, in Melhorn, W. N., and Kempton, J. P., eds., Geology and hydrogeology of the Teays-Mahomet Bedrock Valley System: Boulder, Colorado, Geological Society of America Special Paper 258, p. 51-77.
Brown, S. E., and Laudick, A. J., eds., 2003, Hydrogeologic framework of Marion County, Indiana — a digital atlas illustrating hydrogeologic terrain and sequence: Indiana Geological Survey Open-File Study 00-14, CD-ROM.
Fleming, A. H., Brown, S. E., and Ferguson, V. R., 1993, Hydrogeologic framework of Marion County, Indiana: Indiana Geological Survey Open-File Study 93-05, 67 p.
Gray, H. H., Bleuer, N. K., Hill, J. R., and Lineback, J. A., 1979, Geologic map of the 1o x 2o Indianapolis quadrangle, Indiana and Illinois, showing bedrock and unconsolidated deposits: Indiana Geological Survey Regional Geologic Map 1.
Harrison, W., 1963, Geology of Marion County, Indiana: Indiana Geological Survey Bulletin 28, 78 p.
Hartke, E. J., Ault, C. H., Austin, G. S., Becker, L. E., Bleuer, N. K., Herring, W. C., and Moore, M. C., 1980, Geology for environmental planning in Marion County, Indiana: Indiana Geological Survey Special Report 19, 53 p.
Johnson, W. H., Follmer, L. R., Gross, D. L., and Jacobs, A. M., 1972, Pleistocene stratigraphy of east-central Illinois: Illinois State Geological Survey Guidebook Series 9, 97 p.
Wayne, W. J., 1963, Pleistocene formations in Indiana: Indiana Geological Survey Bulletin 25, 85 p.