Adapted from Fleming and others (1993) and Brown and Laudick (2003)
The pre-Wisconsin surface represents the landscape upon which the late Wisconsin glacial sequences were deposited. As such, its configuration and geology have considerable bearing on the sedimentological characteristics, thickness, and geometry of overlying aquifers and confining units . In addition, a variety of sand and gravel units are present at or just below the surface, and are locally significant aquifers . The proximity of these units to the pre-Wisconsin surface determines the degree to which they may interact hydraulically with shallower aquifers, or may be directly affected by infiltration from the land surface. Hence, the configuration of the pre-Wisconsin surface also affects the sensitivity of these aquifers to contamination.
The pre-Wisconsin surface is a composite, polygenetic feature having considerable relief that complexly truncates all the pre-Wisconsin sequences described above (fig. 1). Consequently, the stratigraphy of materials directly below the surface is not straightforward, and the subjacent units range in age from latest Illinoian to pre-Illinoian . Furthermore, the age of the surface itself appears to vary geographically, and its configuration does not everywhere represent the weathering profile that evolved during the so-called "Sangamon" interglacial between the Illinoian and Wisconsin stages (fig. 2).
Part of a cross section from Brown and Laudick (2003), (verticle exaggeration: 50x) selected to illustrate the large amount of local relief on the pre-Wisconsin surface, denoted by the heavy red line. In this example, the paleosurface cuts across a variety of older units and paleosurfaces, including the bedrock. Countywide, the pre-Wisconsin surface exhibits more than 300 ft (91.4 m) of total relief — comparable to the modern landscape. This is not surprising, because it developed over tens of thousands of years, during which streams presumably incised major valleys into the interglacial landscape and left residual hills. The surface was subsequently modified to a considerable extent by late Wisconsin ice and meltwater, especially in the White River valley, where the cutout of older units is greatest.
Parts of the extensive buried "uplands" on the pre-Wisconsin surface in northern and eastern Marion County are capped by a pervasive, well-developed paleosol , and may indeed represent the true Sangamon paleosurface. In contrast, however, large parts of the pre-Wisconsin surface in central and southwestern Marion County appear to have been severely scoured by meltwater torrents during the initial advance of the late Wisconsin ice sheet. Parts of the surface were also extremely eroded when they were overridden by late Wisconsin ice. It is clear, therefore, that much of the truncation of older units across this surface took place just prior to or during Wisconsin glaciation, and that this truncation exposed units of widely differing ages along the pre-Wisconsin surface. For this reason, it is tenuous to interpret paleosols present along truncated areas of the surface as representing the "Sangamon" interglacial, when in fact they could, and often do, represent exhumed weathering horizons of widely disparate ages.
Top — Gray Wisconsin till overlies a weathering horizon just above stream level in a ravine at Eagle Creek Park. Based on regional mapping, the truncated horizon actually lies in the middle of the Illinoian till sequence and is apparently much older than the true "Sangamon" paleosurface.
Bottom — Stacked paleosols are exposed at Newpoint Quarry in Decatur County, just in front of the late Wisconsin terminal moraine. The arrows denote the tops of two pre-Wisconsin units having distinct weathering horizons. The dark lower unit is organic silt with wood much older than the 50,000-year limit of radiocarbon dating . The upper arrow points to the top of a severely weathered till with a thick orange ultisol . A third unit lies above that, but the weathering horizon is indistinct in the photograph owing to disturbance. The vertical proximity of these weathering horizons illustrates the complications involved in interpreting paleosurfaces in outcrop, much less from scattered borehole samples. Photos by A. H. Fleming.
Pre-Wisconsin Surface Map
The map shows the topography of the pre-Wisconsin surface and the geology of pre-Wisconsin deposits to a depth of approximately 20 ft (6 m) below the contoured surface. The elevation of the pre-Wisconsin surface was determined primarily using several criteria.
- Evidence for a weathering profile , typically manifested by sharp color changes from grey or light-brown (unweathered) Wisconsin deposits to oxidized red, yellow, or gleyed green materials whose colors are suggestive of weathering; by the absence of most or all carbonate minerals from these same materials; and (or) by the abundance of expandable clay minerals at a certain horizon.
- Abrupt changes in hardness, with the pre-Wisconsin materials (especially till units) commonly being substantially harder than the overlying Wisconsin deposits.
- Stratigraphy of till units as determined by their gamma-ray log response, relative grain-size percentages, and other attributes.
- Presence of pervasive organic silt units, wood, or other indicators of a nonglacial landscape along a particular horizon.
- Presence of extensive sand and gravel sheets interpreted to have been deposited as outwash in front of the advancing late Wisconsin ice sheet.
Contours of the pre-Wisconsin surface elevation were drawn chiefly on till and other fine-grained units, and below sand and gravel units that occur along this horizon, because most of the latter are of uncertain age (namely, Wisconsin or pre-Wisconsin). Contours were drawn on top of sand and gravel units only where they could be clearly established as being of pre-Wisconsin age (for example, they are capped by a large, extensive weathering profile or are laterally contiguous with adjacent granular units that are clearly below the pre-Wisconsin surface).
In questionable cases where the pre-Wisconsin surface may be within a large body of sand and gravel, contours were drawn on fine-grained sediment below the granular material. If no fine-grained material is present above bedrock (for example, in large parts of the White River valley), the pre-Wisconsin surface was not contoured.
In accordance with the preceding discussion regarding the contouring of the pre-Wisconsin surface, two general categories of sand and gravel units are represented on the maps (fig. 3):
- Bodies of uncertain age that appear to have been deposited on the contoured pre-Wisconsin surface at various times between the close of Illinoian glaciation and the beginning of the Wisconsin. On the map explanation, these are referred to as "undifferentiated," and are further described according to apparent geometry (for example, channel-like bodies, aprons , discontinuous units);
- Bodies clearly of pre-Wisconsin age that are present within 20 ft (6 m) of the contoured surface. On the map explanation, this category is referred to as "Illinoian and older." Map units within this group are broadly characterized according to their stratigraphic position. That is, units near the top of the overall pre-Wisconsin sequence are referred to loosely as "upper Illinoian," whereas those that appear to be at or below a very large and pervasive weathering profile at depth (presumably the "pre-Illinoian surface") are referred to as undifferentiated basal Illinoian and/or pre-Illinoian. These stratigraphic characterizations should be interpreted in the broadest possible sense, and not as indicating an absolute stratigraphic position, succession, or age.
Figure 3 illustrates a schematic cross section showing the relative stratigraphic positions and geometries of sand and gravel units mapped on and near the pre-Wisconsin surface (from Fleming and others, 1993; Brown and Laudick, 2003). Units A and B appear to lie on the pre-Wisconsin surface (heavy grey line) and are typical of units mapped as "undifferentiated sand and gravel." Units C—G extend beneath till units that are clearly of pre-Wisconsin age; the sand and gravel units thus must also be pre-Wisconsin. These units are physically truncated by the pre-Wisconsin surface and are typical of units mapped as "Illinoian and older." Note the coalescing of the two types of units in several places. Unit A is a large channel whose deep incision into the pre-Wisconsin sequences has caused it to coalesce with unit D. In contrast, unit B is an apron or blanketlike body that cuts sharply across and truncates units C and D along a regional slope on the pre-Wisconsin surface. Such instances of coalesced sand and gravel bodies are indicated by a line pattern on the pre-Wisconsin surface maps.
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.