Location and General Dimensions
A thick complex of sand and gravel is widespread in the subsurface below the northeastern part of the county. The bulk of the complex is centered beneath the valleys of Fall and Mud Creeks in Lawrence Township, and is informally called the Fall Creek aquifer complex (click here to view a geologic map of the complex). The aquifer complex is primarily of pre-Illinoian age and its distribution bears a close relationship to bedrock topography. Portions of the complex have been trenched by the Fall Creek tunnel valley system, producing a composite section of sand and gravel that approaches 200 ft (61 m) thick at places. The Fall Creek aquifer complex is one of the premier groundwater resources in Marion County, serving at least two active public wellfields , as well as hundreds of private wells throughout Lawrence township and the eastern part of Washington Township. Some large-diameter wells developed in this productive aquifer complex are documented to yield in excess of 1,500 gallons (5,678 l) per minute.
Diagram illustrating depositional conditions in northeastern Marion County during the retreat of pre-Illinoian ice just prior to the Yarmouth interglacial stage , and the relation to the ancestral White River valley (from Brown and Laudick, 2003).
Origin and Geology
The bulk of the Fall Creek aquifer complex consists of a thick sheet of sand and gravel that lies directly beneath the pre-Illinoian paleosurface, and which appears to have been deposited as glacial outwash during the retreat of the last pre-Illinoian ice sheet. The outwash forms one or more fans associated with a sizable channel-like body centered over the Oaklandon bedrock valley (fig. 1). The bedrock valley likely formed the axis of a major sluiceway associated with the ancestral White River valley during pre-Illinoian deglaciation , and the adjacent outwash fans appear to have been graded to this sluiceway . Scattered lenses of greenish-brown loam till embedded in and locally capping the outwash probably represent debris flows that emanated from the receding ice front or from unstable parts of the freshly deglaciated landscape surrounding the sluiceway.
The distinguishing feature of the pre-Illinoian outwash is the presence of a deeply weathered paleosol as much as 20 to 25 ft (6 to 7.6 m) thick along its upper surface, commonly marked by strong greenish to orange colors and enriched in chert and other resistant, siliceous lithologies. Immediately below the weathering profile, the pre-Illinoian gravel is characteristically coated in dark-red iron oxides produced by weathering of the overlying deposits and transported downward by soil-forming processes (fig. 2). The presence of such strongly oxidized colors implies that the outwash, or at least the upper several tens of feet, were in an oxygen-rich environment, and thus substantially above the water table , during the Yarmouth interglacial stage.
Iron-stained pebbles affected by pre-Illinoian weathering (bottom) contrast sharply with relatively unweathered late Wisconsin outwash (top). Photo by S. E. Brown.
The outwash, and the paleosol developed on it, were subsequently eroded by ice and meltwater when glaciers again advanced into the area at the onset of the Illinoian glaciation, resulting in laterally variable truncation of the weathering profile and, more importantly, the deposition of locally thick sections of basal Illinoian outwash atop the pre-Illinoian deposits (fig. 3). The coalescing of outwash bodies from these two vastly different periods of glaciation produced extraordinarily thick bodies of coarse sand and gravel, at places exceeding 150 ft (45.7 m). As the Illinoian glaciation proceeded, northeastern Marion County was eventually buried beneath several sheets of till, which today make up a thick, fine-grained cap that confines most of the Fall Creek aquifer complex. The final chapter in the geologic evolution of the complex occurred during the last stages of late Wisconsin glaciation, when the Fall Creek and Mud Creek tunnel valleys were excavated completely through the late Wisconsin and Illinoian sequences and into the top of the aquifer complex. The tunnel valleys were partly filled with late Wisconsin outwash, producing a composite section of gravel that extends downward for as much as 200 feet from the floors of these valleys, and which spans a large part of the Pleistocene glacial record of Marion County.
Cross section illustrating the geologic architecture of the Fall Creek aquifer complex in the vicinity of 75th Street in Lawrence Township. Units are: S- Silurian bedrock; D- Devonian bedrock; WL- West Lebanon lake muds and till; pIx, pIu-pre-Illinoian and early Illinoian sand and gravel; tu-undifferentiated pre-Wisconsin glacial till; uI-younger Illinoian sand and gravel; tt, t1, t2, pc, pd, s2c, s2d-late Wisconsin till and granular units; O3s, a-late Wisconsin outwash and modern alluvium deposited in Fall Creek tunnel valley system. Units pIx and O3s make up the Fall Creek aquifer complex. Recharge to the aquifer complex is likely to be preferentially enhanced at places where the overlying till units are substantially breached by stratigraphically higher channels and other types of gravel bodies, such as at the far right hand side of the diagram. (Diagram adapted from a portion of cross section E-E' by Brown and Laudick, 2003.)
The hydrogeologic map of the Fall Creek aquifer complex shows a reasonably well-defined groundwater flow system characterized by steep hydraulic gradients that closely mirror the dissected surface topography of the overlying landscape. Most of this aquifer complex is strongly confined by between 40 and 150 ft (12 and 45.7 m) of tight glacial till, and water levels typically stand well above the top of the complex, often by several tens of feet, indicating a well-pressurized flow system. A small number of naturally flowing artesian wells are reported from the bottom of Fall Creek Valley. Pronounced elevated regions on the potentiometric surface are associated with the northern and eastern parts of the complex, particularly in the vicinity of Oaklandon and McCordsville, where the top of the complex is intersected by several extensive gravel-filled channels that cut through a large part of the till-confining sequence and act as preferential conduits for recharge from aquifers at higher horizons (fig. 4). The potentiometric surface slopes strongly inward toward the valleys of Fall and Mud Creeks, which act as regional discharge areas; it is highly likely that groundwater discharge from the aquifer complex provides a large percentage of the base flow of these two streams. The potentiometric contours show significant deflections around smaller streams and ravines as well, although there is no evidence to suggest that any of these other streams are in direct hydraulic contact with the aquifer complex.
The strongly confined nature of the complex suggests that groundwater recharge mostly derives from slow leakage through the till- confining units above, with recharge being locally enhanced in a few places by downward flow through channels and other types of granular units that overlie and coalesce with the top of the complex. Interestingly, both the geologic map and the water-level contours clearly indicate that a considerable amount of groundwater in the aquifer complex is flowing into the county from adjacent parts of Hamilton and Hancock Counties, where it presumably originates. Although the subsurface mapping that produced the Marion County hydrogeologic atlas (Fleming and others, 1993) was not carried beyond Marion County, geologic relations leave little doubt that the aquifer complex extends beyond the county line both to the north and east: the complex is well over 40 ft (12 m) thick along the county line at both places, and the potentiometric contours rise in both directions, indicating continuity of flow. Little is known about the thickness, extent, or recharge-discharge relations of the aquifer complex in the adjacent counties, however.
The coarse texture of the late Wisconsin outwash that floors the valleys of Fall and Mud Creeks leads to high infiltration rates and abundant groundwater recharge to the top of the aquifer complex below these valleys. Photo by A. H. Fleming.
The sensitivity to contamination is judged to be relatively low for most of the complex within Marion County, owing to the strong degree of confinement and inferred slow rates of recharge. Moreover, the sloping, dissected surface terrain that characterizes significant parts of the overlying landscape tends to increase surface runoff at the expense of infiltration, further limiting recharge. On the other hand, the top of the complex lies directly beneath the broad valley floors of Fall and Mud Creek at many places. The surface outwash in these valleys is extremely coarse grained, and the topographic relief virtually level, characteristics that lead to abundant direct recharge from precipitation. The sensitivity to contamination is clearly much higher in these areas, although the upward hydraulic gradient documented in this part of the complex suggests that the impact of any contamination that did occur would likely be limited to the uppermost part of the complex.
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.