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Dakota Aquifer Program--Geologic Framework--Stochastic Modeling

Field observations and discussions

Basal Sandstone of the Terra Cotta Clay Member at Kanapolis Lake, Ellsworth County

The basal sandstone of the Terra Cotta Clay Member is widely exposed in vertical cliffs around Kanapolis Lake, especially along the rim of Horsethief Canyon (Appendix A). Architectural-element analysis of these exposures reveals that basal Terra Cotta sandstone in Horsethief Canyon is entirely composed of 3-5 amalgamated channel-fill elements. Limited exposure makes exact widths for these channel-fill elements difficult to determine; however, they are clearly very wide (> 90 m) and moderately thick (1.5-3 m) with very high width/depth ratios (> 60) (Figure 5). These "master channels" are in turn filled with a collection of smaller-scale elements with distinctive fill and geometric form of their own. Elements filling these master channels comprise channel-fill and foreset-macroform elements (Figure 5), and are further discussed below. Paleocurrent data from cross-bedding in master channel fills indicate a northwest flow trend for these basal Terra Cotta channels (see Appendix C).

Approximately 90% of master channel fills is constituted by smaller "minor" channel-fill elements. Minor channel-fill elements appear as vertically and laterally amalgamated concave-up lens 1.2-9.5 m wide and 0.5-1.3 m thick with erosional base (Figure 6). Lithofacies St (60%) and Sp (30%) are by far the dominate lithofacies in minor channel-fill elements. Minor channel-fill elements represent filling of smaller channels developed within the master channel by bar migration and aggradation of bed-bottom deposits. Coarse texture of minor channel-fills infers that channels retained active flow throughout the filling process.

Foreset-macroform elements occupy approximately 10% of master channel fills, and appear as convex lens (Figure 5). These elements are 0.7-2.5 m thick and 2-12 m wide, and are filled mostly with lithofacies Sp and minor St. Cross-bed sets are partitioned by accretion surfaces with down-stream dip directions (Figure 6). Foreset-macroform elements record downstream migration of mid-channel bars in master channels.

Basal Terra Cotta sandstones at Kanopolis reservoir represent deposition from braided streams. The high width depth ratios of Terra Cotta master channels is most typical of braided streams (Schumm, 1968), as is the lack of mudstone lithofacies and the preponderance of cross-bedded sandstone (Rust, 1978). Filling of master channels with multiple amalgamated minor channel-fill elements is also expected in a braided stream where numerous smaller braid channels are constantly shifting and filling within the banks of the larger master channel. Foreset-macroform elements reflect preservation of mid-channel bars that separated braid channels within basal Terra Cotta rivers.

Implications for Terra Cotta Paleogeography

Fluvial deposition shifted from predominately braided to predominantly meandering over the span of Terra Cotta deposition in central Kansas. Work is still too preliminary to make firm assessments as to the cause of this shift in depositional patterns. Some reasonable speculations, however, can be made.

Shifts from braided to meandering patterns in river systems typically are the result of loss of coarse bedload, loss of slope, or shift from flashy to consistent discharge (Schumm, et al., 1987). Although Rocktown Channel strata are substantially finer than Terra Cotta strata on average, this difference is mostly a reflection of the greater abundance of fine-grained elements in Rocktown Channel strata. Grain size for sandstone-rich channel-fill elements in Rocktown Channel and Terra Cotta strata are actually very similar, inferring that grain-size of bedload in the two systems was similar.

Greater abundance of fines in Rocktown Channel strata is mostly a reflection of greater preservation of overbank-fine elements in this unit compared to Terra Cotta strata. Such enhanced preservation of overbank-fine elements is typically more a reflection of greater aggradation rates than greater suspended-load component (Allen, 1978; Shanley, et al., 1992). There is thus no clear evidence to indicate that the shift from braided to meandering patterns during Terra Cotta deposition reflects a shift from coarser to finer load.

Greater aggradation rates during Rocktown Channel/upper Terra Cotta deposition not only explain greater preservation of overbank-fine elements in these strata, but would also be an expected response to rising base level associated with onset of the late Dakota transgression which deposited the overlying marine-influenced Janssen Member. Such rising base level and alluvial aggradation would likely have resulted in lowering of river gradients. This loss of gradient during transgression and associated base--level rise appears to be the most likely cause for the shift from braided to meandering systems during Terra Cotta deposition. This however is only a working hypothesis, bases on speculations from the currently available data.

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Kansas Geological Survey, Dakota Aquifer Program
Updated July 5, 1996.
Scientific comments to P. Allen Macfarlane
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