Rittman Lake and the Overrunning Sequence

The group posing in front of some generations of draglines at the Zollinger Pit in Rittman. Many thanks to the operators for giving us permission to spend a spectacular afternoon at the site.

Figure 1. Map showing some the the local topographic features. A lake resided in the valley prior to the Laurentide ice advance into the basin. The lake at some point drained down the Killbuck Spillway to the west.

Figure 2. This stylized sedimentary sequence from T.V. Lowell provides a framework for the site. Preglacial lacustrine setting overrun by the ice sheet.

Figure 3. The base of the sequence is a series of varve-like sediments consisting of silt-clay couplets. The group determined that is this high sediment charged environment these were likely not annual but diurnal (daily).

Figure 4. The swallows build their nest into the silt and this sequence overall is coarsening upward. Note the vertical downcutting along the boundaries of joint in this unconsolidated sediment pile.

Figure 5.  An introspective moment reflecting on the environment at the bottom of a proglacial lake just prior to advance of the Laurentide ice sheet into the basin.

Figure 6. The group working our the direction of the paleocurrent based on a series of climbing ripples with clay drapes – could they be daily couplets?

Figure 7. This facies model is a good conceptual cartoon of the setting.

Figure 8. A rare moment when the entire class was working. The upper unit record the glacial tills recording the ice advance and retreat from the site.

Figure 10. As the ice advanced and thickened over the site it smear the sediments into a deformation till, and as the ice thickened and effective normal stress increased the till was lodged onto the sediment pile.

Figure 11. The lodgement till at the top of the sequence, which, in turn, is capped by a melt-out till. The class was able to determine the ice was not absent from the site.

Figure 12. The melt-out till showing fluid escape structures and ball and pillow structures indicating loading and melt out.

Figure 13. Closeups of the ball and pillow structures.

Figure 14. Dropstone were evident and here we see a dropstone made of a stone and a till clast. This indicates icebergs in the lake dumping sediments into the lake.

Figure 15. A portion of the class spent much of the time mining gypsum crystals. The presence of gypsum crystals at the pit indicate a desert environment? We explained the gypsum as a case of saturated groundwater and, of course, kinetics.

Figure 16. Some healthy skepticism about the site and the origin of the sediments.

Source: https://woostergeologists.scotblogs.wooster.edu/2024/12/06/rittman-lake-and-the-overrunning-sequence/