Sedimentological Observations from the Tiskilwa Till, Illinois, and Sky Pilot Till, Manitoba

Abstract

We present sedimentological observations from the Tiskilwa Till in northern Illinois, and the Sky Pilot Till in northern Manitoba, that indicate deposition of these tills by subglacial deformation. These generally homogenous tills grade downward into more heterogeneous tills that incorporate underlying sediment into their matrix, indicating entrainment of older sediments by sediment deformation. Deformed sand inclusions within these tills imply deformation of the tills and inclusions prior to deposition. The Tiskilwa Till has relatively high fabric strength throughout its thickness, whereas fabric strength in the Sky Pilot Till generally increases up-section in 2 to 3 m thick increments. Fabric orientations in both tills rotate up-section, possibly due to changes in ice-flow direction associated with the thickening and thinning of ice, and changes in ice-flow divide location. In both the Tiskilwa and Sky Pilot Tills, the change in fabric orientation occurs over intervals of ~1 m, suggesting that the maximum depth of deformation was ≤1 m insofar as any greater depth of deformation would have reoriented till fabric during maximum ice extent and retreat. In the case of the Sky Pilot Till, the up-section increase in macrofabric strength indicates that strain increased up-section. These data suggest that these tills were deposited in a time transgressive manner as strain migrated upwards with the delivery of new till either released from the ice base or advected from up-ice.Les observations sédimentologiques des tills de Tiskilwa, Illinois, et de Sky Pilot, Manitoba, indiquent que ces tills sont issus d’une déformation sous-glaciaire. Ces tills, généralement homogènes, deviennent hétérogènes vers leur base et ils incorporent du matériel sous-jacent dans leur matrice, ce qui indique un déplacement des sédiments plus âgés par déformation. La présence d’inclusions de sable dans ces tills impliquent leur déformation avant leur dépôt. Le till de Tiskilwa présente une matrice très cohérente sur toute son épaisseur tandis que celle du till de Sky Pilot augmente vers le haut tous les 2 ou 3 mètres. La rotation de l’orientation des matrices de ces deux tills est probablement associée aux changements de l’écoulement glaciaire liés à l’épaisseur de la glace et à la migration de la ligne de partage des marges glaciaires. Pour ces tills, le changement d’orientation du matériel se produit sur des intervalles d’environ 1 m, où la profondeur maximale de déformation devrait réorienter le matériel du till durant le maximum glaciaire et le retrait des glaces. Dans le cas du till de Sky Pilot, la section supérieure montre une augmentation dans la force de cohésion du matériel. Ces données indiquent que ces tills se sont déposés de manière diachronique, où la force de tension a migré vers le haut, entraînant le dépôt de matériel basal frais à partir de la base de la glace ou par advection depuis la glace

Internal architecture of a till sheet, Tiskilwa Formation, north‐central Illinois, USA: Application of architectural element analysis

“…It comprises four subunits, including (from the bottom upward): the Oakland facies of the Delevan Member (grey diamict with abundant wood fragments); the main Delevan Member (a silt‐rich heterogeneous grey diamict); the ‘main’ unit (previously described as massive and homogeneous diamict with a distinctive reddish colour); and the Piatt Member (a stratified heterogeneous grey diamict; Fig. ; Willman & Frye, ; Johnson & Hansel, ; Hansel & Johnson, ; Carlson et al ., ). Diamict facies of the Tiskilwa Formation in Illinois have previously been interpreted as subglacial in origin because of their strong fabric direction that is consistent with ice palaeoflow direction (Johnson & Hansel, ; Carlson et al ., ), the presence of bullet‐shaped clasts, boulder pavements and shear planes (Johnson & Hansel, ), a regional spatial distribution related to the Bloomington–Shelbyville Morainic System (which comprises an end moraine complex and till plain located behind the moraine; Fig.…”

“…The Tiskilwa Formation has been described in outcrop studies (e.g. Johnson & Hansel, ; Carlson et al ., ) and regional geological reports (e.g. Willman & Frye, ; Wickham & Johnson, ; Wickham et al ., ; Hansel & Johnson, ) as a homogeneous and massive till with a sandy texture, illite‐rich mineral content and rare coarse‐grained interbeds.…”

“…Diamict facies of the Tiskilwa Formation in Illinois have previously been interpreted as subglacial in origin because of their strong fabric direction that is consistent with ice palaeoflow direction (Johnson & Hansel, ; Carlson et al ., ), the presence of bullet‐shaped clasts, boulder pavements and shear planes (Johnson & Hansel, ), a regional spatial distribution related to the Bloomington–Shelbyville Morainic System (which comprises an end moraine complex and till plain located behind the moraine; Fig. A and D; Wickham et al ., ), over‐consolidated and structureless diamict facies, sharp and erosional facies contacts between diamict beds (Johnson & Hansel, ), and deformation and incorporation of underlying sand facies and wood fragments (Wickham et al ., ; Johnson & Hansel, ; Carlson et al ., ).…”

“…(A) Approximate ice‐marginal position and corresponding glacial landforms and stratigraphic units from Peoria to Chicago (shading indicates deposition of formations). (B) Conceptual idealized log of the stratigraphy (assuming preservation of all units) near Wedron, comprising the Wedron and Mason Groups (the full succession of units here may not be present at all locations; based on data from Johnson & Hansel, ; Hansel & Johnson, ; Carlson et al ., ; Hansel & McKay, ).…”

Surface macro‐fabric analysis of relict rock glaciers in the Cantabrian Mountains (NW Spain)

“…36 In that sense, macro-fabric analysis focusses on the visible features of the coarse fraction of the sediments. [37][38][39][40] This technique has been used in different kinds of sediments, mainly glacial, [41][42][43][44][45][46][47][48][49][50] periglacial [51][52][53] and slope 54,55 deposits. Macro-fabric analyses have been performed to distinguish between different types of deposits such as mudflows and till, 56 different types of slope deposits, 57 rock glaciers and other periglacial landforms such as protalus ramparts 58,59 or landslides, scree and moraines.…”