NEWMARKET TILL AQUITARD: OPTIMUM GRAIN PACKING, WITH A PORE FILLING CEMENT GENERATES PSEUDO-CONCRETE

Newmarket Till is a stoney, sandy (38%) silty (~47%) diamicton, which is of variable thickness (~1 – 69 m) and of widespread distribution (correlated with Catfish Creek Till) in southern Ontario. The Newmarket Till has unusually high densities (2.2 – 2.4 g/cm³); elevated seismic velocities (Vp ~2600 m/s) determined by downhole geophysical studies are characteristic and the Till can be traced across the region as a seismostratigraphic marker. As the Till is highly indurated and has low permeability, it forms a regional aquitard that confines underlying aquifers, and is also a basal aquitard for overlying aquifers (e.g. Oak Ridges Moraine). Given the high sand content of this diamicton, the low permeability and indurated nature is surprising, and could be resultant from over-consolidation due to glacial loading, presence of a secondary cement, or both processes. Clasts larger than coarse gravel consist of grantoids and limestone, however clasts smaller than coarse gravel are dominanted by limestone, with rare granitoids. The mineral assemblage (in decreasing abundance) is quartz, calcite, K-feldspar, plagioclase, dolomite, amphibole and clinopyroxene; these grains are comminuted and range in size from ~1000 mm to ~2 mm, leading to optimum packing. The intra-grain matrix is exceptionally fine (<1 mm, typically 0.25 – 0.50 mm) and not resolvable by optical methods. Higher resolution SEM and FE-SEM backscattered electron and secondary electron images of the intra-grain matrix reveals a complex pore filling cement, with a very low percentage (<1%) of open pore space. The minerals comprising the secondary cement are a challenge to analyze due to their very fine grain size and composition. Preliminary semi-quantitative EDS analyses suggest they consist of hydrated calcium-rich silicate minerals (CSH), with portlandite (Ca[OH]₂), calcite, and minor phyllosilicates. The Ca-rich minerals cement the silt- to sand-sized mineral grains and larger clasts, and results in the Newmarket Till being highly indurated and of low permeability. Hydrated Ca-rich minerals are typical constituents of mortar and concrete, and their presence in a glaciogenic sediment is quite unusual. Additional analytical work is being undertaken to fully characterize the mineralogy of these cements.