New insights on regional stratigraphy and hydrogeology based on surficial and subsurface sediment mapping in Simcoe County, southern Ontario
Authors: Mulligan R P M 
Published: March 2017

The integration of detailed field mapping of surficial sediments and landforms with information obtained from 14 continuously-cored boreholes are providing key insights into the character and distribution of major sediment packages that form regionally
significant hydrostratigraphic units that control groundwater flow systems within Simcoe County.
Six major sediment packages are observed, forming successions &amp;gt;160 m thick. A dense sandy till (LCT) overlies the bedrock at most locations in the study area. This till is locally overlain by glaciolacustrine deposits (LGL), which are generally fine-grained although sandy facies are
encountered and gravel deposits forming units up to 48 m thick have been observed in two boreholes to date. A fine-grained till (LFT) overlies the lower glaciolacustrine deposits forming a continuous, highly undulating marker bed in the subsurface. The upper surface of the till shows evidence of
subaerial weathering at higher elevations (&amp;gt; 170m asl). It is overlain by a fining-upward succession of predominantly sand with lesser gravel and local accumulations of silt and clay-rich diamictons (TF), tentatively assigned to the Thorncliffe Formation. These deposits are overlain by a
dense silty sand to sand till (NT) ranging from 2 to 30 m thick and correlative with the Newmarket Till. Particle size analysis of this till shows a progressive coarsening northward. Significant topographic variation in the Newmarket Till is observed across the study area - its upper surface ranging
from 150 to 310 m asl. This highly undulating unit imparts a strong control on the character of overlying deposits. Where the till is found at lower elevations, laminated silts and clays form the bulk of the deglacial succession (LA). At higher elevations (&amp;gt;230 m asl), sands and gravels
dominate.
The lower parts of the succession (LCT, LGL, LFT) do not commonly host significant aquifers. However, the bedrock interface aquifers can meet the demands for rural domestic use, and where the thick gravel deposits are encountered, significant groundwater resources may be present, especially given
its occurrence at several tens of meters below modern Georgian Bay. Within the study area, the Thorncliffe Formation deposits form a regionally significant confined aquifer. It occurs regionally beneath both uplands and lowlands and, in many areas, artesian conditions are present. The Newmarket Till
forms a regionally significant leaky aquitard, but coarser textures observed in this till in the north appear to significantly decrease its competency as an aquitard. Deglacial sediments generally form thick aquitards in low-lying areas but may form unconfined aquifers at higher elevations.
Additionally, erosion of the Newmarket Till during deglaciation has led to preferential groundwater flow path that govern groundwater seepages and create large piping features. Geophysical studies have been carried out to better constrain the architecture of major sediment packages in the subsurface
and to characterize the physical properties of the major stratigraphic units. In-situ hydrogeologic tests are planned in order to provide preliminary analysis of aquifer capacities and assess hydraulic connection of aquifers in the region.