Before Midwest GeoSciences Group teaches a "Soils Workshop", we generally drill a pilot boring to explore the sedimentary sequence and prepare for the upcoming training event.
BACKGROUND
In collaboration with the University of Minnesota, we conducted a 3-day continuing education course in Golden Valley, Minnesota (about 10 miles west of Minneapolis).
The workshop was comprised of both classroom lectures and afternoon field demonstrations and exercises. Borings MGB-1 was the pilot boring we drilled in Aug 2000 to prepare for the workshop Boring MGB-2 in October 2000.
The stratigraphy in the pilot boring revealed a stacked sequence of glacial till, proglacial sorted sediments (outwash), a deeper glacial till and more proglacial sorted sediments before encountering the top of bedrock. The soil core collected during the prime-time workshop generally matched the pilot boring and both soil borings are presented here.
GEOMORPHIC SETTING
The regional geomorphic setting is mapped as a till plain. However the drilling was in the parking lot on the north side of the hotel, which was the location of the training event.
The soils map reveals that Muskego and Houghton soils are likely present under the QUATERNARY FILL beneath the asphalt parking lot. Both borings encountered a surficial ORGANIC LACUSTRINE / COLLUVIUM that likely filled in a closed depression which is common in the area.
A buried soil horizon was identified within this surficial unit. The A-Horizon was recognized based on the presence of the secondary soil structure, leached carbonates, and organic components. The B-Horizon was faintly developed into the top of the glacial sediments.
WISCONSINAN-AGE SEDIMENTS
The most recent glacial sediments in Hennepin County, Minnesota were deposited by the Des Moines lobe about 14,000 years ago.
The upper glacial deposits are regionally mapped as the NEW ULM TILL MEMBER of the DES MOINES LOBE GLACIAL SEDIMENTS. In both soil cores, there were two distinct variations of this unit differentiated by sedimentary structure and associated sediments: an upper resedimented diamicton facies and a lower subglacial diamicton facies. Both soil boring logs note the contrasting change in the nature of the bedding, uniformity, and variability of soil classifications.
Despite some mottling in the upper portion of the resedimented subfacies, the weathering zone was generally poorly developed with greenish gray colors indicating a Reduced weathering zone profile transitioning into Unoxized colors of dark gray or very dark gray. Jointing was identified in the upper portion of the resedimented subfacies at boring MGB-2.
The next deeper glacial unit is likely associated with the LAKE SUPERIOR GLACIAL LOBE SEDIMENTS. The hypothesis for assigning the sediments to this lobe is purely based on it being a diamicton with reddish colors. Boring MGB-1 revealed the Superior Lobe sediment interval is 1.75 feet thick where the unit is 2.5 feet thick in boring MGB-2. There's not much soil core to definitively confirm that it's Superior Lobe Glacial Sediment, but the unique color of the diamicton and it's similar stratigraphic position lend insight to its lateral continuity between drilling locations.
Sorted proglacial sediments were identified between the Des Lobe Glacial Sediments and the Superior Lobe Glacial Sediments in both borings; mapped at PROGLACIAL SORTED SEDIMENTS (UNIT 2). One general consistency when sorted glaciofluvial sediments are present, they occur as tongues mapped between regionally deposited glacial stratigraphic units which enable us to recognize stratigraphic position and hydrogeologic context. Without stratigraphic control/context, understanding the relationships between borings (where the sorted sediments are present) can seem like a challenge. But with stratigraphic context, it gives tremendous certainty, and we can correlate buried sands without much question.
Beneath the Superior Lobe Glacial Sediments was another sequence of outwash assigned a stratigraphic unit name as PROGLACIAL SORTED SEDIMENTS (UNIT 1).
ORDOVICIAN-AGE BEDROCK
Drilling at both locations were cored a few feet into bedrock. The locally mapped bedrock unit is the SHAKOPEE FORMATION. AND ONEOTA DOLOMITE OF THE PRAIRIE du CHIEN GROUP. Core samples revealed the upper few inches of dolomite is weathered revealing its soft saprolitic condition.
