RI-40 Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota
The Dresbachian Mt. Simon Sandstone, Eau Claire Formation, and Galesville Sandstone
of the Hollandale embayment of southern Minnesota are divisible into eleven major lithofacies
and several subfacies. Because the formations are almost exclusively in the subsurface in
Minnesota, lithofacies descriptions are based on cores, well cuttings, and geophysical logs.
Along the eastern side of the Hollandale embayment, the lower Mt. Simon Sandstone
consists of a thin basal conglomerate lithofacies overlain by medium- and large-scale, crossstratified
and planar-stratified sandstone. Middle Mt. Simon is principally interbedded, coarsely
interlayered sandstone, siltstone, and shale; and thin- to medium-bedded, structureless or crossstratified
sandstone. The upper Mt. Simon is structureless sandstone with Skolithos and shelly
(coquinoid) sandstone. These lithofacies resemble those from outcrops in western Wisconsin
described by Driese and others (1981).
Toward the west and south in south-central Minnesota, at the embayment center, mediumand
large-scale, cross-stratified sandstone dominates in the Mt. Simon. Along the western side
of the embayment, structureless sandstone dominates. There are fewer thin shale and siltstone
beds in the Mt. Simon near the embayment center than along the eastern side of the
Near the embayment center, the uppermost Mt. Simon Sandstone and basal Eau Claire
Formation contain ferroan oolites and coated grains that are scattered in some beds and are the
principal sand-sized particles in others. Ferroan oolites and coated grains are not observed in
Along the eastern side of the Hollandale embayment, the Eau Claire is composed
principally of mixed sandstone and shale lithofacies and greensand lithofacies resembling Eau
Claire lithofacies that crop out in western Wisconsin (Huber, 1975), especially in cores along
structural strike with the Wisconsin outcrops.
Red sandstone and shale lithofacies and dolostone lithofacies are at the base of the Eau
Claire in south-central Minnesota. These are overlain by a ripple-cross-stratified or troughcross-
stratified subfacies of the greensand lithofacies that is much thicker than laterally
equivalent beds of greensand lithofacies to the north and east.
The Galesville Sandstone, mostly structureless, planar-stratified, or trough-cross-stratified
sandstone, appears to be conformable and interbedded with the Eau Claire Formation. The
upper part of the Eau Claire and Galesville appear to be part of an upward coarsening sequence.
There is evidence of slight disconformity between the Galesville and overlying Ironton
The basal conglomerate of the Mt. Simon is interpreted as a braided fluvial deposit.
Medium- to coarse-grained sandstone lithofacies of the lower Mt. Simon are interpreted as braid
plain, braid delta, and littoral deposits. Fine- to medium-grained sandstone beds and shale beds
in the middle Mt. Simon are interpreted as distal braid delta deposits. Sandstone beds in the
upper Mt Simon are interpreted as sand shoals and tidal flat deposits. Beds of interbedded, finegrained
sandstone and shale in the basal Eau Claire that are tidal flat deposits culminate this
initial prograding sequence. Toward the end of the sequence deposition, ferroan oolites formed
nearshore, where some were reworked by shifting tidal channels. Red sedimentary rocks were
deposited in high tidal flat, channel, and deltaic environments. Carbonate rock was deposited in
the southwestern Hollandale embayment as detrital sedimentation ended.
The greensand lithofacies of the medial Eau Claire Formation, which records marine
transgression at the base of the next prograding sediment sequence, is succeeded by shaly
lagoonal deposits, sandy or shaly tidal flat deposits (upper Eau Claire Formation), and sandy
foreshore or shoreface deposits (Galesville Sandstone).
Variation of sandstone composition reflects selective mechanical reduction of contained
potassium feldspar grains as observed by Odom (1975). Medium- to coarse-grained sandstone is
quartzose, and very fine to fine-grained sandstone is highly feldspathic. This variation, reflected
on gamma logs, helps to distinguish lithofacies.
. Sandstone in core from southwestern Minnesota contains accessory minerals, including
diaspore, that indicate the contribution of sediment from the Proterozoic Sioux Quartzite to the
Mt. Simon Sandstone.
Mossler, John H..
RI-40 Sedimentary Rocks of Dresbachian Age (Late Cambrian), Hollandale Embayment, Southeastern Minnesota.
Minnesota Geological Survey.
Retrieved from the University of Minnesota Digital Conservancy,