MRCSP Phase I Geologic Characterization Report - Midwest ...

APPENDIX A: CAMBRIAN BASAL SANDSTONES
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study area (Figure 5). The name Mt. Simon Sandstone was first used
by Walcott (1914) to designate a sandstone exposed near Eau Claire,
Wisconsin (the type section). The Potsdam, described by Emmons
(1838), was named for sandstone cropping out at Potsdam, New
York. Haynes (1891) named the Conasauga Group for exposures of
shale and limestone in the Conasauga Valley of northwest Georgia.
The reader is referred to the following papers which include discussions
on formal and informal nomenclature relative to the lower
Sauk interval in statewide to semi-regional context (Cohee, 1948;
Fettke, 1948; Sloss and others, 1949; Freeman, 1953; McGuire and
Howell, 1963; Catacosinos, 1973; Janssens, 1973; Rickard, 1973;
Calvert, 1974; Wagner, 1976; Bricker and others, 1983; Patchen and
others, 1985a,b; Shaver and others, 1986; Rupp, 1991; Ryder, 1991,
1992a,b; Riley and others, 1993; Ryder and others, 1995, 1996; Harris
and others, 2004).
NATURE OF LOWER AND UPPER CONTACTS
The basal sandstone interval directly overlies the Precambrian unconformity
surface, which locally may have significant topographic
and/or structural relief (most apparent in western Ohio where well
control is greatest). In some local areas, the entire interval is missing
or is thicker due to incised channel development. The thickness of
the interval can vary dramatically over such features, thus affecting
reservoir quality in these areas. The lower contact is generally sharp
due to a regolith found at the base of these rocks, but may appear
gradational on geophysical logs depending on the underlying Precambrian
lithology. A decrease in grain size and an increase in lithic
and feldspathic components at the top of the Mt. Simon make the
upper contact, where it underlies the Eau Claire Formation, appear
gradational in the western part of the MRCSP study area. The upper
contact is moderately gradational in the eastern region where thin
discontinuous dolomitic sandstone layers of the Conasauga Group
and Potsdam mark the top of the interval. The upper contact of the
unnamed basal sandstones in the Rome trough and the adjacent
eastern region is considered disconformable with the overlying
Tomstown and Rome Formations (Harris and others, 2004).
Gamma-ray log response of the Mt. Simon in western Ohio and
the proto-Illinois/Michigan basin is relatively lower than that of the
overlying Eau Claire Formation, indicating sandstone with lower
feldspar, shale, and glauconite content. Gamma-ray curve shapes
range from blocky-, to funnel-, and bell-shaped. Density and photoelectric
(PE) logs are indicative of relatively higher quartz content
of the Mt. Simon. The log response of the lower portion of the Mt.
Simon locally appears cyclic, and may representing alternating beds
of quartz arenite, shale, arkose, and sublitharenite. Conversely, the
gamma-ray log response of the unnamed Conasauga sandstones and
Potsdam Formation is higher than that of the Mt. Simon, an indicator
of higher feldspar, shale, and glauconite content. Gamma-ray
curve shapes for the unnamed Conasauga sandstones and Potsdam
Formation are typically more ”spikey” than those of the Mt. Simon,
indicating the greater cyclicity of lithologic components. PE logs of
the Conasauga and Potsdam also indicate a higher dolostone content
than the Mt. Simon. In eastern Kentucky, core from one well
indicates that basal sandstones in the Rome trough are commonly
arkosic, an attribute that affects the gamma ray log, giving a more
shaley appearance.
LITHOLOGY
The Mt. Simon Sandstone is a white, pink, or purple, fine- to
coarse-grained, moderately to well-sorted, quartz arenite that can be
arkosic. The Mt. Simon also contains thin interbeds of red, green,
gray, or black, sandy to silty shale. Locally, thin beds of tight, silicacemented
quartz arenite occur. Bedding thickness ranges from thin
to medium with many beds containing lamina and wisps of finer
grained materials. Graded beds and cross bedding are common. Bioturbation
is present but generally, poorly developed. However, in
Michigan, preservation of burrowing has been observed to increase
upward towards the contact with the overlying Eau Claire. Grains
are sub-rounded to rounded, commonly etched and generally poorly
cemented and friable. The lower portion is commonly conglomeratic
and shaley, and stained by hematite, some of which is mottled.
In the Michigan subsurface, dolomite cement is common in some
intervals around the basin margin. Deeper in the proto-Michigan
basin, below 7,000 feet, there is pervasive quartz overgrowth cement.
Secondary porosity is common where carbonate cement and
feldspar has been dissolved.
The unnamed Conasauga sandstone and Potsdam Sandstone consist
of interbedded, cyclical-appearing sandstone and dolostone. The
sandstone components are typically white to light-gray and pinkish
gray, and fine- to medium-grained, with moderate- to well-sorted,
sub-rounded to angular grains, although arkosic sandstones occur in
the basal portions of the Potsdam in northwestern Pennsylvania and
south-central New York. Authigenic and primary feldspar, glauconite,
and bioturbated zones are common. Normal and reverse graded
bedding and trough cross-bedding are common thoughout the unit.
Coarse to pebbly, normal graded bedding, and thin zones of rip-up
clasts mark thin intercyclic intervals. Bedding ranges from thin to
medium and massive, to interbedded with laminated and wispy
dark gray shale. Bioturbation, including filled vertical burrows
(Skolithos), graded bedding, and cross-bedding are common. The
dolostone is light to medium gray and pinkish gray, cryptocrystalline,
microcrystalline, medium crystalline, and arenaceous. Minor
beds and laminae of gray to black shale, frosted quartz grains, vugs
filled with selenite and dolomite crystals, ooids, pelloids, disseminated
pyrite, laminae and thin beds of glauconite, apatite, flat pebble
conglomerate, rip-up clasts, mudcracks, and clay-rich zones are also
present (Harris and others, 2004; Baranoski, in preparation).
The basal sandstones of the Rome trough are arkosic and, based
upon a core from a well in Wolfe County, Kentucky, are interbedded
with red and green shale, siltstone, sandy shale, and nodular evaporites.
The unit is locally conglomeratic and may contain poorly
developed cross beds (Harris and others, 2004).
DISCUSSION OF DEPTH AND THICKNESS RANGES
Depth to the top of the basal sandstones interval ranges from approximately
2,000-feet below sea level (bsl) in northern Ohio and
northwestern Indiana on the Ohio-Indiana platform to 14,000 feet
bsl in the center of the Michigan basin, and 20,000 to 25,000 feet
bsl in the Rome trough of southwestern Pennsylvania (Figure A2-3).
Thickness ranges from zero, where local Precambrian topography
(Janssens, 1973; Baranoski, 2002) exists, to more than 2,500 feet in
northwestern Indiana and east-central portion of Illinois, just west
of the MRCSP study area (Figure A2-2). This thick accumulation
of sediments, or depocenter, is unique in its location, not being coincident
with depocenters for any younger stratigraphic units. The
Mt. Simon reaches a thickness in excess of 1,300 feet in east-central
Michigan (Figure A2-2), where again this depositional pattern is
not seen in other geologic units. Eastward, away from the Illinois
and Michigan basins, the Mt. Simon is 50 to 300 feet thick in Ohio
and adjacent central Kentucky. In eastern Kentucky, the Mt. Simon
gradually thins eastward toward the Rome trough. The unnamed