Paleozoic Earth History - The Changing Earth: Exploring Geology and Evolution - page 531

Geo-Focus

Pictured Rocks National Lakeshore

Exposed along the south shore of
Lake Superior between Au Sable Point
and Munising in Michigan's Upper
Peninsula is the beautiful and impos-
ing wave-cut sandstone cliffs called
Pictured Rocks ( Figure 1). The rocks

exposed in this area, part of which
is designated a national lakeshore,
comprise the Upper Cambrian Munis-
ing Formation, which is divided into
two members: the lower Chapel Rock
Sandstone and the upper Miner's Cas-
tle Sandstone. The Munising Forma-
tion unconformably overlies the Upper
Proterozoic Jacobsville Sandstone and
is unconformably overlain by the Mid-
dle Ordovician Au Train Formation.
The reddish brown, coarse-grained
Jacobsville Sandstone was deposited
in streams and lakes over an irregular
erosion surface. Following deposition,
the Jacobsville Sandstone was slightly
uplifted and tilted.
By the Late Cambrian, the trans-
gressing Sauk Sea reached the
Michigan area, and the Chapel Rock
Sandstone was deposited. The prin-
cipal source area for this unit was the
Northern Michigan highlands, an area
that corresponds to the present Upper
Peninsula. Following deposition of the
Chapel Rock Sandstone, the Sauk Sea
retreated from the area.
During a second transgression of
the Sauk Sea in this area, the Miner's
Castle Sandstone was deposited. This

◗

◗ Figure 1 Location of Pictured Rocks National Lakeshore and
stratigraphy of the rocks exposed in this area.

second transgression covered most
of the Upper Peninsula of Michigan
and drowned the highlands that were
the source for the older Chapel Rock
Sandstone.
The source area for the Miner's
Castle Sandstone was the Precambrian
Canadian shield area to the north and
northeast. The Miner's Castle Sand-
stone contains rounder, better sorted,
and more abundant quartz grains than
the Chapel Rock Sandstone, indicating
a different source area. A major uncon-
formity separates the Miner's Castle

Sandstone from the overlying Middle
Ordovician Au Train Formation.
One of the most prominent fea-
tures of Pictured Rocks National
Lakeshore is Miner's Castle, a wave-cut
projection along the shoreline. The
lower sandstone unit at water level is
the Chapel Rock Sandstone, and the
rest of the feature is composed of the
Miner's Castle Sandstone. The two tur-
rets of the castle formed as sea stacks
during a time when the water level of
Lake Superior was much higher
( ◗ Figure 2).

these epeiric seas had covered most of North America, leav-
ing only a portion of the Canadian shield and a few large
islands above sea level (

(composed of fragments of organic remains), contain stro-
matolites, or have oolitic (small, spherical calcium carbonate
grains) textures. Such sedimentary structures and textures
indicate shallow-water deposition.
Recall from Chapter 6 that sediments become increas-
ingly fi ner the farther away from land one goes. Therefore,
in a stable environment where sea level remains the same,
coarse detrital sediments are typically deposited in the near-
shore environment, and fi ner-grained sediments are depos-
ited in the offshore environment. Carbonates form farthest
from land in the area beyond the reach of detrital sediments.
During a transgression, these facies (sediments that represent

Figure 20.6). These islands, col-
lectively named the Transcontinental Arch , extended from
New Mexico to Minnesota and the Lake Superior region.
The sediments deposited both on the craton and along
the shelf area of the craton margin show abundant evidence
of shallow-water deposition. The only difference between
the shelf and craton deposits is that the shelf deposits are
thicker. In both areas, the sands are generally clean and well
sorted, and commonly contain ripple marks and small-
scale cross-bedding. Many of the carbonates are bioclastic

◗