Iowa's Geological Past: Three Billion Years Of Change

3. Cambrian: Sandy Marine Shelves and Shorelines

3 Cambrian Sandy Marine Shelves and Shorelines As to lithological characters the portion ofthe Saint Croix (Cambrian) Sandstone exposed in Iowa is very variable.... With few exceptions the several beds ofthis formation (Cambrian System) vary in character laterally, so much so, that sections taken quite near together would differ greatly in minor details. This whole complex mass of arenaceous strata throughout its entire thickness ... is simply a shore deposit laid down in shallow water upon a subsiding sea bottom. Samuel Calvin (1894) discussing Cambrian strata near Lansing, Iowa By the beginning of Paleozoic time, approximately 545 million years ago, Iowa and the rest of the North American craton were part of a low landmass undergoing weathering and erosion. Weathering and erosion continued in the area into Middle and Late Cambrian time, some 530 million years ago. This old landmass consisted ofa variety ofPrecambrian rocks, including granite ' gneiss, quartzite, rhyolite, tuff, basalt, diabase, gabbro, sandstone, and shale. Weathering altered most of the original silicate minerals in the bedrock, converting them into a variety ofclay minerals. Quartz, a common mineral in much ofthe original bedrock, did not undergo any significant chemical change. Since quartz is very resistant to chemical weathering, it persisted in its original chemical form, silicon dioxide. A few minor constituents, such as durable zircon, tourmaline , and garnet minerals, also survived the long episode of chemical weathering . Some feldspar endured as well. Thus, the weathering products of the old Precambrian landmass were primarily grains ofquartz and feldspar, clay minerals, and minor amounts ofstable silicate minerals. These constituents provided most ofthe ingredients for Iowa's Cambrian record. Quartz became the rounded and sorted quartz sands ofsandstones (fig. 3.1). Clay minerals became important components of argillaceous (clay-bearing) siltstones, sandstones, and carbonate rocks. At the beginning of Cambrian time, the central portion ofthe North American continent was a low-lying landmass. Seas existed on the continental margins where the Appalachian and Rocky mountains stand today (fig. 3.2). During Cambrian time, these marginal seas received a thick accumulation ofmarine sedimentary rocks. As the interior of the continent slowly sank, the seas transgressed overthe craton. ByMiddleto Late Cambrian time, approximately530 million years ago, the seas reached Iowa. Iowa's Cambrian rock record accumulated in such an inland, or epeiric, sea. These deposits are part ofthe Sauk Sequence, 3. I An exposure ofquartz sandstones ofthe Jordan Formation, Allamakee County, Iowa. Photo by the author. the oldest ofsix continent-wide packages ofshallow-water marine strata formed by worldwide sea-level fluctuations. Each of the six sequences represents a major transgressive-regressive (T-R) cycle. Figure 3.3 shows the subsurface thickness and the outcrop belt ofthe Sauk Sequence of rocks. Iowa's Cambrian record lies unconformably on Precambrian rocks ofvarious ages. The gap between the formation ofIowa's youngest Precambrian rocks and Cambrian 49 3.2 An interpretation ofLate Cambrian paleogeography. Iowa's Cambrian rock record accumulated on the sandy floor ofa vast inland sea. Note the inferred position ofthe equator (EQ) during Cambrian time. Iowa's Cambrian rocks formed south ofthe equator in a subtropical setting. Since then, the North American plate has rotated and drifted northward to its present position. Shaded areas represent land, and unshaded areas depict seas. Note also the presence of the Transcontinental Arch and a large area ofland in the Canadian Shield. Symbols: fe = oolitic ironstones, o = carbonate ooids (oolites) and coated grains, m = evaporite crystal molds, s = sulfate evaporites (primarily gypsum and anhydrite). From Witzke 1990b. I i" ~ Miles o 620 LATE CAMBRIAN \)\ \ the start of deposition ofthe Cambrian is at least 370 million years. This represents an immense hiatus for which the state has no record. West ofIowa (in terms ofpresent geography), in what is now the Great Plains region, Precambrian rocks projected above the Cambrian seas as a large island system, a surface reflection of the Transcontinental Arch (fig. 3.2). The Transcontinental Arch, composed ofPrecambrian basement rock, supplied sand and other sediment to the shallow seas that covered the interior ofthe continent. Although the arch influenced sedimentation to some extent during Cambrian time, much ofthe warping and uplift ofthe structure probably took place after Cambrian time. The presence of several major unconformities on and adjacent to the Transcontinental Arch suggests a long history ofperiodic flooding by shallow seas, followed by episodes ofuplift and erosion. Studies ofthe Cambrian in Wisconsin show that the advance ofthe sea there was cyclic. Overall, the pattern ofadvance...