Iowa's Geological Past: Three Billion Years Of Change

5. Silurian: Dolomite and Carbonate Mounds

5 Silurian Dolomite and Carbonate Mounds It becomes very pitted on the surface, but in the mass resists the weather admirably, and tends to stand in vertical, picturesque cliffs and towers, some ofwhich approach 100 feet in height.... Even over the prairies, remote from streams, particularly in the southeastern part of the county, ledges ofthis same horizon project through the thin drift in numberless places. Samuel Calvin (1897) describing exposures ofSilurian dolomite in Delaware County Silurian strata form some ofIowa's most resistant bedrock units (fig. 5.1), and a conspicuous erosional escarpment marks the edge of the Silurian System in northeastern Iowa. Rocks of Silurian age contribute significantly to the landscape and scenery in state parks and lands such as Backbone, Bellevue, Brush Creek, Echo Valley, Maquoketa Caves, Mossy Glen, Palisades-Kepler, and White Pine Hollow. Furthermore, the southeastward course of the Mississippi River from near Dubuque to Clinton may be controlled in part by the trend of the durable Silurian bedrock. Silurian rocks are noteworthy elsewhere for their resistance to erosion, too. For example, dolomites of Silurian age form the majestic waterfalls at Niagara Falls in New York. Silurian rocks ofthe central and southern part ofthe North American craton occur primarily in structural basins that formed before Middle Devonian time. Arches and uplifts now separate these various basins, and Silurian strata have been stripped from the uplifted areas in most places. In Iowa, the thickest Silurian sections are in eastern Iowa, representing the deposits ofthe East-Central Iowa Basin. Silurian strata also thicken to the southwest in Iowa; theyaccumulated in a pre-Middle Devonian basin known as the North Kansas Basin. Marine sedimentation was probably much more widespread across the early North American continent during Silurian time than implied by the present distribution of Silurian rocks. Figures 5.2 and 5.3 are reconstructions of the paleogeography during Silurian time, approximately 409 to 439 million years ago. The region that is now Iowa was located south of the equator in a vast inland sea. Iowa's Silurian rock record consists almost entirely of marine carbonate sediments . Input of mud and sand to the Silurian seaway was very low, except in areas next to the Taconic Mountain belt (figs. 5.2 and 5.3). Emergent lands included the Transcontinental Arch along the northwest corner of Iowa, a portion ofthe Canadian Shield to the northeast, and the Ozark Dome to the south (figs. 5.2 and 5.3). 5. I Resistant Silurian dolomite ofthe Hopkinton Formation forms the Devil's Backbone in Backbone State Park, Delaware County, Iowa. Photo of an outing in the late 1890S by Samuel Calvin. Courtesy ofthe Calvin Photographic Collection, Department of Geology, University of Iowa. S.l The paleogeographic setting during Early Silurian time when Iowa's Mosalem, Tete des Morts, Blanding, Hopkinton, and lower Scotch Grove formations were deposited . Unshaded areas depict seas, and shaded areas represent emergent lands. Note the presence ofthe Taconic Mountains on the eastern seaboard and the emergent Transcontinental Arch along the northwestern corner ofIowa. To the north of Iowa, a portion ofthe Canadian Shield was above the sea, and to the south, the Ozark Dome was emergent. Symbols: m = evaporite crystal molds, 0 = carbonate ooids, s = sulfate evaporites (primarily gypsum and anhydrite), fe = oolitic ironstones, EQ = proposed paleoequator . From Witzke 1990b. I o I I I \ I- - III( Miles 620 I I I \, J- -.j Miles 620 / I EARLY SILURIAN- - Llandovery SILURIAN - - Wenlock - PTidoli S.3 The paleogeographic setting during Late Silurian time, when Iowa's upper Scotch Grove Formation and Gower strata were deposited. Symbols as in figure 5.2 with the addition ofh for halite (rock salt) and k for potash evaporites. From Witzke 1990b. Life was prolific in the Silurian seas and led locally to the development ofcarbonate mounds composed of masses of mud and fossil debris. Although these mounds have been called reefs, they are not at all like the reefs found in modern seas. In Michigan, Ohio, and New York, large basins formed and experienced restricted circulation and high evaporation. This produced waters with elevated salinity and led to the precipitation of evaporites (fig. 5.3) such as gypsum (CaS04'2H20) and rock salt (NaCl). Nature of the Rock Record The Silurian System in Iowa consists primarily of a single rock typedolomite . The Silurian dolomites of the state formed by the dolomitization of limestones, a process discussed previously in chapter 4. In...