Jack L. Gilmore
Supplementary gravity data on an anomaly delineated from an aeromagnetic survey of northeastern Iowa is presented in this report. The gravity survey included 240 observation stations located at section corners from central to western Fayette County. Analysis of the simple Bouguer map, residual gravity maps, and gravity profiles suggests that the observed gravity anomalies probably derive primarily from large fault blocks within the Precambrian crystalline complex and related variations in the thickness of superjacent clastic deposits. Interpretations based on magnetic susceptibility indicate that rocksof the basement complex lie at a depth of 2,000 feet (610 m) to 2,700 feet (823 m) below the surface within the study area and vary laterally in composition from basic to acidic. Two areas that exhibit high gravity and magnetic values may warrant exploration for potential mineralization.
Donald L. Koch
The principal exposures of the Upper Devonian Shell Rock Formation occur along the Shell Rock River in north-central Iowa. Three members, the Mason City, Rock Grove and Nora are recognized. Except for the Upper Devonian State Quarry Limestone in northern Johnson County, the Shell Rock Formation has the smallest areal extent of any formation in Iowa and attains a maximum thickness of only 65 feet.
The Shell Rock Formation lies unconformably between the Middle Devonian Cedar Valley Limestone and the Upper Devonian Lime Creek Formation. Outliers of Cretaceous sand stones and shales lie unconformably upon the Shell Rock Formation along its eastern outcrop margin.
The formation consists of a series of argillaceous limestones, dolomites and shales, and prominent coralline and stromatoporoid biostromes. Stromatoporoid biostromes predominate in the upper part of the formation and indicate successive transgressions of the late Shell Rock sea. During the time of their formation these biostromes extended over shallow banks or shoals and covered a larger area than the lower beds. A discussion of the changes in the physical and mineralogical characteristics of each member as observed in the outcrop section and in the subsurface section outside the type area is included.
Dense limestone (micrite) beds and underlying dolomite beds in the vicinity of Mason City, Iowa, have been correlated by previous workers as facies equivalents of the lower two members of the Shell Rock Formation. However, collation of surface and subsurface data reported here demonstrates that these beds correlate with the underlying Cedar Valley Limestone and that the depositional basin of the Shell Rock Formation was more restricted than previous workers thought.
Fred H. Dorheim, Donald L. Koch, and Mary C. Parker
A new group name, the Yellow Spring Group, is designated for those formations herein removed from the Kinderhook Series and assigned as the uppermost unit in the Upper Devonian Series. The formations comprising the Yellow Spring Group are, in descending order, the English River Formation, the Maple Mill Shale, the Aplington Formation and the Sheffield Formation. In spite of the gradational English River - Maple Mill contact in southeastern Iowa, and the local absence of English River and Maple Mill in northcentral Iowa, these formations have significant lithologic features in common that justify their recognition as a rock-stratigraphic unit with group status.
Recent work by paleontologists has resulted in raising the Devonian-Mississippian boundary from its previous position at the top of the Lime Creek to its presently, designated position at the base of the North Hill Group.
D. H. Hase, Russell B. Campbell, and Orville J. Van Eck
In the spring of 1967, the Iowa Geological Survey established 42 gravity base stations at airports throughout the state to provide a more adequate and extensive gravity base station network than had existed previously. Measurements were made with LaCoste-Romberg geodetic gravity meters and were tied to the U. S. National Gravity Base Net through station Minneapolis L (I. G. C.15443L). For all but one station, the maximum network uncertainty at any station is ±0.041 milligal. One U. S. Air Force and two Woollard and Rose (1963) stations were recovererd, and the differences in observed gravity measured by the Iowa Geological Survey and reported at these stations are —0.005, 0.00, and —0.02 milligal respectively.
R. V. Ruhe, W. P. Dietz, T. E. Fenton, and G. F. Hall
Eugene E. Hinman
The bioherms present in the Silurian of eastern Iowa are confined to the Niagaran Series and are largely restricted to the Gower Dolomite. Those described here are solely Goweran.
Four of the bioherms were chosen for detailed study from the hundreds present, as being typical of the Iowa structure and individually offering exposures of elements which no single bioherm displayed. Typically, they are oval mounds of undisclosed heights some exceeding 180 feet. They range from a few tens of feet to at least a quarter mile in diameter.
The bioherm consists of two parts: a central unstratified core, and peripheral, well stratified flank beds which abut against the core on the one hand and dip radially away from it, merging with the flat-lying inter-reef facies on the other. The core and flanks are petrographically nearly identical but structurally and paleontologically dissimilar.
The core is the most abundantly fossiliferous of the facies, both in variety of species and in numbers present. The coelenterates are the most abundant group, with the pelmatozoans and brachiopods of secondary numerical importance. The flanks have fewer fossils, with pelmatozoans and brachiopods being the most abundant. The presence of stratification in the flanks is the most obvious contrast between them and the core, however.
Both the flanks and core are characteristically porous, the result of solutional activity and attendant fracturing. The initial solution activity at least is a function of the amount and size of the fauna present. For this reason the core is by far the more porous and the more highly fractured.
In some locations collapse breccia and clay fillings form extensive deposits, particularly in the core. Solution activity and the resultant brecciated nature of the core have made this part of the bioherm much less resistant to stream erosion.
The development of the flanks is not entirely dependent upon core debris. Much, if not most, of the flank material is autochthonous.
Although generally similar in architecture and composition, the bioherms display textural, structural, and faunal variations. These variations are ultimately dependent on faunal differences. The most obvious explanation for faunal differences is that the bioherms are a polyphylitic conglomeration unified only by their mound-like out line. Faunal differences also may be explained as the result of growth-stage variations or significant age differences.
The general lack of coarse debris in the bioherm indicates that they must have develped under much less violent energy conditions than those of Illinois. It is concluded that the Iowa structures were formed in a shallow, lagoonal environment near the western shore of the Niagaran sea. The protection thus afforded reduced turbulence.
The bioherms included in this study represent but a small fraction of those present. Certainly they exist in the hundreds within the outcrop belt of the Gower in eastern Iowa.
Donald L. Koch
Adocetocystis williamsi Strimple and Koch 1968, n. gen., n. sp., constitutes the third described genus of Callocystitidae bearing a double hydropore. Only Lipsanocystis Ehlers and Leighly and Strobilocystites White previously were known. Adocetocystis williamsi is further characterized by an ovate theca, four unbranched ambulacra which are apical over most of their length, and an apical mouth, double hydropore and gonopore. The relatively few brachioles of A. williamsi, in comparison with Lipsanocystis and Strobilocystites, are compensated for by their great length.
Adocetocystis williamsi is part of a biocoenose composed dominantly of echinoderms (cystoids and edrioasteroids) and corals (mostly Aulopora). The organisms of this community are attached to a prominent but laterally restricted discontinuity surface. Features which characterize the discontinuity surface are: (1) an irregular upper surface with rounded limestone knobs; (2) prominent borings (post-lithification); (3) a conspicuous zone of pyrite impregnation; and (4) pebbles of the same bore-marked limestone in channelways between the limestone knobs. The origin and form of the discontinuity surface might have been organically controlled but, more likely, the surface was produced by subaerial erosion.
It is supposed that Adocetocystis williamsi lived in an environment of shallow water with at least moderate current activity. The length of stem developed on any individual was dependent upon the position of attachment in relation to the shape and relief of a limestone knob. Strobilocystites calvini, acystoid with short brachioles, is found in the same environment as Adocetocystis williamsi, which has long brachioles.
Mary Combs Parker
A study of well samples from central Iowa shows that a distinct rock-stratigraphic unit is present beneath the Devonian Wapsipinicon Formation and above the characteristic dolomites and cherts of the Silurian Niagaran Series. This subsurface unit consists primarily of medium- to dark-gray chert and light-gray limestone. The unit has been traced into southwestern Iowa and its boundaries have been fairly well defined. The name La Porte City Chert, derived from the town of La Porte City in south eastern Blackhawk County, is proposed for this formation.
The lithologic characteristics and distribution of the La Porte City Chert in Iowa are shown by graphic well sections and a thickness map.
W. Herbert Yoho
Crystalline basement rock has been encountered in the bottom-hole samples of about 45 wells in Iowa. Fairly detailed studies of the samples from most of these wells have been made. Granite is the predominant rock type thus far encountered, followed by diabase in order of abundance. A summary description of the different kinds of crystalline rocks that have been encountered is presented along with a convenient table listing the pertinent data for each well. An index map showing the location and relationship of these wells one to another is included. Detailed descriptions are available in the open files of the Iowa Geological Survey.
A. C. Trowbridge
Stanley E. Harris Jr. and Mary C. Parker
The Osage Series in southeastern Iowa is composed primarily of cherty carbonate rocks. Thin shale beds appear in the middle of the series and increase upward in number and thickness. The lithologic character and thickness of the series are relatively uniform throughout the area. This gives no suggestion of major changes in environment of deposition or tectonics either laterally or vertically.
It is convenient to consider the area in two parts, the south-eastern district and the western district. Subsurface sections in the southeastern district are readily matched to the classic surface exposures of the Burlington Limestone, Keokuk Limestone, and Warsaw Formation near the Mississippi River. The lithologies of the three formations are so similar that small changes, and especially dolomitization toward the west, make tracing of boundaries difficult.
The Burlington Limestone is divided in the southeast into three members that are named and described in this report. The Dolbee Creek Member at the base and the Cedar Fork Member at the top are mainly recrystallized crinoidal bioclastic limestone. The Haight Creek Member in the middle is very cherty and contains much dolomite even in the east. Glauconite at the base of the Haight Creek and disseminated in the Cedar Fork is a persistent horizon marker. Haight Creek and Cedar Fork are found throughout the area; Dolbee Creek is restricted to the southeastern district.
The Keokuk Limestone is characterized by mottled gray bioclastics and chert in the southeast, and brownish-black cherts with white spicules and argillaceous dolomite in the west. The carbonate is argillaceous and the formation contains shale beds throughout the area. No sharp boundary marks the contact with the overlying Warsaw Formation. The latter consists of grayer dolomite beds and much more shale than the Keokuk; chalcedonic chert and crystalline quartz are generally abundant.
The Warsaw Formation is included in the Osage Series be cause of its gradational relationship to the underlying Keokuk Limestone and because of the unconformity above it.
Graphic well sections and photomicrographs illustrate the lithologic nature of each unit. Cross sections, thickness maps, and a lithofacies map of the Keokuk-Warsaw Formations depict the changes and regional variations. Structural features are shown by a map using as a datum the base of the Haight Creek Member of the Burlington Limestone.
The original sediments were marine limestone and shale deposited under stable shelf conditions. The limestones consisted mainly of disarticulated crinoid skeletons which were spread evenly over the sea floor but appear not to have been carried far by waves or currents. There is little evidence of bar accumulation. The occurrence of thicker shale sections peripheral to anticlines suggests that these structures may have been active at the time of deposition. Both dolomite and chert are probably diagenetic.
Iowa was beyond the reach of terrigenous deposits from the Appalachian tectonic belt. Any land in the direction of the Canadian Shield was certainly very low, and the seas extended widely to northwest and southwest.
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