The Sauk Megasequence in the Cratonic Interior of North America: Interplay between a Fully Developed Inner Detrital Belt and the Central Great American Carbonate Bank
Published:January 01, 2012
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Anthony C. Runkel, Robert M. McKay, Clinton A. Cowan, James F. Miller, John F. Taylor, 2012. "The Sauk Megasequence in the Cratonic Interior of North America: Interplay between a Fully Developed Inner Detrital Belt and the Central Great American Carbonate Bank", Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia, James Derby, Richard Fritz, Susan Longacre, William Morgan, Charles Sternbach
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The Sauk megasequence in the far inboard region of the cratonic interior of North America (Minnesota, Wisconsin, and Iowa) is divisible into two packages that fundamentally differ from one another in facies and stratigraphic attributes. A lower Sauk succession package, Marjuman–early Skullrockian in age, is characterized by deposits of the traditional inner detrital belt (IDB) that interfinger hundreds of kilometers seaward with the middle carbonate belt or cratonward margin of the central mid-continent great American carbonate bank (GACB). The IDB contains a typical suite of nearshore siliciclastic facies containing features that document the importance of both wave- and tide-dominated currents in the depositional system. The transitional area between the IDB and the GACB in the Cambrian and earliest Ordovician was a moat, characterized by relatively deep-water deposition, which served as a catchment for mud that was winnowed from landward parts of the shelf and then deposited near the stormwave base. Mixed carbonate and siliciclastic facies in the moat are characterized by condensation features and other attributes indicative of suppressed carbonate productivity and starvation of siliciclastic sand. These facies contrast with shallower water facies that commonly filled available accommodation space in both seaward (central part of the GACB) and landward (cratonic shoreline) directions, the former dominated by typical stacks of oolitic, ribbon-rock, and microbialite lithofacies, and the latter by stacks of nearshore siliciclastic sand-dominated parasequences. Our chronostratigraphic framework provides temporal constraints that support the long-postulated hypothesis that these two depositional systems expanded and contracted in reciprocating fashion: substantial landward migration and expansion of the GACB occurred when siliciclastic input was diminished during the most rapid rates of transgression (marked by maximum flooding intervals in the IDB). Retreat and diminishment in the extent of the GACB corresponded to falls in sea level that led to major progradations of nearshore siliciclastics of the IDB and terrigenous poisoning of the carbonate factory.
An overlying upper Sauk succession package records the establishment of a fundamentally different depositional system in the far inboard regions of the cratonic interior beginning in the later Skullrockian. The Prairie du Chien Group and its equivalents represent a major landward migration and perhaps cratonwide distribution of the oolitic, ribbon-rock, and micro-bialite lithofacies that were previously restricted mostly to the GACB of Missouri and adjacent areas. This change was triggered by a pronounced continental-scale flooding event that led to onlap across much, or all, of the cratonic interior. The resultant burial of terrigenous source regions by carbonate strata is in part responsible for this fundamental change in de-positional conditions.
The Sauk megasequence in the cratonic interior of North America (Minnesota, Wisconsin, and Iowa) includes the most comprehensively understood siliciclastic-dominated package that accumulated as part of the Cambrian inner detrital belt (IDB). Several recent publications have described the results of our effort to construct a regional stratigraphic framework for the Upper Cambrian and lowermost Ordovician part of the Sauk megasequence (Figure 1) across this area, providing new insights into the depositional history of these strata and a general model for sequence-stratigraphic architecture in slowly subsiding cratonic interior settings (e.g., McKay, 1988; Runkel et al., 1998, 2007). More recently, we have expanded our stratigraphic framework in a paleoseaward direction to include strata that record the poorly understood transition from a fully siliciclastic nearshore system of the IDB to the middle carbonate belt, also known as the great American carbonate bank (GACB) of Missouri and surrounding areas. This chapter presents a brief summary of our ongoing work, with emphasis on presenting a preliminary strat-igraphic framework that shows the general chrono-stratigraphic relationships between these two facies belts and provides new perspectives on the deposi-tional interplay between them.
Paleophysiographic and Tectonic Setting
Middle Cambrian-Lower Ordovician strata were deposited across a broad shallow shelf, the upper Mississippi Valley epeiric ramp (UMVER) (Runkel et al., 2007) that extended northward from the Illinois and Ozark Basins across northern Missouri, Iowa, eastern Nebraska, South Dakota, southern Minnesota, and southwestern Wisconsin (Figure 2). The UMVER subsided slowly, at an average rate of less than 10 m (33 ft)/m.y. during the Middle Cambrian-Early Ordovician (Sloss, 1988), and this ramp had a regional shelf gradient of about 0.1 m/km (∼0.5 ft/mi) or less, with no distinct break in slope toward areas with deeper water (Runkel et al., 2007). Thus, both calculated subsidence rates and vertical accommodation were relatively small compared to typical depositional basins outside of cratonic interior regions.
The IDB in the northern part of the UMVER was substantially influenced by the Wisconsin arch and Wisconsin dome, which were positive structural features that flanked the UMVER on the northeast (Figure 2). Together, these features were the principal control on shoreline strike and the distribution of IDB facies. During most of the Middle and Late Cambrian, no features of comparable scale flanked the UMVER to the south and west; instead, age and facies relationships show evidence only for gradual uninterrupted deepening and reduced input of siliciclastic sediment in those directions (Runkel et al., 1998, 2007). Topographically high areas of Precambrian rocks such as the Sioux ridge and Kansas-Nebraska highlands (Figure 2) provided only local sources of siliciclastic material in relatively limited amounts. In the latest Cambrian and earliest Or dovician, uplift along the Transcontinental arch in western Minnesota, Iowa, eastern South Dakota, and Nebraska led to the incipient development of an em-bayment in the northern part of the UMVER (Runkel, 1994; Runkel et al., 2007), referred to as the Hollandale embayment (Austin, 1969).
Paleophysiographic and tectonic features in the GACB of central to southern Missouri (Figure 2) were somewhat more dynamic than those across the IDB. Depositional conditions were controlled to some extent by structures related to the Reelfoot rift, including formation of the rapidly subsiding rift, regional carbonate shelf margins, rift mountains, and intrashelf basins (Palmer, 1989). The Sauk megase-quence strata of this area that we integrated herein into our regional stratigraphic framework were deposited on the St. Francois regional shelf of southeastern Missouri, which was bounded to the southeast by the Reelfoot rift, to the east by the Illinois Basin, and to the north and west by the comparatively subtle central Missouri intrashelf basin. Subsidence across the shelf was on average about twice the rate of the cratonic interior to the north.
Facies and Stratigraphic Attributes
The stratigraphic framework presented here for the Sauk megasequence spanning the region from the IDB of the cratonic interior to the GACB of Missouri (Figure 3) is the result of a multidisciplinary effort combining fa-cies analysis, traditional lithostratigraphic and bio Stratigraphic techniques, and a sequence-stratigraphic approach. Information from many previous investigations was combined with our contributions, which included (1) extension of previously established biostratigraphic correlations and facies relationships across outcrop belts into the subsurface using information from drill core and boreholes, (2) improvement of biostratigraphic resolution by making collections that fill in biostratigraphic gaps in previous studies of the macrofauna and by the addition of relatively high-resolution conodont biostra-tigraphy in upper Sunwaptan and Skullrockian strata,(3)correlation of diagnostic chemostratigraphic (δ13C) signatures (e.g., Saltzman et al., 2004; Cowan et al., 2005), (4) definition of para sequences and mapping of their geometry using petrophysical logs, and (5) identification and tracing of sequence boundaries and maximum flooding intervals. Much of this effort for the strata deposited in the cratonic interior of Minnesota, Wisconsin, and Iowa is summarized in McKay (1988), Smith et al. (1993), Smith and Clark (1996) and Runkel et al. (1998, 2007). The strata of the GACB of Missouriare integrated into the regional framework of Runkel et al. (2007) basedonfacies and stratigraphic information contained in publications by Kurtz (1971, 1975a, b), Howe et al. (1972), Palmer (1989), and core logging conducted by authors Runkel and Cowan (additional information provided in caption to Figure 3). Palmer et al. (2012) provide additional detail on the Missouri section, but we were unable to incorporate all of their information into this chapter.
Lower Sauk Succession
The Sauk megasequence across the UMVER is broadly divisible into two stratigraphic packages, referred to here informally as a lower and upper Sauk succession (Figures 1, 3), that fundamentally differ from one another in facies and stratigraphic attributes. The lower Sauk succession, Marjuman–early Skullrockian in age, is dominated by siliciclastic deposits of the IDB that inter-finger hundreds of kilometers seaward with deposits of the GACB (Figure 3). The IDB is composed of a typical suite of nearshore siliciclastic facies that were described and interpreted in several publications, most of them summarized by Dott et al. (1986) and Runkel et al. (2007). Nonmarine facies are composed of unfossilifer-ous fine- to coarse-grained quartzose sandstone with adhesion structures, channel-shaped erosional features, and other attributes that indicate it was deposited as part of an eolian sandsheet and braided stream system immediately landward of the paleoshoreline (Dott et al., 1986). Nearshore marine facies are similarly dominated by fine- to coarse-grained quartzose sandstone, but the strata contain trace fossils (most commonly Skolithos) and skeletal fossils diagnostic of marine conditions. These facieshavea suite of sedimentary structures such as swaley, trough, and planar cross-strata that are typical of deposition at or above the fair-weather wave base in a shoreface setting. Rhythmically bundled cross-bedded foresets are also common in nearshore marine facies, indicating that tide-generated currents were markedly more important in the depositional system than commonly believed (Tape et al., 2003). Offshore shelf deposits that formed in the transition zone between the IDB and GACB include a siliciclastic facies comprising feldspathic, variably glauconitic, very fine grained sandstone, siltstone, and shale; interbedded carbonate facies range from packstonetomudstone. The interbedded carbonate and siliciclastic offshore shelf fa-cies of the lower Sauk succession are most common from central Iowa to northern Missouri. Both the siliciclastic and carbonate offshore facies are dominated by normally graded tempestites with hummocky cross-strata that represent storm-generated deposition below the fair-weather wave base. The deposits are variably bio-turbated with common bedding plane–parallel trace fossils. Evidence for condensed sedimentation along specific horizons includes intraclasts associated with firmground and hardground surfaces (some with pyrite or phosphate) that have high percentages of glauconite and lags of skeletal material (trilobites, brachiopods, and echinoderm debris). Thrombolites, and more rarely stromatolites, are locally associated with such horizons. Additional description and interpretation of these offshore transitional facies are provided in the next section of this chapter.
Lower Sauk succession strata in central to southern Missouri (Figure 3) are composed mostly of typical GACB facies, summarized most recently by Palmer (1989). Strata on carbonate platforms of southern Missouri are dominated by ooid-intraclast packstones– grainstones, microbialites, and supratidal laminites, which are commonly deposited as part of shoal complexes. Subtle intrashelf basins differ from platforms by also containing deeper subtidal facies, such as ribbon rock and intraclastic skeletal packstone and grainstone, with intercalations of shale, mudstone, and siltstone. Palmer (1989) described these facies in greater detail, including the relationship of facies distribution to tectonic and sea level controls in lower Sauk succession strata of this area.
Five sequence boundaries are recognizable within the lower Sauk succession (Figures 1, 3). Of these, three major boundaries are associated with regional sheet sandstones and are considered to correspond to sub-aerial erosion surfaces that extend across much of the IDB deposits of the cratonic interior (Runkel, 1994; Runkel et al., 1998, 2007). The lowest boundary is the basal Sauk megasequence unconformity that separates various Precambrian rocks from the Marjuman, Mount Simon, and Lamotte sheet sandstones. A major unconformity is also recognized within the Wonewoc Sandstone, and it corresponds to the Sauk II-III subsequence boundary of Palmer (1981). In a paleoseaward direction, from approximately central Iowa to the GACB, the Sauk II-III unconformity appears to merge with a correlative conformity (Runkel et al., 1998; Saltzman et al., 2004). A younger major sequence boundary (SB5;Figures 1, 3) caps the Sunwaptan to basal Skullrockian Jordan Sandstone in the IDB and may be a subaerial unconformity across the entire UMVER, although the magnitude of the lacuna diminishes basinward to such a degree that it is irresolvable within the resolution of conodont biozones (Runkel et al., 2007) within the offshore shelf succession. This sequence boundary has not been confidently traced across the GACB, but the drawdown that created it is most likely that which produced the Gunter Sandstone Member at the base of the Gasconade Formation (Howe et al., 1972; Repetski et al., 2000). Two minor sequence boundaries in the lower Sauk succession (SB3, SB4; Figures 1, 3) are recognized only in the most extreme paleolandward areas of the IDB, within the Tunnel City Group. Each lies within shoreface-dominated facies in the part of the Mazomanie Formation that represents the most paleolandward part of the IDB deposits that are still preserved (Figures 1, 3). Like the major sequence boundaries, these minor sequence boundaries separate regressive from transgressive stacks of nearshore parasequences but are considered minor sequence boundaries because the shoreface-dominated sandstone bodies extend relatively short distances basinward (Runkel et al. 2007).
Upper Sauk Succession
The upper Sauk succession records the establishment of a fundamentally different depositional system in which carbonate facies dominated across the entire geographic extent of preserved Sauk strata in the UMVER. The Prairie du Chien Group and its equivalents record a major landward migration of the oolite, ribbon-rock, and microbialite facies that previously were restricted to more seaward areas on the GACB that had circumscribed Laurentia during the Cambrian. No record of a distinct IDB exists at this time. The IDB that had persisted for millions of years in the Cambrian may have existed landward of today’s preserved Sauk megase-quence, or alternatively, the IDB was replaced completely by a carbonate system that covered the entire Lau-rentian craton at the peak of transgression in the Early Ordovician. Features indicative of peritidal to shallow subtidal deposition are well documented in upper Sauk succession strata, including abundant oolites, desiccated planar microbial laminites, and molds and silicified nodules after anhydrite (Smith et al. 1993; Overstreet et al., 2003). Our ongoing work has led to the discovery of coeval deposits dominated by hummocky cross-stratified ribbon rock and meter-scale thrombolitic reef complexes that formed in deeper subtidal environments than the previously documented peritidal and shallow subtidal facies.
Two major sequence boundaries are associated with upper Sauk succession strata (Figures 1, 3). A middle Lower Ordovician sequence boundary (Smith et al., 1993) separates the Oneota Dolomite from the Shakopee Formation across the UMVER of Wisconsin, Minnesota, and much of Iowa. This sequence boundary is a sub-aerial unconformity locally marked by paleokarst and silcrete cement below and eolian facies above. It has not been confidently recognized in the GACB succession of Missouri but likely approximates the base of the Roubidoux Formation (Repetski et al., 1998), aquartzose sandstone-rich succession in the otherwise carbonate-dominated Lower Ordovician section of that area. The Sauk-Tippecanoe unconformity, which caps Lower Or-dovician carbonates regionally across the UMVER, is a first-order sequence boundary thatis recognized across North America.
The top of the Sauk megasequencein Minnesota isat the unconformable contact of the Shakopee Formation with the St. Peter Sandstone. Younger rocks are present beneath the St. Peter Sandstone on the southern and eastern flanks of the Ozark dome, where the upper Sauk succession includes the Roubidoux, Jefferson City, Cotter, Powell–Smithville–Black Rock, and Everton units in that stratigraphic order (Ethington et al., 2012; Palmer et al., 2012). The Shakopee Formation is equivalent to some lower part of this succession, but sparse invertebrate faunas and long-ranging conodonts in these units preclude correlation with high resolution. The Jasper Member of the Everton Formation ofnorthern Arkansas contains conodonts of the Histiodella holodentata Biozone, which demonstrates the latest early Whiterockian age for the top of the rocks of the GACB in that region. No faunal evidence is available there for the age of the base of the St. Peter Sandstone. The boundary between the Sauk and Tippecanoe megasequences may be a correlative conformity in the Reelfoot rift of southeastern Missouri and northeastern Missouri, but this has not been demonstrated.
Relationship Between the Inner Detrital Belt and the Great American Carbonate Bank
Our refined chronostratigraphy and incorporation of subsurface data reveal the presence of a relatively deep-water sediment-starved moat that persistently occupied a transitional area between the IDB and the GACB during deposition of lower Sauk succession strata (Figures 3, 4). The suite of lithologies deposited in the moat reflects minimal siliciclastic input and chronically low carbonate productivity (cf. Walker et al., 1983). Moat facies were deposited generally above the storm wave base and are dominated by shale, mudstone, and dolostone. The shale is typically laminated to structureless, with sparse body and trace fossils. Thin to very thin beds of very fine grained sandstone and siltstone display small-scale hummocky cross-stratification. Carbonate inter-beds are dolomudstone to skeletal dolograinstone and include beds of thrombolites. The dominant skeletal grains are echinoderm and trilobite fragments. Pack-stone and grainstone beds are commonly intraclastic; many are flat-pebble conglomerates. Fine-grained carbonate lithologies include ribbon rock displaying hummocky cross-stratification or less common, thin to very thin, graded tempestites. Moat facies exhibit several features that indicate episodic suppressed carbonate productivity and starvation of siliciclastic material: an abundance of hardgrounds (many with Fe encrustation), moderate to high glauconite content, amalgamated beds of heterolithic intraclasts, and replacement by phosphate or pyrite. Thick intervals dominated by shale and siltstone but lacking such features appear to be present in what were relatively rapidly subsiding areas of the moat that at times served as catchments for mud that was winnowed from landward parts of the shelf and deposited near the storm wave base.
The UMVER moat facies indicate that, through most of the Late Cambrian, deposition occurred well below the fair-weather wave base, with depths consistently exceeding approximately 30 m (∼100 ft) (Runkel et al., 2007). Maximum paleobathymetry is difficult to estimate, but the common presence of storm-generated sedimentary features, the less abundant occurrence of microbialites, and the projection of estimated shelf-slope gradients for the UMVER suggest that, at most times in most areas, moat bathymetry was unlikely to have markedly exceeded a depth of 100 m (328 ft). Deposits in the moat contrast with shallower water facies that commonly filled available accommodation space in both seaward (central part of the GACB) and landward (IDB shoreline) directions. Seaward areas were dominated by typical tropical carbonate facies and landward areas by laterally shingled, shoreface, siliciclastic parasequences. Such facies dominate in outcrop belts of Missouri and across southern Minnesota and Wisconsin (cores of equivalent strata from the intervening region received limited attention). Previous studies incorrectly concluded that deposition was dominated by very shallow sub-tidal and peritidal conditions everywhere across the region (e.g., Lochman-Balk, 1970; Westrop and Adrain, 2001).
Our stratigraphic framework provides temporal constraints that support the hypothesis that the regional-scale siliciclastic- and carbonate-dominated systems expanded and contracted in reciprocating fashion (cf. Aitken, 1966; Palmer, 1971). The retreat and diminished extent of the GACB resulted from falls in sea level that led to major progradations of nearshore siliciclastics of the IDB. Substantial landward migration and expansion of the GACB occurred when siliciclastic input was diminished during the most rapid rates of sea level rise (Figures 3, 4). Pronounced landward expansion of the GACB occurred several times, each marked by maximum flooding intervals in the IDB, as evidenced in the lower Eau Claire Formation (near the base of the Crepicephalus trilobite Zone), in the lower Lone Rock Formation (near the base of the Taenicephalus trilobite Zone), in the lower St. Lawrence Formation (lowermost Illaenurus trilobite Zone), and in the lower Oneota Dolomite (upper Cordylodus angulatus–lower Rossodus manitouensis conodont Zones). Each successive transgression was progressively more pronounced, resulting in a progressively greater landward expansion of the GACB through time. The flooding event seen in the C. angulatus–R. manitouensis Zones led to onlap across much, or all, of the cratonic interior by deposition of the Lower Ordovician Oneota Dolomite and equivalents and has been recognized across much of North America. Taylor et al. (1992, 2004) refer to this event as the Stonehenge transgression. In the UMVER, the Stonehenge transgression produced the fundamental change in depositional conditions described in the previous section, where by conditions characteristic of the classic GACB prevailed across the entire preserved extent of the cratonic interior Sauk megasequence. Burial of detrital sources across the Superior craton is likely to be at least in part responsible for this fundamental change in depositional conditions.
Studies at this scale, which include the strata representing the IDB paleoshoreline, can provide insights necessary to decipher long-standing sequence-stratigraphic problems associated with cratonic successions (e.g., Sloss, 1996) such as the GACB. Comparison of GACB strata to the stratigraphic architecture of the cratonic shoreline deposits allows the interplay of accommodation change, sediment supply, and productivity to be more fully understood. For example, our ongoing work may help decipher the long-standing dilemma of grand cycles in the GACB (cf. Aitken, 1966; Palmer, 1971). Carbonate facies are susceptible to changes in productivity that can mask changes in accommodation space and, therefore, it is possible that shaly half cycles of grand cycles correspond to delivery of siliciclastics to the GACB from the IDB during sea level fall, instead of representing drowning during sea level rise. Given that bathymetry may remain relatively deep during these regressions because of suppressed carbonate productivity (poisoning) and low terrigenous input (distal paleo-geography), akin to the chronic conditions in the moat facies, such intervals in the GACB may be easily misinterpreted as representing sea level rises. An assessment of large-scale stratigraphic relationships, as we present in this chapter, and a reevaluation of the bathymetric interpretations of GACB facies (cf. Cowan and James, 1992) will be part of our ongoing effort to better understand the genesis of grand cycles.
Great American Carbonate Bank: The Geology and Economic Resources of the Cambrian—Ordovician Sauk Megasequence of Laurentia
The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.