Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 258, part 2
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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 258, part 2
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Next Page--Stratigraphic Controls
Since the earliest studies of the Dakota Formation in the type area of Iowa and Nebraska, the formation has been considered to be mostly, if not entirely, of fluvial origin. It is true that the Dakota Formation is dominated by fluvial and flood-basin depositional facies, commonly overprinted by pedogenic alteration. However, evidence for marine-influenced depositional facies in upper Dakota strata was presented by Witzke et al. (1983) and Witzke and Ludvigson (1994), who interpreted deltaic, nearshore-mud, and coastal-swamp environments associated with transgression of the Greenhorn seaway across the eastern coastal plain during the middle Cenomanian. However, evidence for older Albian and early Cenomanian marine influence in the Dakota type area was not recognized in those reports.
The discovery of marine palynomorphs and tidal rhythmites in lower Dakota strata (Albian) of eastern Nebraska (Witzke et al., 1996c; Brenner et al., 2000) significantly changed our understanding of Dakota sedimentation in the region. The resulting paradigm shift recognized the possibility that marine-influenced deposition, particularly that associated with brackish-water estuaries, was part of the regional depositional mosaic of Dakota sedimentation (Witzke, Ludvigson, Brenner, et al., 1996, p. 11). Further studies have supported this new perspective on Dakota deposition, recognizing marine influence at multiple stratigraphic positions within the Dakota succession at positions far inland from the Western Interior seaway's eastern shoreline (Witzke et al., 1999; Brenner et al., 2000, 2003; Joeckel et al., 2005; Gröcke et al., 2006). This paradigm shift has necessitated changes in interpretations of coastal geographies along the eastern margin of the seaway. Influenced by descriptions of Cretaceous deltaic systems along the western margin of the seaway, earlier Dakota depositional models portrayed constructional deltas advancing seaward from the eastern margin (e.g., Witzke and Ludvigson, 1994). The subsequent recognition of large estuarine systems in the Dakota depositional mosaic now seems to better constrain a broadly embayed coastline, not one dominated by constructional deltas (in other words, an "innie" vs "outie" eastern coastline; Ludvigson, Witzke, et al., 1998).
Several kinds of paleontologic information provide definitive evidence for marine influence during Dakota deposition. As discussed earlier, the presence of marine palynomorphs, including acritarchs and dinoflagellate cysts, provides compelling confirmation that not all of the Dakota Formation was deposited in nonmarine environments. Many of the recovered acritarchs and dinocysts comprise biofacies that most likely represent brackish-water associations, as these do not typically occur with invertebrate fossils or foraminfera that are considered to inhabit open-marine environments (normal marine salinity). Trace-fossil assemblages have been recognized within all three major stratigraphic sequences of the Dakota Formation, and these include burrow forms that generally are considered to be marine- or brackish-water indicators, including Skolithos and thalassinoid networks. Some particularly well developed trace-fossil assemblages have been recognized in basal Dakota strata of Jefferson County, Nebraska, and Guthrie County, Iowa (see Brenner et al., 2003, p. 445-446), but the characterization of ichnotaxa and their environmental interpretation awaits further study. Small horizontal burrows have been recognized in silt laminae and silt interbeds associated with tidally modulated units within all three sequences in the formation. Upper Dakota units with evidence of marine influence are noted to contain a variety of horizontal, subvertical, vertical, and 'u'-shaped burrows (Witzke and Ludvigson, 1987).
Although invertebrate fossils generally are not common in the Dakota Formation, some mollusk fossils also provide compelling evidence for marine influence. Some siltstone and sandstone beds in the upper Woodbury Member in the uppermost part of the formation in northwestern Iowa and northeastern Nebraska locally contain molds of high-spired gastropods and marine bivalves, including veneroids, arcoids, nuculoids, ostreids, and inoceramids (Witzke and Ludvigson, 1987, 1994; Tester, 1931). Hattin (1967) described a relatively diverse molluscan association from the upper Janssen Clay Member in the upper Dakota sandstone beds in central Kansas, including oysters (Crassostrea, Exogyra), mytilaceans (Brachidontes, Modiolus), and a variety of other bivalve genera (Breviarca, Corbula?, Parmicorbula, Anatimya, Cymbophora, Geltena, Isocardia, Laternula, Linearia, Lucina, Yoldia?). He interpreted these faunas to be shallow-water brackish to marine assemblages. Some of the brackish faunas are intermixed locally with freshwater forms (unionids). Brackish to marine mollusk fossils, including Crassostrea, also have been noted in lower Dakota strata of Nebraska and Kansas (Franks, 1975, 1979; Witzke et al., 1983). The occurrence of Brachidontes within leaf-rich carbonaceous mudstones in uppermost Albian strata of Jefferson County, Nebraska (Upchurch and Dilcher, 1990), is particularly noteworthy, as this is the only known occurrence of brackish-water mollusks in middle Dakota strata of the region (our revised palynostratigraphy places these strata within the Albian Muddy cycle, and not within the Cenomanian as originally reported). This occurrence is even more surprising considering that extensive palynological sampling of this unit failed to produce any associated acritarchs or dinocysts. Although brackish to marine mollusk assemblages are not commonly encountered in the Dakota Formation, when identified they provide compelling evidence for marine-influenced deposition (i.e., saltwater environments).
Some marine-influenced mudstone units in the upper Dakota Formation contain low-diversity assemblages of agglutinated foraminifera, although sampling has been sparse and preliminary. Such assemblages are interpreted to live in nearshore, estuarine, and coastal marsh environments. Tibert and Leckie (2004) interpreted the occurrence of agglutinated foraminifera in coal-bearing Turonian strata along the western margin of the Western Interior seaway as evidence for high-frequency estuarine sea-level cycles. van Hengstum et al. (2007) recently reported on the occurrence of agglutinated foraminifera from carbonaceous shale leaf beds in the Yankee Hill claypits in Lancaster County, Nebraska, from strata included in Palynostratigraphic Unit 2. While they interpreted the enclosing strata as lacustrine in origin, the occurrence of marine palynomorphs in carbonaceous shales from this locale (fig. 2, table 6), and the interpreted tidal modulation of carbonaceous shaly rhythmites at this locale (see next section) may suggest that the agglutinated foraminifera inhabited brackish estuarine environments.
Further study needs to be undertaken, but it seems likely that such assemblages will be recovered in other parts of the Dakota succession, as well. Some sandstone and siltstone lenses in the uppermost Dakota Formation of Iowa and Nebraska locally contain concentrations of marine-vertebrate fossils, primarily shark teeth and teleost scales, bones, and teeth (including ichthyodectids).
Numerous sedimentary structures in the Dakota Formation commonly are associated with marine to brackish paleontologic indicators, and many of these are suggestive of, although not necessarily definitive for, marine-influenced sedimentation. These sedimentary structures provide supporting evidence for sedimentary processes associated with estuarine, deltaic, and coastal environments (Witzke and Ludvigson, 1994). A variety of silt-sand sedimentary bedforms in upper Dakota strata are most consistent with marine-influenced deposition in shallow nearshore to outer estuarine settings, especially hummocky and flaser bedding, but also including silt-sand laminae, symmetrical starved ripples, and low-angle cross stratification (Witzke and Ludvigson, 1987, 1994). Such structures may result from storm currents and tidal flow. Compelling evidence of marine influence is seen where Dakota sandstone and siltstone beds interfinger with marine shale facies of the Kiowa and Graneros shales. Some nearshore-marine Dakota sandstones locally are glauconitic (or chloritic). Some marine-influenced Dakota mudstones locally contain carbonate concretions (commonly septarian).
Inclined heterolithic strata (IHS) are identified at multiple stratigraphic positions within the Dakota succession, including 1) lower Dakota units in Cass and Jefferson counties, Nebraska (fig. 4), and Guthrie County, Iowa; 2) middle Dakota units in Lancaster and Jefferson counties, Nebraska; and 3) and upper Dakota units in Woodbury County, Iowa, and Thurston and Jefferson counties, Nebraska (fig. 6). These IHS units show laterally accreted sets of dipping sandstone, siltstone, and mudstone (commonly carbonaceous), and some contain leaf compressions and carbonized logs. Many IHS units are pyritic, some contain marine burrows, some have produced marine palynomorphs, and some contain tidal rhythmites. The bulk of the Dakota IHS units are considered to have formed by lateral accretion within inner estuarine and bayhead settings, and many appear to have been tidally influenced. Lockley et al. (2006) illustrated IHS units from the Middle Mesa Rica Sandstone Member of the Dakota Group along the Colorado-Kansas border, from strata included in the "Muddy cycle." Mud-draped crossbedded sandstone bodies are seen in some marine-influenced facies that likely record alternations between high- and low-energy flow regimes within fluvial-estuarine settings (Brenner et al., 2000, p. 874).
Tidal rhythmites are strata that display distinctive bundles of thickening-and-thinning laminae, typically organized into groups of 14 and 28 laminae that are interpreted to record neap-spring tidal cycles (see Brenner et al., 2000, p. 873). Such rhythmic laminae are identified in the Dakota Formation within several lithologies: 1) foreset laminae within crossbedded sandstones (e.g., Witzke and Ludvigson, 1994, p. 49), 2) laminated siltstones, and 3) silt/sand-laminated mudstones (Brenner et al., 2000, p. 873). Tidal rhythmites commonly are associated with IHS units and with other laterally accreted cross strata in the Dakota Formation. Such occurrences are interpreted to relate to deposition in tidal estuaries, most of which represent fills within incised channels. Tidal rhythmites are now identified within all three sequences of the Dakota succession at localities in Iowa and Nebraska. The best examples are from basal Dakota strata of the Kiowa-Skull Creek marine cycle at the Ash Grove pit in Cass County, Nebraska, where analysis of the tidal bundling indicated mesotidal ranges (Zawistowski et al., 1996; see also Brenner et al., 2000, 2003; Joeckel et al., 2005). Tidally modulated laminites also are known from the same general stratigraphic position in Jefferson County, Nebraska, and Guthrie County, Iowa (fig. 4). The recognition of tidal rhythmites in middle Dakota strata (Muddy cycle) of Jefferson and Lancaster counties, Nebraska (Korus, 2000), is particularly noteworthy because the latest Albian has previously been considered to be a time of major seaway withdrawal across much of the Western Interior.
Much of the Dakota Formation has sparse to absent sedimentary iron sulfides (pyrite), but other iron minerals, especially siderite and iron oxides, are common in some beds. However, pyrite is locally abundant at some stratigraphic positions, variously seen as discrete pyrite nodules, disseminated framboids and crystals, and pyrite cements. Pyrite is most commonly recognized in the Dakota Formation within IHS units and associated with carbonaceous shales and lignites. Although pyrite and other iron sulfide deposition is not exclusive to marine or marine-influenced environments, a ready and available supply of dissolved sulfate is needed to produce large quantities of sulfide through anaerobic-bacterial sulfate reduction. Because seawater is a readily available source of abundant sulfate in marine-influenced depositional settings, an abundance of sedimentary pyrite in certain strata is strongly supportive of marine influence during deposition (Ward, 2002). White et al. (2000) considered the abundance of pyrite in Albian carbonaceous mudstones and lignites of Ontario to be most easily explained by derivation from brackish estuarine sulfate sources, a reasonable conclusion based on the recognition of other paleontologic and geochemical indicators of marine influence in associated strata.
Geochemical information can provide additional evidence for marine influence during deposition and early diagenesis of Dakota sediments. Analysis of organic matter in many Dakota mudstones is potentially significant, as the organic geochemistry can reveal compelling evidence for marine or marine-influenced sedimentation (Young, 2002; White et al., 2000). Most Type II organic matter analyzed by Rock Eval pyrolysis (Espitalie et al., 1977) is derived from marine phytoplankton, which contrasts with organic matter derived from terrestrial plants (Type III) and freshwater algae (Type I) (Espitalie et al., 1977). Calculations from Rock Eval pyrolysis yield values for the Hydrogen Index (HI) and Oxygen Index (OI) of the organic matter in a sample, and the relative importance of the three types of organic matter is reflected by these values. Young's (2002) evaluation of organic matter from Dakota Formation cores in Iowa and Kansas showed intervals with mixed terrestrial and marine values, consistent with paleontologic and sedimentologic indicators previously discussed. White et al. (2000) used evidence from organic geochemistry, pyrite distribution, and palynology to demonstrate estuarine influence in coeval Albian strata of Ontario.
Geochemical study of carbonate minerals in the Dakota Formation provide additional lines of evidence for the mixing of meteoric freshwater and marine to brackish ground water in coastal and estuarine phreatic ground-water systems. Early diagenetic synsedimentary calcite cements in siltstone and sandstone beds are noted at certain stratigraphic positions within the Dakota Formation, including a dinosaur track site in Jefferson County, Nebraska (Joeckel et al., 2004; Phillips et al., 2003; Phillips, 2004; Phillips et al., 2007). Detailed stable isotope geochemistry of the carbonate cement paragenetic sequence, as shown by data arrayed along fluid mixing curves in δ18O and δ13C isotope space, demonstrate that some of the Dakota calcite cements were precipitated in mixed ground-water systems with brackish influence (Phillips et al., 2007). Such investigations demonstrate early diagenetic processes associated with phreatic "subterranean estuary" systems (sensu Moore, 1999), a term that refers to solute transport and mixing in ground-water-flow systems that discharge to submarine environments. Radioisotope measurements along modern coasts have indicated that ground-water flux must be about 40% of the flux from riverine discharge (Moore, 1996), a far greater proportion than previously realized. Processes related to the existence of subterranean estuaries are recognized in authigenic cements in strata of the Dakota Formation, and certain calcite-cemented horizons likely correspond to fluid-mixing zones beneath marine-flooding surfaces (Phillips et al., 2007; Ufnar et al., 2004a).
The Dakota Formation is replete with well-developed paleosols, and many paleosols show an abundance of small siderite spherulite (iron carbonate) concretions (termed spherosiderite) that precipitated in the ancient wetland soils. Geochemical data recovered from these spherosiderites also have shown marine or brackish influence in the coastal phreatic systems. Siderite that precipitated in freshwater settings is commonly very pure (>95% FeCO3; Mozley, 1989), whereas marine siderite is impure (incorporating other cations, especially Ca)(White et al., 2005). This relationship led to the recognition of brackish influence on siderite precipitation in middle Dakota strata (Albian Muddy cycle) of Iowa (as also indicated by studies of Ludvigson et al., 1995; Ludvigson et al., 1996, discussed below), with stratigraphic variations likely recording cyclic shifts in the updip migration of brackish ground water along the eastern coastal plain, probably reflecting parasequence-scale changes in sea level (Ludvigson et al., 1996).
Chemostratigraphic excursions in δ18O recorded by stratigraphic successions of spherosiderite-bearing paleosols may also reflect the mixing of marine and meteoric ground waters (Hart et al., 1992). Ludvigson et al. (1995) and Ludvigson et al. (1996) interpreted mixing-zone spherosiderites from lower Dakota strata of Nebraska and middle Dakota strata of Iowa, which displayed positive covariant trends in δ18O and δ13C. These trends likely resulted from the mixing of meteoric phreatic and modified marine phreatic fluids. Ufnar et al. (2001) interpreted spherosiderites from Albian paleosols in Canada to show evidence of meteoric-marine fluid mixing in an interval with enriched δ18O and increased minor element concentrations (Mg and Ca). By contrast, Phillips (2004, p. 142) identified other early diagenetic siderite and calcite cements from the same stratigraphic interval with yet even more depleted δ18O values and increased Mg and Ca concentrations that he suggested may indicate meteoric-marine mixing. Although further work on siderite geochemistry is needed, it seems clear that evaluation of stable isotopic and minor element chemistry may afford additional means for evaluating marine influence in the Dakota paleoground-water systems.
As noted above, abundant syndepositional pyrite is considered to be a likely sedimentary indicator for marine influence, and pyrite is common in some Dakota facies. However, it should be noted that pyrite and siderite are generally not considered to be coeval mineral phases, because siderite growth is restricted to environments in which sulfide activity is absent. Sulfate availability is likely the limiting factor in the formation of pyrite over siderite (Carpenter et al., 1988), and marine and brackish waters typically contain abundant sulfate unless modified by sulfate-reducing bacteria. Any Dakota siderite precipitation that occurred within brackish ground water must have been in phreatic systems with mixing ratios of less than 25% seawater (see Ufnar et al., 2004a, p. 139), or systems in which sulfate reduction already had either progressed largely if not entirely to completion (White et al., 2005). Microscopic pyrite inclusions at the centers of some Dakota spherosiderites provide evidence of this process (White et al., 2005, p. 15).
Marine-influenced depositional facies are recognized at various stratigraphic positions within all three Dakota sequences (D0, D1, D2 of Brenner et al., 2000) in areas of Iowa, Nebraska, and Kansas. The D0 sequence is coeval with the Kiowa-Skull Creek marine cycle of the Western Interior, and marine-influenced facies are best developed in the lower half of this sequence (fig. 4), apparently recording the eastward expansion of estuarine environments coincident with maximum transgressive expansion of the Kiowa-Skull Creek seaway. Tidal rhythmites, IHS units, marine burrows, pyritic mudstones, and marine palynomorphs are observed in the sequence eastward into eastern Nebraska, as displayed in outcrop in Jefferson and Cass counties. Two or three intervals within the Nebraska sequence locally are marked by marine indicators, separated by paleosols or nonmarine sandstone channels.
The discovery of IHS units, tidal rhythmites, burrowed laminites, pyritic mudstones, and pyrite nodules in lower Dakota strata of Guthrie County, Iowa, is particularly noteworthy (Witzke and Ludvigson, 1998; Phillips and White, 1998). The sedimentary evidence indicates that during the Kiowa-Skull Creek interval, estuaries and tidal influence extended as far as central Iowa, an area that lies 450 km (280 mi) east of the marine Kiowa shale edge (this shale edge marks the approximate margin of the Western Interior sea at that time).
As noted by Joeckel et al. (2007), a southwest-northeast transect of the Dakota outcrop belt in the midcontinent shows that the transition from Kiowa marine shales to estuarine facies occurs abruptly at Salina, Kansas, and extends to Guthrie County, Iowa, near Des Moines (fig. 1) along a line roughly approximating the Cretaceous paleoslope. The large lateral extent of this paleoestuary was governed by two factors: 1) flooding of low-gradient paleovalleys by sea-level rise during the Kiowa-Skull Creek marine transgression (Brenner et al., 2003); and 2) amplification of tidal ranges in tidally influenced rivers discharging to estuaries. For a recent example, Kvale and Archer (2007) noted that tidal influences extend over 500 km (310 mi) inland along the Amazon River, the river system with the largest fluvial discharge in the modern world.
Of note, the apparent absence of marine palynomorphs in Guthrie County, Iowa, may suggest deposition in the inner estuary where freshwater influx was highest and salinities lowest. The geographic distribution of marine-influenced and estuarine facies in the lower Dakota succession provides compelling evidence for very large estuaries indeed, among the largest known in the modern or ancient world. The distribution of lower Dakota sedimentary facies supports a paleogeographic interpretation of a Late Albian coastal plain along the eastern margin of the Western Interior sea with deeply embayed estuaries. Such large estuaries apparently developed at times of maximum-marine flooding (maximum sea level) of the Kiowa-Skull Creek sea. During times of falling sea level and shore-line progradation, the estuaries likely were filled by nonmarine fluvial sediments.
As discussed previously, Middle Dakota strata (sequence D1 of Brenner et al., 2000) are correlative with the latest Albian Muddy Sandstone and Mowry Shale of Colorado and Wyoming and accordingly are included within the newly recognized "Muddy-Mowry marine cycle" of the Western Interior. Indicators of marine-influenced deposition have been noted in this sequence in western Kansas (Hamilton, 1994; Scott et al., 1998), western Nebraska (Graham, 2000), eastern Nebraska (Witzke et al., 1999; Korus, 2000; Phillips et al., 2007), and Iowa (White et al., 2005). The recent report of agglutinated foraminifera from this interval in eastern Nebraska (van Hengstum et al., 2007) further supports this interpretation. Marine palynomorphs have been identified in several stratigraphic positions in eastern Nebraska (fig. 5), and tidal laminites have also been identified in the lower and middle parts of the sequence in south-eastern Nebraska (fig. 5; also Korus, 2000). Oboh-Ikuenobe et al. (2007) have similarly noted the widespread distribution of marine palynomorphs in fluvial-estuarine strata in the Western Interior basin during the interval of the Late Albian "Muddy marine cycle." Holbrook (2001) proposed that the Cucharas Canyon and Huerfano Canyon alloformations in southeastern Colorado are similarly re-lated to the "Muddy marine cycle," while Holbrook et al. (2006) further subdivided the interval into even smaller-scale cycles. These units are all included in the newly recognized "Muddy-Mowry marine cycle."
Previously discussed calcite cements from the lower part of this interval in Jefferson County, Nebraska, show geochemical evidence of a marine-influenced phreatic ground-water system; the sandstone bed contains dinosaur footprints and is interpreted to represent a marine-flooding surface (Phillips et al., 2007). Pyritic beds, siderite geochemistry, and burrows suggest marine influence as far east as western Iowa during deposition of upper Muddy-Mowry cycle strata.
Indicators of marine-influenced sedimentation are recognized in Dakota sequence D1 in at least three stratigraphic positions. The lower interval with marine palynomorphs, tidal rhythmites, and burrows is the most widespread and best-developed interval with evidence for marine influence (fig. 5). Middle strata of this sequence contain marine palynomorphs and tidal rhythmites in Lancaster and Burt counties, Nebraska (fig. 5), and upper strata contain indicators of marine influence at localities in Nebraska and Iowa, including marine palynomorphs from as far east as Burt County. Each of these three intervals with evidence for marine-influenced and estuarine deposition is separated from underlying and overlying units by nonmarine strata with paleosols and sandstone channels, suggesting several cyclic expansions and contractions of the interior seaway during sequence D1.
The latest Albian Muddy-Mowry interval generally has been considered to mark a general contraction of the Western Interior seaway northward to Montana and Canada, but the extent of the seaway during that time has remained problematic. Similarly, Mowry marine shales generally have been interpreted to have been deposited in a relatively constricted seaway, cut off to the south across northern Colorado and extending no further east than western Nebraska (Reeside and Cobban, 1960). However, the identification of marine-influenced deposition as far east as eastern Nebraska and western Iowa during portions of the Muddy-Mowry cycle suggests that estuaries extended eastward from an interior seaway that may have been of significantly greater geographic extent than previously recognized (Witzke et al., 1999; Witzke, 2007). Even if immense estuaries extended eastward as far as 400-500 km (248-310 mi) into the Dakota coastal plain, the adjoining seaway must have expanded to at least as far as central Nebraska and western Kansas. Of note, several authors have postulated ephemeral through-going seaway connections in the Western Interior during the latest Albian, joining Boreal and Tethyan realms (Holbrook et al., 2002; Scott et al., 2004; Oboh-Ikuenobe et al., 2004; Oboh-Ikuenobe et al., 2007). It is here suggested that these ephemeral seaway connections expanded even farther eastward into Kansas and Nebraska, but pedogenesis and erosional channeling during sea-level lowstands may have obscured much of the sedimentary evidence for marine deposition in those areas. The isolated preservation of restricted marine and estuarine strata of the Muddy-Mowry interval in local interfluve positions beneath multiple incised sequence boundaries in southeastern Colorado (Holbrook et al., 2006) is illustrative of these phenomena.
Multiple phased transgression of the Greenhorn marine cycle during the early and middle Cenomanian resulted in the progressive eastward incursion of estuarine and marine-influenced deposition that encroached across the Dakota coastal plain. There are multiple stratigraphic levels in upper Dakota strata of Iowa, Nebraska, and Kansas with evidence for marine-influenced deposition (fig. 6), many of them separated by episodes of pedogenesis and channeling, especially in the lower part. Dakota nonmarine, estuarine, and coastal facies were displaced by open-marine facies of the Graneros and Greenhorn formations during the middle Cenomanian as the seaway expanded eastward.
The stratigraphic distribution of estuarine and marine-influenced sedimentation in the Dakota sequences is schematically portrayed in fig. 7. Each sequence corresponds to a third-order marine cycle of the Western Interior basin, but smaller-scale cycles are apparent in the Dakota succession that are interpreted to be parasequences. These smaller-scale cycles are marked at their base by marine-influenced facies (primarily estuarine), corresponding to marine flooding events. The upper portion of each parasequence is marked by nonmarine deposition, erosional channeling (fluvial downcutting), and soil formation. These phases of deposition correspond with episodes of falling base levels in the fluvial systems, likely coincident with falling sea level in the adjoining Western Interior sea.
Figure 7--Schematic representation of the stratigraphic distribution of marine-influenced and estuarine facies in the Dakota sequences of Iowa, Nebraska, and Kansas. Distributions of illustrated sedimentary facies and fluvial channeling are generalized from the stratigraphic sections shown on figs. 4-6. Shifts between marine influence and nonmarine fluvial channeling and paleosol development likely reflect parasequence-scale changes in sea level/base level.
A basic stratigraphic and depositional model of tide-dominated estuarine deposition within the Dakota Formation was presented by Brenner et al. (2000), and this model need not be reiterated in detail here. The recent recognition of an estuarine facies tract within the Dakota sequences has significantly modified our earlier understanding of Dakota sedimentation. The Dakota estuarine facies tract can be subdivided into several zones (Brenner et al., 2000; Dalrymple et al., 1992; White et al., 2000): 1) The outer zone is located adjacent to the estuary mouth and is dominated by marine processes (waves, tides). It can include barrier and shoreface sands, flood-tidal deltas, and tidal sand bodies; 2) The central zone, or estuarine bay, is a relatively low-energy area of mixed fluvial and marine processes; 3) The inner zone is dominated by fluvial processes, and this area has the lowest salinity waters within the estuary. A freshwater tidal zone is recognized in the upstream reaches of the inner zone. Bayhead deltas and tidal bar-channel complexes occur within this zone. The inner to outer estuarine-bay margins can be flanked by tidal wetlands and laterally accreting tidal flats and point bars.
Dakota estuarine sedimentation can be accommodated within a sequence stratigraphic framework, here partly modified from that presented by Brenner et al. (2000) and Brenner et al. (2003). The Transgressive Systems Tract (TST) is marked by marine transgression and eastward estuarine flooding of the older Dakota river valleys. With the accompanying rise in sea level during transgression, base levels rose in the fluvial systems resulting in progressive upstream aggradation of fluvial systems (fig. 8). Because Dakota sediment accumulation rates were relatively low, the maximum eastward expansion of estuaries is interpreted to generally correspond with absolute sea-level highstand in the adjoining Western Interior sea.
Figure 8--Schematic representation of fluvial-estuarine facies of the Dakota Formation during marine transgression and offlap. Rising base levels in the Dakota fluvial systems accompanied sea-level rise, leading to eastward flooding (A) of former fluvial valleys by estuaries and upstream aggradation of fluvial channels. Falling base levels accompanied sea-level fall, resulting in fluvial-deltaic infilling of the former estuaries and upstream incision of fluvial channels (B).
Following maximum transgression, sea levels began to fall resulting in the westward retreat of the seaway and shoreface. This resulted in falling base levels within the Dakota fluvial systems and a general downstream migration of fluvial and flood-basin aggradation. With the fall in base levels, progressive fluvial downcutting would proceed from distal to proximal areas of the fluvial systems, and stacked soils could develop in the fluvial flood basins. Estuaries would contract seaward as sea levels fell, and the former estuaries would be infilled by bayhead deltaic and fluvial deposits (fig. 8). The episode of seaward progradation and downstream fluvial aggradation has generally been included within the so-called "Highstand Systems Tract" (HST) in many sequence stratigraphic models. However, in regions of low net sediment accumulation, progradation and downstream fluvial aggradation may not necessarily correspond to times of actual sea-level highstand, and HST may not be an appropriate label in these cases (Witzke et al., 1996d). Some stratigraphers have included other terminology for this systems tract, including the "Regressive Systems Tract" (RST) or "Falling Sea Level Systems Tract" (FSLST) (Witzke et al., 1996d).
As the seaway continued to retreat and sea level reached its absolute lowstand in the the adjoining seaway, the Dakota fluvial systems would be marked by their lowest absolute base levels. This would correspond to a time of maximum incision of the fluvial systems and the deepest weathering and soil development in the adjoining interfluves. In areas of low net sediment accumulation, the "Lowstand Systems Tract" (LST) in nonmarine sequences would be predicted to show the deepest levels of erosional channeling and complex stacks of deeply weathered paleosols. Because of the increased potential for extensive pedogenic alteration and erosional incision during lowstand episodes, it is possible that evidence for earlier transgressive and estuarine deposits in the sequence may be significantly modified or even removed.
Parasequences are recognized in the Dakota sequences, each marked by smaller-scale changes in sea level or base level within the estuarine and fluvial systems. Many Dakota parasequences show an eastward expansion of estuarine facies at their base, succeeded by the deposition of nonmarine facies and ultimately by paleosol development or fluvial incision (fig. 7). Joeckel (1987) interpreted sea-level change, with resulting base-level change in nonmarine areas, as the cause of soil horizonation in Dakota paleosols of Nebraska. White et al. (2005) suggested that bundles of stacked paleosols in the Dakota sequences may represent parasequence sets.
Some final considerations for Dakota estuarine sedimentation also need to be discussed. Regional paleogradients of the Dakota fluvial systems are difficult to constrain, but Witzke and Ludvigson (1996) argued for low gradients across the eastern coastal lowlands, at maximum 0.3-0.6 m/km and probably less. Because the Dakota estuaries extended eastward up the former river valleys tens to hundreds kilometers (up to 300-400 km [186-248 mi]) from the adjoining sea, the postulated stream gradients would require sea-level changes on the order of 10-100 m (33-328 ft) to create such large estuary systems. Sea-level changes in the mid-Cretaceous on the order of 100 m (328 ft) may be excessive, which, if true, would indicate even lower fluvial gradients within the Dakota river systems. In general, nonmarine sediment-accumulation rates are very low in the eastern-margin area of the Western Interior, which for lower to middle Cenomanian strata of the upper Dakota Formation averages only about 15 mm (0.6 inch)/1000 yr. Even though runoff from the eastern landmass-dominated freshwater influx to the Western Interior seaway (Slingerland et al., 1996; Witzke and Ludvigson, 1996), sediment supply was correspondingly low. The persistence and widespread extent of estuarine facies in the the Dakota depositional systems were likely enabled by such low rates of sediment influx. The eastward extent and duration of estuarine facies would be constrained by the relative interplay of sea-level change (base-level change) and rates of sediment accumulation, that is creation versus infilling of accommodation space. In a simple sense, the less the sediment influx, the longer it would take to infill the large Dakota estuaries.
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