Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 258, part 2
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Sequence Stratigraphic Controls on Carbonate Cementation in the Dakota Formation

Regional studies of the occurrence and stable isotopic geochemistry of authigenic carbonates in continental to marginal-marine facies of the mid-Cretaceous Western Interior basin have already yielded new insights on perturbations of the global hydrologic cycle during this warm period in earth history (Ludvigson, González, Metzger, et al., 1998; White et al., 2001; Ufnar et al., 2002, 2004b). In short, changes between the Cretaceous and modern δ¹⁸O gradients of pedogenic siderites along equator-to-pole transects have been used to argue for an intensified hydrologic cycle in the Cretaceous greenhouse world. This whole line of basic research was pioneered through studies of spherosiderites in the Dakota Formation (Ludvigson et al., 1995; Ludvigson et al., 1996; Ludvigson, González, Metzger, et al., 1998).

Authigenic carbonates in the Dakota Formation commonly occur in other sedimentary contexts. One especially noteworthy mode of occurrence is as thin, laterally extensive sheets of hard-cemented rock that are enclosed in otherwise soft, friable sedimentary strata. Similar phenomena have been noted by Taylor et al. (1995, 2000) elsewhere in Cretaceous units of the Western Interior basin. Taylor et al. (2000) attributed the sheet cementation to early carbonate diagenesis controlled by sequence stratigraphic processes, a model that is applicable to some examples from the Dakota Formation. In the cases cited by Taylor et al. (1995, 2000), cementation occurred in shoreface sandstones immediately overlain by marine-flooding surfaces. Similar examples of authigenic cementation of fluvial-estuarine deposits immediately below marine-flooding surfaces have also been reported from Cenozoic deposits (Laenen and De Craen, 2004).

Here, we report on the field relationships, petrography, and stable isotopic paleohydrology of thin sheets of laterally extensive carbonate cements in fluvial-estuarine clastic units of the Albian Kiowa-Skull Creek interval of the Dakota Formation.

Field Relationships

Siderite-cemented Mudstones at the Ash Grove Cement Quarry, Nebraska

Exposures of Albian fluvial-estuarine strata from the basal Dakota Formation at the Ash Grove cement quarry at Louisville, Nebraska, have been described by Witzke, Ravn, et al. (1996), Witzke, Ludvigson, Brenner, et al. (1996), and Joeckel et al. (2005). Tidal rhythmites in estuarine mudrocks containing marine palynomorphs from this locality (fig. 9) were also described by Brenner et al. (2000).

Figure 9--Location map and graphic logs of stratigraphic sections showing the positions of Albian fluvial-estuarine units with authigenic siderites that were sampled for petrographic, stable isotopic, and diagenetic investigations of the paleohydrology of the Kiowa-Skull Creek interval of the Dakota Formation.

Carbonate cements are described from the lower of two marine palynomorph-bearing estuarine mudrock units (unit 2 or Kd2; figs. 9, 10). These include a massive, hard-cemented horizon (AGP-LU2) with sheet geometry just above the unit 1-unit 2 contact (the Kd1-Kd2 contact in fig. 11A), and a hard-cemented mudstone bed (AGP-MU2) with sheet geometry from inclined heterolithic strata from the middle part of unit 2 (fig. 9; Kd2 in figs. 10 and 11A). Discrete meter-scale concretionary bodies also occur in unit 2 (Kd2), but have not yet been studied. The hard-cemented zones at the base and middle part of unit 2 are of special interest. Exposures in the north and south quarry walls show the lateral continuity of the sheet-cemented zone at the base of unit 2, and the drilling of three research cores of units 2-5 by hollow-stem auger in 1995 all were terminated by auger refusal at the top of this hard-cemented zone. This sheet-cemented zone ranges between 30 to 60 cm (12-23 inches) in thickness. It is probably analogous to the thin, laterally extensive carbonate-cemented horizons beneath marine-flooding surfaces described by Taylor et al. (1995, 2000).

Figure 10--Panoramic photomosaic of a high wall in the Kiowa-Skull Creek interval of the Dakota Formation in the Ash Grove cement quarry in Louisville, Nebraska, in summer 2002, looking north showing the cemented zone (arrows) at Kd1-Kd2 contact. Photomosaic is shown in the upper panel, and an interpretive line tracing is shown in the lower panel. Kd1 is a conglomeratic fluvial sandstone unit, and Kd2 is an estuarine heterolithic unit that has yielded abundant marine palynomorphs and is characterized by IHS lateral accretion beds that extend throughout the unit (Brenner et al., 2003; Joeckel et al., 2005). The Kd1-Kd2 contact is interpreted to be a marine-flooding surface. Joeckel et al. (2005, p. 464) illustrated Skolithos-type burrows at the top of Kd1. Joeckel et al. (2005) further showed that both of these units fill a paleovalley that is delimited by the extent of the workings at the Ash Grove cement quarry. Note that Kd1 is overlapped by Kd2 in the left side of the photomosaic, and that both units rest on Pennsylvanian limestone strata. Kd3 is a crossbedded gravelly sandstone that intertongues with estuarine mudrocks elsewhere in the quarry (Brenner et al., 2003, p. 445; Joeckel et al., 2005, p. 462).

Figure 11--A) Close-up view of cemented zone at the Kd1-Kd2 contact (arrows) at the Ash Grove cement quarry looking at the south wall of the quarry workings in spring 2001. The Jacob staff is 1.5 m (5 ft) in length. Sample AGP-LU2 was taken from this freshly exposed outcrop. Geologists Phillips (back to camera) and Ludvigson (facing camera) are shown to the right. Sample AGP-MU2 was taken from siderite-cemented mudstones in an IHS unit shown directly above LudvigsonŐs head. B) Cross-polarized light micrograph of sample AGP-LU2 showing microcrystalline (10-20 microns) siderite cements pervading mudstone matrix. C) Same field of view, plane-polarized light micrograph. Note angular detrital quartz-silt grains (white).

Siderite-cemented Mudstone at the Long Creek Section

Exposures of fluvial-estuarine mudstones from the base of the Nishnabotna Member of the Dakota Formation at the Garst Farm Resorts (recently redesignated as the Whiterock Conservancy) were described by Witzke and Ludvigson (1996, 1998) and Brenner et al. (2003). An especially instructive exposure of estuarine mudrocks along Long Creek in the Garst properties was first described by Phillips and White (1998). These mudrocks contain a sparse marine-influenced ichnofauna and tidally modulated heterolithic siltstone-mudstone strata that show marine influences on deposition (Brenner et al., 2003). They represent the most landward marine influences of the Late Albian Kiowa-Skull Creek marine cycle that are currently known.

A hard-cemented ledge (sample LC-1, fig. 9) at the contact between a lateral accretion set of inclined heterolithic strata (unit 1 in fig. 9; Kdla in fig. 12A) was examined for this study. This ledge extends throughout the width of the section. A scanned image of a thin section cut from this sideritic ironstone (fig. 12B) shows that carbonate cements pervade a burrowed laminite. Petrographic examination of this bed (fig. 12C, D) shows that it is a siltstone-claystone laminite cut by Cylindrichnus burrows, all of which are perva-sively cemented by early diagenetic blocky equant siderite.

Figure 12--A) Field photograph of the siderite sheet-cemented zone capping the IHS lateral accretion unit Kdla (red arrows) at the LC-1 exposure along Long Creek at the Garst Farms Resort. Sample LC-1 was taken from this hard-cemented bed. B) Scanned image of a thin section of siderite-cemented ironstone from sample LC-1, showing cemented Cylindrichnus (?) burrow (cb) and clay-drape laminae distorted by soft-sediment deformation (white arrows). Note that in the margins of the ironstone sample, oxidation from modern weathering has replaced the pervasive siderite cements by opaque oxides. C) Plane polarized-light micrograph showing a siltstone lamina enclosed by mud-drape laminae. Note the high intergranular volume (IGV) greater than 50% in this unit. D) Same field of view, cross-polarized light micrograph.

Siderite-cemented Nishnabotna Sandstone in the Hawarden (D-7) Core

This important stratigraphic reference section was described by Whitley and Brenner (1981), Witzke and Ludvigson (1994), and Brenner et al. (2000). The core (fig. 9) fully penetrates the Dakota Formation, and thin bands of siderite-cemented sandstones were recovered from the lower part of the Albian Nishnabotna Member that were the subject of an earlier brief report (Phillips et al., 1998).

Petrographic Relationships

Witzke and Ludvigson (1994) and Ludvigson (1999) showed that sandstones in the Dakota Formation are mineralogically mature quartzarenites. Detrital carbonate-rock fragments are not present in the unit (Ludvigson, 1999). For this reason, authigenic carbonates in the Dakota Formation are easily isolated for isotopic studies through micromilling of polished thin-section billets with carbide dental burrs.

Siderite cements examined for this study (fig. 9) are all interpreted to be early diagenetic, synsedimentary features, based in part on field evidence for the sedimentary erosion and entrainment of carbonate-cemented intraclasts, but mainly on petrographic evidence from sandstone-siltstone units that the cements filled an open framework of primary sediment porosity prior to any significant reduction of intergranular volume through compaction (Paxton et al., 2002). These units probably had original intergranular volumes (IGVs) of about 40% (Houseknecht, 1987), and the IGVs (also known as minus-cement porosities) in them, as determined by point counting, range between 36% to 68% (table 8). Taylor et al. (1995) and Taylor et al. (2000) reported high IGVs around 40% from their sequence-bounding sheet-cemented zones and argued for synsedimentary carbonate cementation. The occlusion of large volumes of original intergranular pore space by the siderite cements is consistent with near-surface, early synsedimentary diagenesis. For example, the 56.5% and 68% IGVs measured from ACP-LU2 and LC-1 (table 8), respectively, are compared with published data sets from Houseknecht (1987) in fig. 13. The main point shown by this comparison is that the IGVs from these beds are well outside the norms that are reported from ancient coarse-grained clastic rocks. The abnormally large IGV values in these rocks (fig. 13) probably resulted from displacive growth of siderite crystals under low confining pressures, coincident with near-surface cementation.

Figure 13--Histogram of intergranular volumes, comparing published datasets from ancient sandstones (Houseknecht, 1987) and those measured from siderite-cemented zones from parasequence-bounding surfaces in the Kiowa-Skull Creek interval of the Dakota Formation.

Stable Isotopic Investigations

Methods

Rock samples examined for this study were impregnated with blue-dyed epoxide resins and cut into micropolished thin sections and thin slabs for microsampling. They were assessed by polarized light and cathodoluminescence petrography, and some thin sections were stained with alizarin Red-S and potassium ferricyanide to confirm identifications of carbonate minerals. While multiple siderite cements were distinguished by crystal morphology in some successive pore-filling sequences, no other carbonate cements were identified in these samples. Selected siderite cements were microdrilled by dental carbide burrs of 0.5-mm diameter on a microscope-mounted drill assembly, and resulting powdered samples of several micrograms were roasted for one hour in vacuum to remove volatile contaminants. Samples were reacted at 73°C with anhydrous phosphoric acid in a Finnigan Kiel III automated carbonate reaction device coupled to the inlet of a Finnigan MAT 252 stable isotope mass spectrometer at the Paul H. Nelson Stable Isotope Laboratory at the University of Iowa. Data are reported in standard delta (δ) notation in parts per mil relative to the VPDB isotope standard. Daily analyses of interlaboratory standards show a precision of better than 0.1‰ for both carbon and oxygen isotopes. The δ¹⁸O values for siderite were corrected for the temperature-dependent fractionation between acid-liberated CO2 and siderite using the experimental data of Carothers et al. (1988). Estimates of paleoground-water δ¹⁸O values relative to SMOW were calculated using the experimental siderite-water fractionation relation of Carothers et al. (1988).

Results

Carbon and oxygen isotope plots of data from siderite cements in the Dakota Formation of Iowa and Nebraska are shown in fig. 14. While more detailed assessments of these data are pending in future publications, some general relationships are discussed here. Syndepositional cements in the Dakota Formation throughout the region have δ¹⁸O values that tightly cluster around -5‰, while the δ¹³C values are much more variable, ranging between -17 to -1‰. The aggregate diagenetic trend for these data can be simply referred to as a meteoric siderite line. The general meaning of such a diagenetic trend is that the cements in question precipitated from a shallow, stable ground-water system in which the fluid δ¹⁸O compositions and temperatures changed very little, while the δ¹³C variations indicate that dissolved inorganic carbon in early diagenetic systems was contributed from multiple sources. Meteoric siderite lines, meteoric spherosiderite lines (Ludvigson, González, Metzger, et al., 1998), and meteoric calcite lines (Lohmann, 1988) are all useful paleoclimatic indicators, as they are interpreted to result from the isotopic equilibrium crystallization of authigenic carbonates from shallow meteoric ground waters. Accordingly, they integrate the local mean annual δ¹⁸O of paleoprecipitation, at the local mean annual temperature (Hays and Grossman, 1991).

A covariant trend toward δ¹³C and δ¹⁸O enrichment at the 614.4-ft to 616.1-ft level in the Hawarden (D7) core (fig. 14) is interpreted to indicate possible fluid mixing between meteoric- and marine-source waters. Pyrite inclusions and euhedral pyrite pore fillings are consistently noted as minor cement phases in cemented Nishnabotna sandstones (see "other cements" in table 8), suggesting that anaerobic reduction of seawater sulfate from brackish pore fluids overlapped with the deposition of these cements. The precipitation of siderite cements as the dominant reduced iron mineral phase, and the tangential approach of the covariant isotopic trend to the meteoric siderite line trend (note that δ¹⁸O values from the D7 core samples range only up to about -4‰) indicate that the diagenetic fluids were dominantly meteoric in origin and probably consisted of much less than 25% seawater (see Ufnar et al., 2004a, p. 139).

Discussion

The meteoric siderite line δ¹⁸O value of -5‰ from fluvial-estuarine deposits of the Nishnabotna Member compares to a meteoric spherosiderite line value of -4.2‰ from spherosiderites in a plinthic paleosol developed in correlative fluvial-estuarine deposits at the Camp Jefferson section (Brenner et al., 2000, p. 873) in southeast Nebraska. Spherosiderites from plinthic paleosols in fluvial-estuarine deposits of the overlying Late Albian Muddy cycles at the Yankee Hill claypits west of Lincoln, Nebraska, yield meteoric spherosiderite line values ranging between -4.4 to -3.6‰. Farther to the north in the Sioux City, Iowa, area, spherosiderites from the Muddy cycles and the lower part of the overlying Late Albian-Turonian Greenhorn marine cycle have meteoric spherosiderite line values that range between -5.3 to -2.9‰ (White et al., 2005).

Figure 14--Carbon-oxygen isotope plot of data from Albian authigenic siderites in the Kiowa-Skull Creek interval in the Dakota Formation. The δ¹⁸O and δ¹³C values are in per mil variations from the VPDB carbonate isotope standard.

In short, siderites from cemented zones in fluvial-estuarine facies of the Dakota Formation have δ¹⁸O values that overlap with, but are generally lighter than those from spherosiderites in the interbedded paleosols. Phillips et al. (2007) argued that differences between the fluid δ¹⁸O compositions determined from pedogenic spherosiderites and regional fluvial-estuarine paleoaquifers in the "Muddy cycle" of the Dakota Formation resulted from differences in the sizes of the catchment basins that recharged their respective paleoground-water systems. The lighter δ¹⁸O values of cements in Dakota fluvial-estuarine paleoaquifers are attributed to "continental effects" (sensu Rozanski et al., 1993) on the ¹⁸O composition of precipitation over the interior of the large eastern landmass that drained into the Western Interior foreland basin. This far inland precipitation was delivered by runoff and shallow ground-water flow into the large-scale fluvial systems that drained into the Western Interior seaway. Dutton et al. (2005) presented actualistic isotopic data that verify this partitioning of North American local precipitation (heavier δ¹⁸O values) and regional runoff (lighter δ¹⁸O values), and referred to this phenomenon as "catchment effects." The practical importance of this phenomenon is that river waters in large continents have lower δ¹⁸O values than the locally recharged ground waters beneath their immediately adjacent interfluve areas.

Estimates of the ground-water δ¹⁸O compositions from which the siderite cements in fluvial-estuarine facies of the Kiowa-Skull Creek-Nishnabotna interval were precipitated are shown in fig. 15. These estimates depend on independent estimates of the temperatures of the shallow paleoground waters from which the siderites precipitated--usually a close approximation of the mean annual paleotemperature. We here use the envelope of empirical estimates of Cretaceous paleotemperatures at each paleolatitude from Barron (1983). At estimated zonal mid-Cretaceous mean annual temperatures for 34°N paleolatitude ranging between 21 to 31°C, and using the siderite-water ¹⁸O fractionation relation of Carothers et al. (1988), these ground waters are estimated to have had δ¹⁸O compositions ranging between -8.5 to -4.2‰ SMOW. These estimated ground-water values compare to an estimated coastal paleoprecipitation value of -8.1‰ at 34°N paleolatitude by White et al. (2001), based on a mid-latitude linear regression of meteoric spherosiderite δ¹⁸O compositions, but used a lower climate model-derived paleotemperature of about 15°C. At this same lower paleotemperature estimate of 15°C, the Iowa siderite cements yield even lower estimated ground-water compositions ranging between -10 to about -8‰ SMOW. Once again, the estimated ground-water δ¹⁸O values from fluvial-estuarine paleoaquifers of the Kiowa-Nishnabotna interval overlap with, but are generally lighter than those from spherosiderites at this paleolatitude. The significance of this observation is that authigenic minerals analyzed from differing sedimentary facies in the same fluvial depositional system will yield differing estimates of the δ¹⁸O of ground water.

Figure 15--Calculated ranges of Albian paleoground waters determined from authigenic siderites in the Kiowa-Skull Creek interval of the Dakota Formation. The δ¹⁸O values are in per mil variations from the SMOW water isotope standard. They were calculated using the average δ¹⁸O values from siderites at specific stratigraphic positions (see figs. 9 and 14), the range of estimated Cretaceous mean annual surface temperatures, and the temperature-dependent siderite-water ¹⁸O fractionation relationship of Carothers et al., 1988. A--Nebraska locales; B--Iowa locales.

As discussed by Ludvigson, González, Metzger, et al. (1998), White et al. (2001), and Ufnar et al. (2002), mid-Cretaceous spherosiderites have δ¹⁸O values that are notably lighter than those calculated for the same latitudes using modern temperature and precipitation data (Rozanski et al., 1993). White et al. (2001) argued that these data call for substantially increased precipitation in the Cretaceous, and that average mid-Cretaceous precipitation rates in this region ranged between 2,500 to 4,100 mm/yr. Oxygen-isotope mass balance modeling experiments simulating the hydrologic cycle of the Cretaceous northern hemisphere (Ufnar et al., 2002) suggest that the estimates of White et al. (2001) are probably conservative, and that even substantially higher Cretaceous precipitation rates are required to accommodate these data. These relationships are highly relevant to the paleoenvironmental reconstruction of the ancient fluvial systems of the Dakota Formation. Analogies to modern fluvial systems need to account for major mass balance changes in the global hydrologic cycle in the greenhouse world of the mid-Cretaceous.

Of special note is the fact that thin, laterally extensive sheet-cemented zones (see arrows in figs. 10, 11, and 12), are interpreted to have formed just beneath marine-flooding surfaces as in Taylor et al. (2000), but precipitated from pore waters that are interpreted to have been purely meteoric in origin (fig. 15). We suggest that early carbonate cementation was coeval with fresh ground-water discharge upward through the estuarine-sediment column. This could even include scenarios involving the far offshore instruion of fresh ground-water systems, with discharge through submarine springs beneath the brackish-water columns of contemporary estuaries. This general concept represents a variation on the idea of the "subterranean estuary" as explained by Moore (1999). Greatly enhanced Cretaceous mid-latitude precipitation rates interpreted from the oxygen isotopic data (Ufnar et al., 2002) suggest that coastal ground-water-recharge rates, flow-through volumes, and discharge rates also increased, so that ground-water fluxes into offshore shallow-marine deposits of the cratonic margin of the Western Interior seaway might have been much greater than those indicated by any modern studies.

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