Comparison/Correlation of the geochemical results

The following plots (Figure 23, 24, 25) show average data of all three measured geochemical parameters (CaCO₃, TOC, δ¹³C_org) which were analyzed for the sections. They have been plotted onto a N-S transect to determine whether latitudinal changes or trends occur.

The geochemical data for TOC and CaCO3 show trends along the N-S transect (see Figure 23 and 24), but only three sections show definite cyclicity. The δ¹³C_org data (Figure 25) do not show any general N-S trend.

There is a general decline from about 1.4 wt. % TOC at the northern-most location CAN-III-00 to values of ~0.1 wt. % TOC at the Mexican locality. A major exception is section MT-I-00, with a TOC content of ~3.8 wt. %. These high values, well-known for the Cone Member of the Marias River Shale and may explained by large scale watermass mixing and/or local factors. The Marias River Shale is considered to be a good potential source rock for oil (DYMAN, 1987), because of the relatively high hydrogen index values and above average organic carbon values (Clayton et al., 1983).

Distances (km) between the sections are shown below:

Figure 23: Average TOC data [wt. %] from north to south (to scale).

The CaCO₃ content (see Figure 24) rises from north to south, from 20 wt. % (CAN-III-00) to 74 wt.

% (MX-I-00). Again there is a notable exception, the highest value is not at the southernmost location as might be expected, but at locality NM-I-00 (Emery Gap in NW-New Mexico).

Figure 24: Average CaCO₃ data [wt. %] from north to south (to scale).

A general correlation between lithology and geochemistry can only be observed at the New Mexican and Texan sections. Here, CaCO3 and TOC of limestones and shales correlate negatively; the limestones have higher carbonate- and lower TOC content than the shales.

The northern sections (CAN-II-00) do not indicate a cyclic variation of any of the three geochemical proxies close to that produced by Milankovitch parameters, except the carbonate content at section CAN-II-00. The values do vary with large amplitudes. It is possible that information was lost due to the relatively large sampling interval at this sections (3 m). Section CAN-I-00 shows variation of the CaCO₃ content with a period of about 122 ka, which is close to the eccentricity signal. The next two sections to the south (MT-I-00; NM-I-00) do all show large variation of one or more geochemical parameters, but these shifts occur highly irregular. This changes at NM-II-00, where a cyclicity of 36 cm for TOC and of 50 cm for CaCO3 can be seen. These cyclic variations occur between 2 and 4 m in the section and give periodicities of 16 ka and 22 ka which indicate that these variations could have been produced by changes of the precessional parameter. Whereas the southernmost location in Northern Mexico does not show any resemblance between Milankovitch-type

periodicities and the geochemical data, the Texan section shows a periodicity of ~96 cm for the CaCO₃ content, which could have been influenced by changes in obliquity (41 ka cycle).

The geochemical transects also suggest important oceanographic changes. Section MT-I-00 has the highest TOC value of all sections. It could have been located near an oceanic frontal system,

7. Geochemistry 72

as proposed by FISHER et al. (1994) for the area near the Black Hills area in South Dakota.

Another important indicator is the increase of the carbonate content south (~70-80 wt. %) of MT-I-00; north of MT-I-00 the carbonate values are around 20 wt. %.

The Cenomanian oceanic front in the WIS was first described as a classic facies change by BRAMLETTE and RUBY (unpublished data, available at the USGS, Denver, CO), but they did not interpret ist cause. MOORE (1949) pointed out that the continuous bentonites were deposited as a series of ash falls and can therefore be used as stratigraphic datum levels. After the facies change was correlated lithostratigraphically it was clear that the calcareous and the noncalcareous shales were deposited at the same time, but it was still not interpreted. Biofacies analysis (FISHER et al., 1994) of the abrupt facies change in southeastern Montana indicated that these changes record the boundary or oceanic front between two water masses with distinctly different

paleoceanographic conditions. One water mass entered the seaway from the Arctic and the other from the Tethys/Proto-Gulf of Mexico. The southern water mass supported calcareous

microplankton and calcareous benthic foraminifera as can be seen in carbonate content south of MT-I-00. The lack of calcareous microplankton in the northern seaway is due to ecologic exclusion, probably due to low salinity of the water mass. The high TOC values could be explained by a freshwater lid on the northern seaway which limited oxygenation of the bottom water and therefore enhanced preservation of organic matter. Therefore section MT-I-00 was probably located on the northern side of the oceanic front, mainly influenced by the fresher and cooler northern water mass from the Arctic region.

Figure 25: Average δ¹³Corg data ($) from north to south (to scale).

The ¹³C_org data (see Figure 25) does not reflect a connection between isotopic fractionation and latitude. The average value for all sections is –25.6 +/-1.1 $. The independence of latitude and isotopic fractionation is supported by the fact that the sections with the heaviest (NM-II-00; -23.0

$) and the lightest (TX-I-00; -27.1 $) isotopic values are in sections next to each other, separated by only 750 km. The proximity between the heaviest and the lightest isotopic values reveals an interesting pattern. The paleogeography proposed by ROBINSON ROBERTS and KIRSCHBAUM (1995) in Figure 27 could give an explanation. If the proposed narrow gateway between the Western Interior Seaway and the Tethys is correct, these sections would have been influenced by different water masses. The New Mexican section was under the influence of the water of the seaway, whereas the Texan section would have been influenced by warmer and more saline subtropical waters of the Gulf of Mexico.

As described earlier, the C/T OAE (Oceanic Anoxic Event) is characterized by a large positive global carbon-isotope excursion in both carbonate and organic matter, caused by a major perturbation of the global carbon budget, most probably due to the extensive burial of organic matter in black shales (ARTHUR et al., 1987). The isotopic measurements of this study indicate lack of a positive excursion in the sections. The C/T excursion can only be detected at Burnt Timber Creek, Las Vegas, and Hot Springs but even at these four localities the amplitude of the excursion changes. These inconsistencies are probably due to the sampling of differing time slices.

As can be seen in Figure 11, the isotopic excursion begins with a sharp rise towards more positive in the uppermost Cenomanian. In the Turonian values slowly get lighter gain. The isotopic data, together with biostratigraphic data, suggest that the sections at Thistle Creek, Big Horn River, Cone, Landfill, Emery Gap, and La Boca Canyon are already in the early Turonian. Another reason for changes of isotopic signature could be local effects on fractionation, which have been described for the C/T OAE (SCHLANGER et al., 1987).