Kansas Geological Survey, Subsurface Geology 12, p. 59-60
by
Robert H. Goldstein, Randall C. Carlson, Mark W. Bowman, and James A. Anderson
The University of Kansas
Developing realistic models of sedimentary systems depends on constraining the controlling variables. One important variable is the history and magnitude of relative sea-level change. During a low stand in sea level, shallow-water carbonate sediments commonly are exposed subaerially and subjected to diagenesis by meteoric fluids. Identification of surfaces on which subaerial exposure has occurred and the paleotopography along these surfaces helps to reveal the history and magnitude of sea-level change. Paleosols, stable-isotopic shifts, and cement-stratigraphic discontinuities are useful indicators for identifying ancient surfaces of subaerial exposure.
A good example of the utility of paleosols is the Pennsylvanian Holder Formation of southern New Mexico, which consists of about 20 carbonate-siliciclastic cycles deposited on the edge of the Pedernal uplift. The limestone units in the cycles show evidence of shallowing upward, but for the most part, appear to have been deposited entirely in the subtidal realm. For many cycles, paleosols cap the carbonate units. Rhizoliths, tangential needle fibers of calcite, alveolar texture, ribbon spar, irregular coatings on grains, micritized grains, glaebules, desiccation cracks, and laminated crusts provide good evidence of paleosols and thus indicate repeated subaerial exposure during deposition of the Holder. As most of the paleosols are developed on subtidal carbonate rocks, simple aggradation into the subaerial realm must have caused subaerial exposure; a relative fall in sea level is required. Some paleosols are laterally continuous and can be traced from the shelf to more basinal positions. Onlapping relationships of overlying beds and original topography in bioherms reflect 30-50 in (99-165 ft) of paleotopography on marine rocks capped by a single paleosol, indicating a relative fall in sea level of at least 30-50 in (99-165 ft; Goldstein, 1988a).
Subaerial exposure, however, is not always recorded unambiguously in the stratigraphic record. Paleosols may be eroded during exposure or subsequent transgression. Meteoric diagenesis associated with subaerial exposure may occur well below the actual exposure surface. Features resembling paleosols may occur at the water table and paleokarst may occur far below the subaerial surface.
Stable isotopic analysis of whole-rock samples commonly is used to identify ancient surfaces of exposure. The method, developed by Allan and Matthews (1982), predicts that stabilization of marine-carbonate sediment in the subaerial realm could yield a light-carbon signature from soil-gas CO2, a heavy-oxygen signature from evaporation, and an overall shift in oxygen isotopic composition because of different diagenetic histories across the surface. Whole-rock samples across paleosol-capped cycles of the Holder Formation provide data by which this method can be evaluated. The light carbon shift is not consistently present, the heavy oxygen signal is missing, and overall oxygen shifts across the surfaces of subaerial exposure are absent (fig. 1). The light-carbon and heavy-oxygen signatures are present, however, in soil-precipitated microcomponents. These data suggest that the whole-rock isotopic method for subaerial surface identification should be supplemented with petrogaphic observations and isotopic data on soil-precipitated microcomponents.
Figure 1--Stable-isotopic composition of soil-formed microcomponent and whole-rock samples relative to stratigraphic position of two surfaces of subaerial exposure in the Holder Formation.
When cyclically deposited stratigraphic units are consecutively subjected to subaerial exposure, infiltration of meteoric water may result in low-Mg calcite cementation. Low-Mg calcite cement can provide a record of the history of subaerial exposure, which in turn, allows interpretation of the history of sea-level change. Obvious indicators of subaerial exposure are calcite cements with gravity-asymmetric or meniscus fabrics. Cement stratigraphy of cathodoluminescent zonation in calcite may provide a more subtle record of subaerial exposure. Cross-cutting relationships are essential to show that cement zones are related to events of subaerial exposure. Merely tracing of cathodoluminescent zones relative to stratigraphic surfaces may be misleading because of the complexities of paleoaquifer chemistry.
In calcite cements of the Holder Formation limestones, cross-cutting relationships with paleosol features, early fractures, and clast boundaries in intraformational conglomerate relate zones of calcite cement to events of subaerial exposure. Vertical tracing of different sequences of cement zones indicates at least 15 events of subaerial exposure of limestones. The cements are best developed in a shelf-crest setting and tend to die out in a basinal direction (Goldstein, 1988b). Sandstones directly underlying limestones capped by surfaces of subaerial exposure may contain early meteoric calcite cement that correlates with the cements in limestones. In contrast, subaerial exposure in the clastic phase of a cycle does not result in calcite cementation of underlying limestones. Subaerial exposure of limestone appears to be required for development of calcite cement in underlying strata (Bowman, 1987).
The Pennsylvanian Lansing-Kansas City groups of northwestern Kansas consist of carbonate-siliciclastic cyclic strata that show evidence of repetitive subaerial exposure (Watney, 1980). Most of the cathodoluminescent cement zones cannot be related to events of subaerial exposure and much cement post-dates compaction. Fluid-inclusion data indicate calcite cementation from brines rather than freshwater (Anderson, 1989). Deposition was in a local paleotopographic low area which was not conducive to development of abundant meteoric calcite cement. Thus, cement stratigraphy in this setting is not closely related to sea-level history.
In contrast, carbonate sediments deposited and exposed on paleotopographic high areas show preferred development of meteoric calcite cements. An example is the Lisburne Group in the Sadlerochit Mountains of northeastern Alaska. During the Pennsylvanian, as many as 40 shallowing-upward carbonate sequences developed in this shelfcrest setting (Carlson, 1987). Major and minor events of subaerial exposure within the Lisburne may have provided meteoric waters that precipitated nonferroan calcite cement with complex cathodoluminescent zonation. The upper limit of early cement zones commonly coincides with the tops of shallowing-upward sequences. Upward pinchouts of zoned calcite cement also occur at major lithologic discontinuities in which rocks containing Microcodium (indicate subaerial exposure) bound different types of shallowing-upward sequences. These early cements commonly occlude as much as 50% of the original porosity and are associated with uncompacted textures. Some cement zones can be traced for less dm 10 m (33 ft), whereas others range over stratigraphic intervals of up to 70 m (231 ft). Further research will seek cross-cutting relationships to better relate cementation to events of subaerial exposure. Discontinuities in cement zonation can be used as a guide for locating potential ancient surfaces of subaerial exposure and interpreting history of sea-level change.
Cement stratigraphy, stable-isotope trends, and paleosols provide useful diagenetic evidence of subaerial exposure that, when applied properly, may help constrain history and magnitude of sea-level change.
Allan, J. R., and Matthews, R. K., 1982, Isotope signatures associated with early meteoric diagenesis: Sedimentology, v. 29, p. 797-818
Anderson, J. E., 1989, Diagenesis of the Lansing and Kansas City groups (Upper Pennsylvanian), northwestern Kansas and Southwestern Nebraska: M.S. thesis, The University of Kansas, 259 p.
Bowman, M. W., 1987, Sandstone diagenesis in an interbedded carbonate-siliciclastic sequence, Virgilian Holder Formation, New Mexico: M.S. thesis, The University of Kansas, 166 p.
Carlson, R. C., 1987, Depositional environments, cyclicity, and diagenetic history of the Wahoo Limestone, eastern Sadlerochit Mountains, northeastern Alaska: M.S. thesis, University of Alaska, 189 p.
Goldstein, R. H., 1988a, Paleosols of Late Pennsylvanian cyclic strata, New Mexico: Sedimentology, v. 35, p. 777-803
Goldstein, R. H., 1988b, Cement stratigraphy of Pennsylvanian Holder Formation, Sacramento Mountains, New Mexico: American Association of Petroleum Geologists, Bulletin, v. 72, p. 425-438
Watney, W. L., 1980, Cyclic sedimentation of the Lansing-Kansas City groups in northwestern Kansas and Southwestern Nebraska: Kansas Geological Survey, Bulletin 220, 70 p. [available online]
Kansas Geological Survey
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Web version May 11, 2010. Original publication date 1989.
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