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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 241, part 1
Allostratigraphic and Sedimentologic Applications of Trace Fossils to the Study of Incised Estuarine Valleys--page 5 of 13

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Depositional Environment

According to Lanier et al. (1993), the Buildex succession was deposited on a tidal flat, close to or at the fluvial-estuarine transition of a macrotidal estuarine paleovalley. Analysis of the systematic variations in bed thickness demonstrates the influence of tidal processes (fig. 5) and indicates that these deposits are tidal rhythmites, with thicker sets of strata representing deposition during spring tides and thinner sets recording deposition during neap tides (Lanier et al., 1993). Lanier et al. (1993) also suggested that evidence of brief periods of subaerial exposure existed in the strata based upon on the presence of certain bedding surface structures (e.g., raindrop impressions, rill marks, runnel marks) and the absence of desiccation cracks or oxidized horizons. Although small desiccation cracks were found in one of Bandel's slabs (KUMIP 25131), they are rare in this section.

Fig. 5. Thickness of beds 1 to 258 in the Tonganoxie Sandstone Member at the Buildex Quarry. (A) Unit A1, (B) Unit A2, (C) Unit A3, (D) Facies CL (adapted from Lanier, 1993; adapted from Lanier et al., 1993).

Architecture and Sequence Stratigraphy of the Tonganoxie Valley Fill

The Tonganoxie Sandstone Member occurs at the base of a large estuarine paleovalley incised in the underlying Weston Shale and Lansing Group (Lins, 1950; Sanders, 1959; Lanier, 1993; Gibling et al., 1993; Lanier et al., 1993; Archer, Lanier et al., 1994; Archer and Feldman, 1995; Feldman et al., 1995). At Buildex, the basal erosion surface is marked by a rooted coal (Ottawa coal) developed on the Weston Shale, indicating that the paleovalley walls were subaerially exposed (Archer, Lanier et al., 1994). The Tonganoxie paleovalley was oriented NE-SW and was about 41 m (135 ft) deep, 11 km (6.8 mi) wide, and 240 km (149 mi) long (Archer, Lanier et al., 1994). The valley was formed during the latest Missourian sea-level fall and filled during a subsequent transgression in the earliest Virgilian (Gibling et al., 1993; Archer, Lanier et al., 1994; Feldman et al., 1995).

Archer, Lanier et al. (1994), Archer and Feldman (1995), and Feldman et al. (1995) discussed the problems associated with the recognition of system tracts within the Tonganoxie incised valley system (fig. 6). In the present study, the definitions of Dalrymple et al. (1992) were used to determine boundaries between system tracts. However, if the terminology of Van Wagoner et al. (1990) were used, the boundary between the lowstand and transgressive system tracts would be placed further up in the sequence (at the base of the Westphalia Limestone Member). The basal erosion surface represents a type 1 sequence boundary (Van Wagoner et al., 1990) resulting from sea-level fall and subaerial exposure. The lowstand system tract is recorded by coarse-grained, fluvial channel deposits at the base of the Tonganoxie Sandstone Member (type I sandstone of Feldman et al., 1995). Dalrymple et al. (1992) suggested that the boundary between lowstand and transgressive system tracts should be placed at the lowest evidence of marine influence. Accordingly, Archer, Lanier et al. (1994) and Feldman et al. (1995) placed the boundary near the top of the type I sandstone package within the Tonganoxie Sandstone Member, where clay-draped bedforms indicate the onset of estuarine deposition. Upwards in the sequence, however, a widespread transgression is suggested by deposition of the Westphalia Limestone Member. Above this limestone, retrogradational parasequence sets are stacked (Westphalia Limestone, Vinland Shale, and Haskell Limestone Members), culminating with an open-marine, condensed section in the lowermost part of the Robbins Shale Member (Archer, Lanier et al., 1994). Because they judged this condensed section to represent the maximum flooding surface, Archer, Lanier et al. (1994) regarded the remainder of the Robbins Shale Member as the highstand system tract.

Fig. 6. Sequence stratigraphic model for the Tonganoxie paleovalley (adapted from Archer, Lanier et al., 1994). Placement of system-tract boundaries based upon the definitions of Dalrymple et al. (1992).

The Buildex Quarry succession is situated at the eastern edge of the Tonganoxie paleovalley (Feldman et al., 1995) and is part of the transgressive system tract. At this locality, the boundary between the Tonganoxie Sandstone Member and the Ottawa coal represents a coplanar surface of lowstand erosion and flooding.

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