Estuarine valleys are typically incised during a sea-level fall (fig. 30). They may begin to fill during a lowstand (fig. 31), but sediments accumulate during the subsequent sea-level rise (Zaitlin et al., 1994). If lowstand, coarse-grained sediments are deposited and preserved, they are replaced vertically by finer-grained facies of the transgressive system tract (fig. 32). Commonly, during a lowstand, the valley acts as a bypass zone (fig. 30), or lowstand deposits are eroded and reworked during the subsequent transgression (MacEachern and Pemberton, 1994). In such cases, transgressive deposits directly overlie the sequence boundary, resulting in the formation of coplanar surfaces. Finally, during highstand, progradation of the coastal plain occurs (fig. 33).
Fig. 30. Ichnofacies model of a lowstand (fan) system tract in an incised estuarine valley system. (A) Cross section; (B) plan view (adapted from Zaitlin et al., 1994). Boundaries of system tracts are based on definitions by Dalrymple et al. (1992). The lowstand (fan) system tract is characterized by valley incision and sediment bypass. Trace fossils (Cruziana ichnofacies) are present only in open-marine sediments.
Fig. 31. Ichnofacies model of a lowstand (wedge) system tract in an incised estuarine valley system. (A) Cross section; (B) plan view (adapted from Zaitlin et al., 1994). Boundaries of system tracts are based on definitions by Dalrymple et al. (1992). The lowstand (wedge) system tract is dominated by fluvial deposition in the incised valley. Trace fossils are restricted to open-marine settings (Cruziana ichnofacies) and high-energy mouth bars of the lowstand delta (Skolithos ichnofacies).
Fig. 32. Ichnofacies model of a transgressive system tract in an incised estuarine valley system. (A) Cross section of estuary funnel; (B) cross section of upper estuary channels; (C) plan view (adapted from Zaitlin et al., 1994). Boundaries of system tracts are based on definitions by Dalrymple et al. (1992). In the trangressive system tract the estuary funnel and upper estuary channels are separated. At an early stage, freshwater conditions coexist with tidal influence in the upper estuary channels, where a mixed Scoyenia-Mermia ichnofacies is present. A mixed Cruziana-Skolithos ichnofacies typifies the brackish-water estuary funnel. As transgression proceeds, brackish-water conditions reach the upper estuary channels and the mixed Cruziana-Skolithos ichnofacies migrates landward. Normal salinity waters in the estuary funnel may progressively allow the establishment of a more marine Cruziana ichnofacies.
Fig. 33. Ichnofacies model of a highstand system tract in an incised estuarine valley system. (A) Cross section; (B) plan view (adapted from Zaitlin et al., 1994). Boundaries of system tracts are based on definitions by Dalrymple et al. (1992). The highstand system tract is characterized by progradation of the coastal plain. Fully marine trace-fossil assemblages are dominant. Progressive increase in energy associated with coastal plain progradation may lead to the vertical replacement of the Cruziana ichnofacies by the Skolithos ichnofacies.
Buildex-type ichnofaunas characterize not only deposition in the upper part of the inner estuary (segment 2 of Zaitlin et al., 1994), but also the basal trangressive deposits immediately overlying the coplanar surface. In this specific setting and at this particular stage of estuarine valley evolution, freshwater conditions coexist with tidal influence. As transgression proceeds, backstepping brackish-water deposits accumulate. The ichnologic signature of such a change in depositional conditions is reflected in the upward replacement of a mixed Scoyenia and Mermia ichnofacies (Buildex-type ichnofaunas) by a mixed Skolithos and impoverished Cruziana ichnofacies (fig. 32). The presence of burrows in the strata overlying the Buildex Quarry section is suggestive of brackish-water conditions and a transgressive infill of the estuary. The mixed Skolithos and depauperate Cruziana ichnofacies is, for example, clearly displayed in cores described by Wightman et al. (1987) and Pemberton and Wightman (1992) as structures of infaunal burrowers, such as Gyrolithes, Thalassinoides, Teichichnus, and Chondrites. Because the mixed Scoyenia and Mermia ichnofacies is dominated by surface or shallow subsurface traces, the core expression of such an assemblage is parallel-laminated deposits with minimal or no bioturbation (fig. 27A,C).
MacEachern and Pemberton (1994) noted that the Glossifungites-demarcated surfaces are restricted to the limits of marine influence within the valley system. Tracemakers of the Glossifungites ichnofacies are unable to colonize freshwater portions of the estuary. The coplanar surface (flooding surface and sequence boundary) at the base of the Tonganoxie sequence at Buildex Quarry lacks the Glossifungites ichnofacies. In contrast, this surface is characterized by coals and paleosols with upright plant remains (Lanier, 1993; Lanier et al., 1993) and represents a surface of erosional truncation and nondeposition close to the valley interfluves. This rooted horizon may be regarded as the landward equivalent of the Glossifungites ichnofacies (fig. 32).
Kansas Geological Survey
Web version March 19, 1998
http://www.kgs.ku.edu/Current/1998/buatois/buatois10.html
email:lbrosius@kgs.ku.edu