Tiering Extent and Depth of Bioturbation and Disruption of Original Physical Sedimentary Structures

A critical factor determining the nature of ichnofabric is tiering, or the vertical distribution of organisms above and below the sediment-water interface (Ausich and Bott-jer 1982). In the infaunal realm, trace fossils can provide data on depth of bioturba-tion and vertical distribution of animals and their activity in the sediment. Infaunal tiering results in the juxtapositioning of several trace fossils as animals burrow to different depths. This produces an ichnofabric composed of crosscutting burrows.

Because infauna are strongly tiered, the upward migration of the sediment column creates what has been termed a "composite ichnofabric" (Bromley and Ekdale 1986) where burrows of organisms in the lower tiers crosscut burrows in the shallower tiers with steady-state accretion. In some sedimentary settings, under certain conditions, the original tiering pattern is preserved. This is termed a "frozen tier profile" (Savrda and Bottjer 1986). Such profiles provide a "snapshot" view of the tiering structure of the infaunal community. Frozen tiered profiles result when (1) organisms do not move vertically upward following sedimentation, and (2) sediments are not subsequently reburrowed. Thus, the documentation of original tiering relationships from composite ichnofabric, through analyses of crosscutting relationships, provides information otherwise not available about the ecology of the infaunal habitat.

Tiering complexity, as well as depth of bioturbation, varies across environments. In nearshore and shallow marine Cambrian sandstones, Skolithos, Diplocraterion, and Monocraterion are common and have depths of up to 1 m (Droser 1991) (figures 7.1

Bioturbation Examples

Figure 7.1 Examples of Cambrian ichnofab-ric. A, Skolithos piperock from Lower Cambrian Zabriski Quartzite (Emigrant Pass, Nopah Range, southeastern California, USA) with an ichnofabric index of 4 (ii4); scale bar 4 cm. B, Small Skolithos burrows in the Lower Member of the Eriboll Sandstone (Skaig Burn, Ordinance Survey #15, Loch Assynt, Scotland); scale bar is in millimeters. C, Cross-sectional view of Skolithos ichnofabric in the Eriboll

Sandstone (Skaig Bridge, Loch Assynt, Scotland); scale bar 15 cm. D, Ichnofabric of the Upper Cambrian Dunderberg Shale (Nopah Range, California, USA); ichnofabric index 3 is recorded from this thin-bedded limestone and mudstone unit; scale bar 5 cm. E, Ichnofabric of Lower Cambrian Poleta Formation (White-Inyo Mountains, California, USA); differential dolomitization enhances burrows in this limestone; scale at base of photo in centimeters.

and 7.2). This may or may not reflect original depth of bioturbation (because animals adjust to sediment deposition and erosion). Nonetheless, these burrows clearly represent the deepest tiers of the Cambrian. Additionally, Teichichnus occurs as a relatively deep tier burrow in the earliest Cambrian and remains important throughout the Cambrian. Other than these burrows, Cambrian infaunal tiering in general was relatively shallow; recorded depth of bioturbation is most commonly under 6 cm.

The extent to which original sedimentary structures will be disrupted and destroyed by bioturbation is a function of sedimentation rate and rate of bioturbation. If sedimentation rate is slow enough, then shallow or even horizontal bioturbation will result in the complete destruction of physical sedimentary structures. A totally bioturbated rock simply shows that the rate of biogenic reworking exceeded that of sedimentation. Thus, thorough bioturbation is possible in virtually any setting. Environmental control is very important, and we see that ichnofabrics vary accordingly. It is critical to examine similar facies when comparing changes in amount or depth of bioturbation through time (Droser and Bottjer 1988). By way of characterizing the Cambrian, complete to nearly complete disruption of physical sedimentary structures is common in only a few settings: (1) in high-energy sandy settings where vertical burrows were common, and (2) in finer-grained sediments when rate of sedimentation was slow enough for shallow-tiered animals to keep up with sedimentation.

Cambrian infaunas produce ichnofabrics that are comparatively simple when contrasted with those of later times but are far more complex than those of the Precam-brian. Skolithos, Diplocraterion, Teichichnus, and Monocraterion all commonly produce a monospecific ichnofabric with a record ichnofabric index (ii) of up to 4 or 5 (see figures 7.1 and 7.2). Shallow-tiered burrows may have been present but are not commonly preserved in these ichnofabrics. Ichnofabrics produced by these burrows are present in lowermost Cambrian strata, and although there may be wide variability— even within the Cambrian—these monotypic ichnofabrics remain essentially unchanged throughout their stratigraphic ranges.

Outside the realm of Skolithos, Teichichnus, and Diplocraterion, ichnofabrics are in general less well developed than environmentally comparative ones of later times. In pure carbonates, for example, until the advent of boxwork Thalassinoides in the Late

Figure 7.2 Examples of Cambrian ichnofabric. A, Treptichnus pedum ichnofabric from the Uratanna Formation from the Castle Rock locality, Flinders Ranges, South Australia; scale bar in centimeters. B, Densely packed Diplocra-terion, producing an index of ii5 in the Lower Cambrian Parachilna Formation (Parachilna Gorge, Flinders Range, Australia); scale bar 6 cm. C, Glauconite-rich sandstone from Upper Cambrian St. Lawrence Formation (Upper Mississippi Valley, Wisconsin, USA), showing sediment-starved ripple lamination and small horizontal bioturbation. Source: Photograph courtesy of Nigel Hughes. D, Tommotian Pe-trosvet Formation (middle Lena River, Siberian Platform, Russia) with a Teichichnus ichnofab-ric; preserved ripple lamination also occurs;

scale bar 3 cm. E, Diplocraterion ichnofabric from the Lower Cambrian Hardeberga Formation (Scania, Sweden); scale bar 6 cm. F, Outcrop view of Tommotian Petrosvet Formation (middle Lena River, Siberian Platform, Russia); note that overall bedding is preserved but within beds, primary stratification is commonly completely destroyed by Teichichnus; field of view approximately 50 cm across. G, Laminated sandstones interbedded with bio-turbated finer-grained sediments from the Upper Cambrian St. Lawrence Formation (Upper Mississippi Valley, Wisconsin, USA); burrows are nearly all horizontal, but individual finegrained beds are destroyed, although overall bedding is preserved; scale bar 5 cm. Source: Photograph courtesy of Stephen Hesselbo.

Ordovician, tiering was relatively simple, and although complete disruption of original sedimentary fabric occurred (Droser and Bottjer 1988), centimeter-scale bedding is generally still discernible. In shallow marine subtidal terrigenous clastics, tiering was similarly shallow, and although mudstones may be thoroughly bioturbated, sedimentary packages representing storm deposition are commonly preserved.

Cambrian trace fossils are well known, and Cambrian trace fossil assemblages have been extensively documented (e.g., Jensen 1997). These assemblages likely produce distinct ichnofabrics. For example, a type of Cambrian ichnofabric is produced by the Plagiogmus-Psammichnites-Didymaulichnus group. Although these burrows are shallow, they are relatively large and generate a great deal of sediment destruction (S. Jensen, pers. comm., 1997). These burrows are widespread, but the resulting ichnofabric has not been described. Trace fossil assemblage data are useful; however, ichnofabric studies of these assemblage-bearing strata will provide even more insight into interacting physical and biological processes and the ecology of Cambrian infaunal metazoans.

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