Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 494

of the primary sedimentological fabric ( Knaust, 2012; Taylor and Goldring,
1993 ). The bioturbation index reaches fromBI

0, which equates to zero biotur-

¼

bation and extends up to BI 6, which indicates 100% complete biological

reworking (e.g., homogenization) of the primary sedimentary fabric. The mea-
surement of ichnodiversity (number of ichnotaxa per unit) is influenced by
(1) taphonomy (preservation), (2) sediment textural and mineralogical maturity,
(3) colonization history, (4) BI, and (5) the taxonomic philosophy of the inves-
tigator (sedimentologist or specialist ichnologist; e.g., “lumper” or “splitter”).

¼

2.2 Ichnofabric and Ichnofacies Analysis
The ichnofabric is a characteristic approach used to categorize the ichnology, diver-
sity, bioturbation level, and colonization order of bioturbated beds (see Ekdale
et al., 2012; McIlroy, 2008; Taylor et al., 2003 for detailed methodology). Ancient
deltas can also be categorized using Seilacherian ichnofacies (see MacEachern
et al., 2007a, 2012 for detailed methodology). Ancient fluvial-, tide-, and wave-
dominated deltas may contain archetypal, stressed, proximal, distal, and mixed
expressions of the Skolithos and Cruziana ichnofacies (Supplementary Table 1:
http://booksite.elsevier.com/9780444538130 ). In addition, “ Glossifungites sur-
faces” are recognized at the bases of distributary channels ( McIlroy, 2004b )and
at sequence-stratigraphic surfaces with submarine erosion such as the transgres-
sive surface of erosion ( Gingras et al., 2002 ; MacEachern andGingras, 2008; Pem-
berton et al., 1992 ). The Curvolithus Ichnofacies characterizes shallow deltaic or
marginal-marine environments with rapid deposition in quiet-water settings
( Lockley et al., 1987 ), in contrast to rapid deposition in high-energy settings that
characterize the Skolithos Ichnofacies. Alternatively, the study of ichnofabrics
(e.g., McIlroy, 2004b, McIlroy et al., 2005 ) can be incorporated with sedimentol-
ogy, and the results presented as an ichnological analysis (see McIlroy, 2008 ).

3. COMMON DELTAIC TRACE FOSSILS
The assemblage or suite of trace fossils in an ichnofabric or ichnofacies, when
considered in their sedimentological context, may be used to refine subenviron-
ments of marine deltas. The most abundant trace fossils in ancient marine deltas
are also the most widely distributed in siliciclastic environments. Planolites ,
Thalassinoides , Palaeophycus , and Teichichnus are documented in all deltaic
types and occur in all subaqueous deltaic facies associations (lower delta plain,
delta front, and prodelta; see Supplementary Table 1: http://booksite.elsevier.
com/9780444538130 and Fig. 2 ).
Thediversityof a trace-fossil assemblage (ichnodiversity) shouldnot bedirectly
linked to species diversity or abundance on the sea floor. Factors that may
obscure or enhance apparent ichnodiversity are (1) preservational biases
(taphonomy), which favor mid- to deep-tier traces (mixed layer may be
removed by erosion); (2) burrow overprinting (e.g., palimpsest surfaces);