Mixed Siliciclastic/Carbonate Systems - Trace Fossils as Indicators of Sedimentary Environments - page 786

low-diversity proximal Cruziana and Skolithos assemblages. Bioturbation is
less commonly observed within the bioclastic grainstone and packstone units,
although Skolithos and Diplocraterion have been recognized. In all cases, the
bioclastic units represent deposition in wave-dominated settings (upper shore-
face and foreshore), and thus, it is likely that the drop in bioturbation intensity is
due to deposition within a more proximal setting than coeval siliciclastic facies.
2.2 Moenkopi Formation (Early Triassic), Nevada and Utah
Similar to the Montney Formation, shallow-marine successions in the Moen-
kopi Formation are characterized by mixed siliciclastic/carbonate deposition
that is the result of lateral facies mixing in both nearshore and offshore environ-
ments. Bioclastic limestone, characterized by variably abundant crinoid ossi-
cles, echinoid skeletal debris, and bivalves, occurs interstratified with
mudstone in proximal offshore environments and gradationally overlays sand-
stone beds in upper shoreface to tidal-inlet environments. The trace-fossil dis-
tribution is strongly controlled by grain size and inferred depositional
environment, with proximal offshore/offshore transition sandstone beds con-
taining low-diversity assemblages of dominantly Arenicolites and Planolites ,
with rare Diplocraterion , Helminthopsis , and Rhizocorallium ( Fig. 2 ). Shore-
face successions of amalgamated and non-amalgamated sandstone beds contain
more diverse assemblages that include Arenicolites , Asteriacites , Asterosoma ,
Conichnus , Diplocraterion , Gyrochorte , Lockeia , Palaeophycus , Planolites ,
and Rhizocorallium ( Fig. 2 ). Bioclastic units contain lower-diversity assem-
blages, limited primarily to Planolites and Thalassinoides ( Fig. 2 ).
Offshore bioclastic packstone units consist primarily of disarticulated cri-
noid ossicles and were deposited as storm-induced concentrations of skeletal
debris in proximal offshore through offshore-transition successions and as such
were the result of rapid deposition and high-energy conditions. Nearshore bio-
clastic packstone and grainstone successions, restricted to upper shoreface and
tidal-inlet paleoenvironments, lack evidence of bioturbation, and the absence of
trace fossils may be due to the coarse bioclastic nature of the deposits, as well as
the persistent high-energy conditions present during deposition. Differences in
trace-fossil assemblages between carbonate and siliciclastic depositional units
are inferred to be the product of both, differences in depositional energy
between the two end-members as well as grain-size and grain-shape constraints
on bioturbator activity and the preservation of individual trace fossils. Many
taxa, such as those that constructed Diplocraterion and Rhizocorallium , likely
avoided substrates characterized by coarse skeletal debris, preferring instead
sand-dominated successions, in which it would have been easier to construct
burrows. Bedding-plane traces, such as Asteriacites and Gyrochorte , have a
low preservation potential in coarse substrates. Whether the constructors of
these traces avoided coarse bioclastic substrates is uncertain, but their preser-
vation potential, and if preserved, recognition potential, is low.