Slopes - Trace Fossils as Indicators of Sedimentary Environments - page 612

Well-exposed successions of channel-levee deposits locally crop out along the
intertidal zone of several of the southern Gulf Islands.
Channel deposits consist of amalgamated, thick-bedded (meters to tens of
meters) granule, pebble, and cobble conglomerates that are interstratified with
normally graded, massive to planar-laminated medium- to coarse-grained sand-
stones. A few beds toward the top of the channel fill succession show dm-scale
trough cross-stratification. Most of the channel-fill deposits are unburrowed
(BI

1, manifest by isolated, thin-
walled Ophiomorpha and small-diameter Skolithos .
Levee deposits ( Fig. 10 ) display sharp-based, thin- to moderately thin-
bedded (cm- to dm-scale), low-density turbidites ( T b-e divisions of Bouma,
1962 ). Most levee turbidites display convolute bedding ( Fig. 10 B and F), with
rarer aggradational current ripples ( Fig. 10 E), and uncommon mudstone rip-up
clasts. Load structures are locally associated with some basal contacts
( Fig. 10 C). Slumped intervals also are prevalent, associated with downslope
displacement and rotational folding of beds. Boudinage structures are rare.
Levee units typically display BI

0). Sandstone beds locally display BI

¼ ¼

2-3. Bio-
genic structures include common fugichnia, lesser Planolites , Thalassinoides ,
Macaronichnus , Chondrites , and Phycosiphon , as well as very rare Asterosoma ,
Taenidium , Scolicia and an unidentified vertical burrow with fecal-pellets
packed into meniscae (possibly attributable to Compaginatichnus ; cf.
Pickerill, 1989 ; Fig. 10 F). Suites show a predominance of infaunal responses
to event deposition (e.g., escape or rare equilibrichnia), and opportunistic
colonization of the bed by largely mobile deposit-feeders (e.g., Planolites ,
Macaronichnus , and Scolicia ). Subordinate behaviors are reflected by sessile
deposit-feeding fauna (e.g., Thalassinoides , Chondrites , and Phycosiphon ).
Such associations are consistent with high sedimentation rates and variable
depositional energies. Longer pauses between turbidite emplacements on the
levees led to horizons conducive to sessile deposit-feeding, although dominated
by the more opportunistic elements (e.g., Phycosiphon ) of that behavioral
group. Chondrites constitutes a subordinate element.

¼ 0-1, with isolated zones of BI ¼

FIGURE 9 Trace fossils in slope-channel deposits of the Late Cretaceous Tres Pasos Formation at
Laguna Figueroa, Magallanes Basin, Chile. (A) Axial abandonment deposits, BI ¼ 3-4, with an
opportunistic trace-fossil suite consisting of Ophiomorpha nodosa ( On ) , Thalassinoides ( Th ), and
Planolites ( P ) . (B) Proximal-margin deposits with uncommon Thalassinoides ( Th ). (C-F) Distal-
abandonment deposits with a relatively diverse assemblage of trace fossils representing the
Cruziana Ichnofacies. BI ranges from 0 to 4. Ichnogenera include Rhizocorallium ( Rh ) , sand-filled
Planolites ( P ) , small, silt-filled Planolites montanus ( Pm ) , Gyrochorte ( Gy ), and Asterosoma ( As ).
(G) Bypass-drape deposits, BI ¼ 1-3, with a low-diversity suite of trace fossils consisting of Plano-
lites ( P ) and Neonereites ( Ne ) . (H-L) Distal-margin deposits, BI ¼ 0-4, containing a moderately
diverse trace-fossil assemblage dominated by opportunistic traces including Ophiomorpha nodosa
( On ) with distinctive laminated fill, sand-filled Planolites ( P ), silt-filled Planolites montanus ( Pm ) ,
Teichichnus ( Te ), and Thalassinoides ( Th ) . Scolicia ( Sc ) occurs locally.