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4.4 Slope Minibasin Deposits, Cretaceous Tres
Pasos Formation, Chile
Deposits of a slope system are preserved in the Late Cretaceous Tres Pasos For-
mation of southern Chile, recording infill of the Magallanes foreland basin
( Hubbard et al., 2010 ). A series of slope clinoforms, characterized by hundreds
of meters of bathymetric relief and lengths
25 km, are well exposed in outcrop
( Fig. 7 A). The upper parts of the clinoforms (i.e., the shelfal top-set deposits) are
characterized by shelf-edge deltaic complexes, with trace-fossil suites primarily
assigned to the Skolithos and Cruziana ichnofacies ( Covault et al., 2009; Hubbard
et al., 2010 ). Slope strata are dominated by mudstone, punctuated locally by sand-
stone deposits. A sand-prone deposit at El Chingue Bluff,
>
60 m thick and char-
acterized by upward coarsening and bed thickening, is present along a clinoform
surface ( Shultz and Hubbard, 2005 ). This anomalous unit is interpreted to record
progradation of turbiditic deposits into a growth-fault-controlled intraslope mini-
basin. The minibasin formed in a middle- to lower-slope position.
Thinly interbedded turbidites (5-10 cm thick; cf. Bouma, 1962 ) yield two dis-
tinctive trace-fossil suites ( Fig. 7 B-E). Mudstone layers characterized by Bouma
T d and T e divisions contain Chondrites , Planolites , and Thalassinoides ( Fig. 7 B).
Sandstone beds possess Arenicolites , Glockerichnus , Ophiomorpha annulata ,
Palaeophycus , Phycosiphon , Planolites , Spirophyton , Taenidium , Teichichnus ,
Thalassinoides , and Zoophycos ( Fig. 7 C-E). Shultz andHubbard (2005) assigned
these trace-fossil suites to the Zoophycos Ichnofacies. The variation in trace-fossil
assemblages was attributed to the changes in food and oxygen availability in the
slope environment, with more abundant resources introduced to the setting by
higher energy turbidity currents.
Thick-bedded, high-density turbidites (cf. Lowe, 1982 ) are increasingly sig-
nificant toward the top of the succession. Chondrites , Glockerichnus , Planolites ,
Spongeliomorpha ,and Thalassinoides are moderately abundant within thin silt-
stone interbeds ( Fig. 7 B and C). Biogenic reworking of turbiditic sandstone is
reflected by the presence of Arenicolites , Chondrites , Gordia , Ophiomorpha
annulata , O. nodosa , Palaeophycus , Phycodes , Phycosiphon , Planolites ,
Skolithos , Taenidium , Teichichnus , Thalassinoides ,and Zoophycos ( Fig. 7 D).
This suite is assigned to the archetypal Cruziana Ichnofacies, although a series
of trace-fossil forms typical of the Skolithos Ichnofacies (e.g., Arenicolites ,
O. nodosa ,and Skolithos ) are notable in discrete beds ( Shultz and Hubbard,
2005 ). In general, the trace-fossil suite is more comparable to ichnofacies typical
of shelf and distal shoreface environments. Organisms survived in slope settings
that contained sufficient oxygen levels and food resources yet were also adaptable
to recurring high-energy sediment gravity flows and associated sediment influx
(cf. Crimes, 1977 ). Shultz and Hubbard (2005) compared this adaptation to fluc-
tuating physical conditions to that experienced by organisms in shelf settings that
are subjected to periodic storms (e.g., Frey and Pemberton, 1987 ; Howard and
Frey, 1984 ; Pemberton and MacEachern, 1997 ).
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