Hollows (Element HO)
Fig. 6.40. LA element composed of very fi ne-grained sand
and silt. Note the steep accretionary dip, and the flat
benches formed by the development of horizontal separafi lls. Wizevich (l992b) designated them as DA2 - a
distinctive type of downstream-accretion element.
6.8 laminated Sand Sheets (Element LS)
Sheets of laminated sand (lithofacies Sh, Sl) with
minor Sp, St, or Sr commonly dominate in some
ancient rock sequences, and have been interpreted
as the product of flash floods depositing sand under
upper flow-regime plane bed conditions (Miall 1977,
1984b; Tunbridge 1981, 1984; Sneh 1983). The flood
deposits of Bijou Creek, Colorado (McKee et a!.
1967), are invariably quoted as a close modern analog; they provided the basis for the Bijou Creek
model of Mia!! (1977). Ephemeral streams of the
Lake Eyre Basin also contain local accumulations of
this assemblage (Williams 1971).
The characteristic architecture of this element has
been best described by Tunbridge (1981), Sneh
(1983), and Stear (1985). Individual sand sheets are
163
tion eddies at the downstream ends of tight bends (Nanson
and Page 1983; see Chap. 8). Eureka Sound Formation
(Tertiary), Ellesmere Island) Arctic Canada. (Miall 1979a)
0.4-2.5 m thick, and rest on flat to slightly scoured
erosion surfaces. They may be capped, gradationally, by Sp, St, or Sr, indicating waning flow conditions at the end of a flood event. Individual sheets
may be traced laterally for more than 100 m. At the
edges they thin and split into thinner units dominated by fi ner-grained sands and silts of lithofacies
Sr. These beds probably represent the margins of
individual fl ood sheets. Channel cut banks are rare to
absent. Stacked sequences may reach tens of meters
in thickness.
6.9 Hollows (Element HO)
Research by Cowan (1991) in the Westwater Canyon
Member of the Morrison Formation has indicated
that some architectural units previously classified as
channels require a quite different interpretation,
and justify the erection of a new type of architectural
element. Although the features to be described here
Fig. 6.40. LA element composed of very fi ne-grained sand
and silt. Note the steep accretionary dip, and the flat
benches formed by the development of horizontal separafi lls. Wizevich (l992b) designated them as DA2 - a
distinctive type of downstream-accretion element.
6.8 laminated Sand Sheets (Element LS)
Sheets of laminated sand (lithofacies Sh, Sl) with
minor Sp, St, or Sr commonly dominate in some
ancient rock sequences, and have been interpreted
as the product of flash floods depositing sand under
upper flow-regime plane bed conditions (Miall 1977,
1984b; Tunbridge 1981, 1984; Sneh 1983). The flood
deposits of Bijou Creek, Colorado (McKee et a!.
1967), are invariably quoted as a close modern analog; they provided the basis for the Bijou Creek
model of Mia!! (1977). Ephemeral streams of the
Lake Eyre Basin also contain local accumulations of
this assemblage (Williams 1971).
The characteristic architecture of this element has
been best described by Tunbridge (1981), Sneh
(1983), and Stear (1985). Individual sand sheets are
163
tion eddies at the downstream ends of tight bends (Nanson
and Page 1983; see Chap. 8). Eureka Sound Formation
(Tertiary), Ellesmere Island) Arctic Canada. (Miall 1979a)
0.4-2.5 m thick, and rest on flat to slightly scoured
erosion surfaces. They may be capped, gradationally, by Sp, St, or Sr, indicating waning flow conditions at the end of a flood event. Individual sheets
may be traced laterally for more than 100 m. At the
edges they thin and split into thinner units dominated by fi ner-grained sands and silts of lithofacies
Sr. These beds probably represent the margins of
individual fl ood sheets. Channel cut banks are rare to
absent. Stacked sequences may reach tens of meters
in thickness.
6.9 Hollows (Element HO)
Research by Cowan (1991) in the Westwater Canyon
Member of the Morrison Formation has indicated
that some architectural units previously classified as
channels require a quite different interpretation,
and justify the erection of a new type of architectural
element. Although the features to be described here