Geology Reference
In-Depth Information
Fig. 13.14
Depositional model for a tidal shelf ridge, based on
the Precambrian of north Norway. The ridge is interpreted to be
shore-parallel and separated from the coastline by an area of
wave erosion. The upper part of the ridge is composed of cross-
bedded sandstone created by the along-strike migration of dunes
('sand waves'). Channels cut by bidirectional currents dissect
the ridge crest. The ridge growth and migration is related to
storm-enhanced tidal activity. It passes gradually offshore to
muddy, storm-influenced facies (From Johnson
1977
)
13.5.3 Ridges in the Rock Record
(Fig.
13.14
) is more consistent with modern examples
because of the presence of lateral accretion and
upward coarsening, but the suggestion that migration
is always in an offshore direction is inconsistent with
many modern examples. The absence of a basal ero-
sion surface is also not what is seen in modern shelf
ridges. Erosionally based shelf sand bodies have been
documented from the Cretaceous Western Interior
Seaway of Northern America (Tillman and Martinsen
1984
), but they have been reinterpreted as forced-
regressive shorefaces and deltas, detached from the
coast by subsequent transgressive ravinement (Walker
and Bergman
1993
; see Chap. 17).
There are very few detailed case studies in the ancient
that argue convincingly for the existence of tidal-cur-
rent ridges. The Mutti et al. (
1985
) model for a 'tidal
bar' shows an upward-coarsening succession and the
presence of dune cross bedding as predicted from
modern tidal ridges, but it shows forward accretion
instead of the lateral accretion documented from mod-
ern examples. Therefore, the bedforms described by
Mutti et al. (
1985
) are more likely to represent very
large compound dunes (cf. Fig.
13.4
; Dalrymple
2010b
). The Johnson (
1977
) model for tidal ridges
