Environmental reconstruction - Fundamentals of the Physical Environment - page 562

seasonal) breaks, or unconformity representing longer,
often erosional, gaps ( Figure 23.1 ; Plate 23.3 ). The terms
bed and facies are sometimes used interchangeably but
not without confusion! Cross-bedded sediments in
meandering rivers and deltas (see Figures 14.15 and 17.14 )
contain many individual beds, and even laminae, but their
collective sediment bundles are recognized generically as
floodplain or deltaic facies. Facies represent discrete events
or periods of accumulation and are therefore also basic
units of chronostratigraphy and succession or sequence .

Geology employs a hierarchy of progressively longer time
from ages, epochs, periods to eras and aeons for the age
of rock supergroups (whole assemblages forming geo-
logical terranes ) and their progressively smaller compon-
ent groups, formations and members . The last of these is
usually composed of several beds which may also con-
stitute a facies (see also Chapter 23).
Dynamic links between process, genetic facies and
time at the largest geological scales underpin sequence
stratigraphy , connecting tectonic, sea-level and climate
change in the history of entire sedimentary basins.
Onshore this is seen in the tectonic uplift and extension
of southern California, with vigorous erosion of elevated
fault blocks infilling down-faulted trenches with coarse
detritus ( Plate 23.4 ). Uplift was high enough to permit
Quaternary glaciation and extensive enough to isolate
inland areas from Pacific maritime climate. These influ-
enced sediment fluxes and styles, with the intro-
duction of glacigenic, desert evaporite and aeolian facies.
In addition, local transtension triggered minor volcanic

(a) Unconformity (b) Nonconformity

(c) Disconformity

Figure 23.1 Principal types of unconformity.

Plate 23.3 An angular unconformity created by the
deposition of Devensian permafrost deposits on a wave-cut
rock platform eroded across tilted Carboniferous limestone,
Gwr coast, south Wales. The platform was cut by higher
Ipswichian interglacial sea levels, 125 ka ago; the limestone is
over 325 Ma old.
Photo: Ken Addison

Plate 23.4 The eastern flank of the half-graben fault basin of
Death Valley, California, with typical depths of 3-7 km of
mostly Quaternary sediment infill, sourced here from the
Amargosa range. The sharp boundary of light-coloured trench
sediments running across the foreground is part of the
Furnace Creek fault zone.
Photo: Dee Trent