Page 236 - Sedimentology and Stratigraphy

Criteria for the Recognition of Sandy Shallow-marine Sediments 223

Tidal shelf causes of sea-level changes and the responses to them
recorded in sedimentary successions deposited on
Scale Lithology MUD SAND GRAVEL Structures etc Notes shelves are further discussed in Chapter 23.
clay silt vf m vc gran pebb cobb boul Sand bodies formed as sand ridges may preserve the
c
f
overall dimensions of the ridge if a relative sea-level
rise occurs, leaving the sands in deeper water and
Foreshore: stratified
sands therefore inactive. Mud deposited over the surface of
the ridges will wholly enclose them, preserving them
as large, elongate lenses of sandstone. These bodies
Shoreface: cross-
bedded sands of
tidal sand bars make attractive oil and gas exploration targets
(18.7.4) because they are made up of relatively well-
sorted sandstone (a suitable reservoir rock) sur-
rounded by mudstone (a suitable reservoir seal).
One particular response to sea-level change on
sandy shelves is the deposition of a thin layer of gravel
Sands reworked by during sea-level rise. These transgressive lags form
tidal currents, waves
10s metres and storms as coarse sediment deposited on the shelf during peri-
ods of low sea level is reworked by wave action: as the
sea level rises (Plint 1988; Hart & Plint 1995),
the gravel is moved by waves in a landward direction,
resulting in a thin (usually only a few tens of centi-
metres) conglomerate bed within the succession. The
clasts in the bed are likely to be well sorted and well
rounded, and hence resemble pebbly beach deposits
(13.2): the context will, however, be different, as
Offshore transition:
hummocky cross- transgressive lag deposits will be associated with
stratified (HCS)
sands interbedded deeper water facies of the shoreface in contrast to
with bioturbated mud
the foreshore associations of a beach deposit.
14.5 CRITERIA FOR THE RECOGNITION
OF SANDY SHALLOW-MARINE
SEDIMENTS

The environments of deposition on continental
shelves vary according to water depth, sediment sup-
ply, climate and the relative importance of wave, tide
and storm processes. The products of these interacting
processes are extremely variable in terms of facies
character, sediment body geometry and stratigraphic
succession. There are, however, certain features that
can be considered to be reliable indicators of shallow
Offshore: marine environments. First, the physical processes are
bioturbated mud
generally distinctive: for example, extensive sheets
and ridges of cross-bedded sand deposited by strong
currents are easily recognised and cannot be the depos-
its of any other environment, especially if there is
Fig. 14.10 A schematic sedimentary log through a tidally evidence that the currents were tidal; hummocky
influenced shelf succession. and swaley cross-stratification are distinctive sedi-
mentary structures that are believed to be unique to