Page 224 - Sedimentology and Stratigraphy

Estuaries 211

Fig. 13.16 Distribution of depositional settings in a tidally dominated estuary.

salt marsh areas cut by tidal creeks that act as the bedforms are created and migrate with the tidal
conduits for water flow during the tidal cycles. The currents to generate cross-bedded sandstone beds. Evi-
processes and products of deposition in these settings dence for tidal conditions in these beds may include
are the same as found in macrotidal settings. mud drapes, reactivation surfaces and herringbone
cross-stratification (11.2.4). The mud drapes form as
the current slows down when the tide turns, and the
Tidal bars
reactivation surfaces occur as opposing currents
The outer part of a tide-dominated estuary is the erode the tops of dune bedforms. Herringbone cross-
zone of strongest tidal currents, which transport bedding is relatively uncommon because the ebb and
and deposit both fluvially derived sediment and mate- flood tidal flows tend to follow different pathways,
rial brought in from the sea. In macrotidal regions with the flood tide going up one side of the estuary
the currents will be strong enough to cause local and the ebb tide following a different route down the
scouring and to move both sand and gravel: bioclastic other side. Dune bedforms that form on elongate
debris is common amongst the gravelly detritus depos- banks (and hence the cross-beds) will be mainly
ited as a lag on the channel floor (Reinson 1992). Dune oriented in either the flood tide direction or with the

Fig. 13.17 A tidally dominated estua-
rine environment with banks of sand
covered with dune bedforms exposed at
low tide.