Geology Reference
In-Depth Information
biofi lms on tidal sedimentation is presented by Decho
(
2000
). These organic materials may assist in preser-
vation and can be recognized in ancient stratigraphic
sequences.
Tidal rhythmites are typically associated with
intertidal fl ats that border marine or estuarine water
bodies. Similar sequences that accumulate under tidal
conditions may occur in a fl uvial, fresh-water envi-
ronment. These are present in both modern systems
and ancient sequences (Kvale and Mastalerz
1998
) . It
is common for tidal environments and tidal sediment
transport to extend beyond the salt-fresh water bound-
ary. Examples include the St. Lawrence Seaway in
Canada, Hudson River in New York and the Gironde
River in France. Recognition in the ancient record
involves the preserved biogenic skeletal material and
the placement of the sequence in the context of the
entire depositional system.
Heterolithic tidal beds are also produced in fl uvial
deltaic environments with a strong tidal infl uence.
They have been reported and described from the Fly
River delta of Papua New Guinea (Harris et al.
1993
;
Dalrymple et al.
2003
) and the Mahakam River delta
of Indonesia (Gastaldo et al.
1995
). Similar ancient
tidalite sequences have been reported from delta front
deposits in the Mississippian of the Appalachians
(Adkins and Eriksson
1998
; Miller and Eriksson
1997
) .
The preservation potential of tidal bedding is high
where tidal fl ats are aggrading and prograding. As is
typical for intertidal fl ats, the sediment is delivered by
tidal currents from the adjacent subtidal environment(s).
Such intertidal deposits are still vulnerable to erosion
by wind tides and storms in general. Raising the water
level above its normal condition would also be
accompanied by wave action. This combination will
rework the tidal rhythmites. Although there are pre-
served sequences that have several hundred continu-
ous tidal cycles, most show scour areas and local
unconformities.
Fig. 3.20
Photo of tidal bedding sequences from the Baraboo
Quartzite (1.7 BY) in Wisconsin (scale is in centimeters). These
rhythmites are monolithic except for some iron staining that
gives them their defi nition
Fig. 3.21
Diagram showing spring-neap cycle and the change in the accumulated sequence as you move across and up the tidal fl at
(From Archer
1998
)
