Geology Reference
In-Depth Information
genetically related strata bounded by unconformities and their correlative conformi-
ties (Mitchum 1977; Van Wagoner et al. 1988). A parasequence is a subunit within a
sequence that is bounded by marine flooding surfaces (Van Wagoner et al. 1988) and
the approximate time equivalence along flooding surfaces makes them suitable for
structural mapping. A maximum flooding surface (Fig. 1.7; Galloway 1989) can be the
best for regional correlation because the deepest-water deposits can be correlated across
lithologic boundaries. At the time of maximum flooding, the sediment input is at a
minimum and the associated sedimentary deposits are typically condensed sections,
seen as radioactive shales or thin, very fossiliferous carbonates.
1.3.4
Welds
A weld joins strata originally separated by a depleted or withdrawn unit (after Jackson
1995). Welds are best known where a salt bed has been depleted by substratal dissolu-
tion or by flow (Fig. 1.9). If the depleted unit was deposited as part of a stratigraphically
conformable sequence, the welded contact will resemble a disconformity. If the de-
pleted unit was originally an intrusion, like a salt sill, the welded contact will return to
its original stratigraphic configuration. A welded contact may be recognized from
remnants of the missing unit along the contact. Lateral displacement may occur across
the weld before or during the depletion of the missing unit (Fig. 1.9b). Welded con-
tacts may crosscut bedding in the country rock if the depleted unit was originally cross-
cutting, as, for example, salt diapir that is later depleted.
1.3.5
Intrusive Contacts and Veins
An intrusion is a rock, magma, or sediment mass that has been emplaced into another
distinct unit. Intrusions (Fig. 1.10) may form concordant contacts that are parallel to
the layering in the country rock, or discordant contacts that crosscut the layering in
the country rock. A single intrusion may have contacts that are locally concordant and
discordant.
Fig. 1.9.
Cross sections illustrating the formation of a welded contact.
Solid dots
are fixed material
points above and below the unit which will be depleted.
a
Sequence prior to depletion.
b
Sequence
after depletion:
solid dots
represent the final positions of original points in a hangingwall without
lateral displacement;
open circles
represent final positions of original points in a hangingwall with
lateral displacement
