Page 130 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 121

BOUNDING SURFACES This wedge is a lowstand tract whose shelf break is slightly
lower than the preceding shelf break but whose flat top
Sequence boundary extends over the preceding shelf break.

Vail et al.’s (1977) definition of the sequence boundary Type-3 sequence boundary implies no fall of relative sea level.
as an unconformity that separates conformable successions It forms when sea level rises faster than the system can
of strata is broad and leaves room for further specification. aggrade such that a transgressive systems tract directly
In a pioneering paper, Vail and Todd (1981) presented six overlies the preceding highstand tract, often with a sig-
ways of producing unconformities by subaerial or marine nificant marine hiatus (Saller et al., 1993; Schlager, 1998;
erosion, three of them were deemed suitable to serve as Schlager, 1999b). Marine erosion frequently accentuates
sequence boundaries. Two of the three became widely this sequence boundary, particularly on drowned carbonate
accepted. I recently proposed to add a third type (not platforms.
identical with the type 3 of Vail and Todd, 1981). The three
types are briefly characterized below and subsequently From a sedimentologic perspective, the three types have
evaluated in more detail. rather different qualities. The type-1 unconformity is in
many ways the ideal sequence boundary. It has a good
Type-1 sequence boundary forms when relative sea-level geometric expression because of the downstepping of the
falls below the shelf break of the preceding sequence. The shelf break and, in carbonates, the karst morphology that is
new sequence starts with a lowstand tract whose flat top sometimes visible even in seismic data. Type-1 unconformi-
is distinctly lower than the youngest shelf surface of the ties also have a distinct lithologic signature in the form of
underlying highstand tract. terrestrial overprint on marine deposits (Microcodium,plant
roots, calcrete soils). Finally, the type-1 unconformity is un-
Type-2 sequence boundary forms when relative sea-level falls ambiguous evidence of a significant relative sea-level fall.
to somewhere between the old shoreline and the shelf Type-2 boundaries are similar to type-1 boundaries but
break. Consequently, only the inner shelf becomes exposed less pronounced. The geometric expression is subdued be-
and the new sequence starts with a “shelf-margin wedge”. cause the difference in elevation between the old highstand

sand shoals

boundstone (automicrite, sponges, cement)

lime mud, sand
BOUNDSTONE PROGRADING
BASIN ONLAPPING

marl + turbidites

SLOPE FAILURE slump
LANDSLIDE
debris tongue outrunner
boulders turbidites debris sheets

Fig. 7.19.— Model of slope failure by increased shear stress from growth of rigid automicrite lenses or mounds. Model based on
observations on Triassic slopes in the Southern Alps. Upper panel: automicrite boundstone grows and overloads slope. Lower panel:
slope failure removes excess load; slumps and debris tongues of megabreccias are emplaced on lower slope and basin floor. After
Schlager et al. (1991), modified.