Page 131 - Carbonate Sedimentology and Sequence Stratigraphy

122 WOLFGANG SCHLAGER

tract and the shelf-margin wedge is small and standard basic pattern is a deepening of the depositional environment
seismic data may not resolve it. Lithologic signatures are to below the photic zone and thus below the production
less pervasive than with type-1 unconformities because only zone of the T factory. The drowning process need not go
part of the shelf is exposed and subaerial alteration pene- to completion. Platforms may be flooded and submerged
trates less deeply. The type-2 unconformity is evidence for to less-than-optimal but still photic conditions. In these in-
a minor fall of relative sea level. It is crucial that an uncon- stances, one should speak of "incipient drowning" (Read,
formity is assigned type-2 status only if there is evidence for 1982) or pronounced flooding. Such flooding events often
exposure of the inner shelf. It has become common practice lead to backstepping and re-orientation of the platform mar-
to classify unconformities with only questionable evidence gin. The record of flooding or drowning may show a grad-
of exposure as type-2 sequence boundaries. I recommend ual transition from shoalwater to deepwater deposition but
to describe such unconformities as sequence boundaries in major gaps and abrupt changes are more common. A com-
general and abstain from further specifying the boundary mon reason for the punctuated record is current amplifi-
type (see below). cation by sharp topography. The drowned platform rein-
Type-2 boundaries are not common on rimmed carbon- forces the (generally sluggish) oceanic tides and this may
ate platforms because the rim tends to build to sea level. trigger intensive and long-lasting marine erosion. The re-
Under these circumstances, the platform top has no over- sult are major hiatuses between the drowned platform and
all seaward dip and a sea-level fall of only a few meters its (hemi)pelagic cover or within this cover. Many of these
will expose the platform out to the shelf edge and gener- hiatuses exceed 10 My, some exceed 100 My in duration (Fig.
ate a type-1 unconformity. However, Sarg (1988) pointed 2.25; Schlager, 1999b).
out that on certain rimmed platforms, such as the Permian The drastic changes in sediment composition and dis-
of the Guadalupe Mountains, the outermost platform does persal that accompany platform drowning commonly pro-
have a distinct seaward dip that leaves room for the devel- duce an unconformity called drowning unconformity by
opment of a shelf-margin wedge during a modest fall in sea Schlager (1989). Geometrically, this unconformity may re-
level. The reason for the “hanging shoulders” of these plat- semble a lowstand unconformity because of basin-restricted
forms probably lies in the nature of the rim: it consists of the sediment bodies that onlap the slope of the drowned plat-
deeper-water automicrite community and extensive marine form (Figs 7.20, 7.21). In reality, however, the unconfor-
cement; this system does not necessarily build to sea level mity must form during a rise or highstand of relative sea
– the topographic crest of the platform may be formed by level because drowning can occur only if the platform top
sand shoals that lie landward of the deeper-water rim. is flooded. Drowned platforms and drowning unconformi-
Type-3 boundaries reflect the effects of two independent ties are common in the geologic record and some of them
processes that operate independently or in conjunction have been interpreted as the result of major lowstands. This
to generate marine unconformities: (1) amplification of may explain some discrepancies between the sea-level curve
oceanic tidal waves by sharp topography and (2) platform from sequence stratigraphy and curves derived by other
flooding or drowning, i.e. demise by submergence below techniques (Fig. 7.22). Raised rim and empty bucket are
the photic zone. Superposition of both processes can common features of drowning unconformities and provide
generate marine hiatuses that may exceed 100 My and rep- good criteria for their recognition.
resent some of the most prominent seimic unconformities Drowning may be a drawn-out affair whereby the produc-
on record (Fig. 2.25, 7.20, 7.21). It should be noted that tive platform surface shrinks gradually or in discrete steps.
type-3 unconformities qualify as sequence boundaries only The last step in this process often shows the most productive
if one accepts the original definition of Vail et al. (1977). part of the former rim disintegrating into a chain of patches.
They are not unconformities (nor sequence boundaries) These patches are mound-shaped because they are deeply
in the sense of Van Wagoner et al. (1988). To stay in line submerged and no longer planed by waves (Fig. 7.20, 7.21,
with Van Wagoner et al.’s definition, J.F. Sarg (written 7.23).
communication) proposed an alternative interpretation of Drowning events often appear more prominently in seis-
the drowning unconformity in Liuhua (Fig. 7.20). He views mic data than exposure events. In fact, seismic reflectors and
the top of the platform as a maximum flooding surface unconformities resulting from drowning are so prominent
and puts the sequence boundary at the exposure horizon that they are often picked as sequence boundaries in seis-
within the platform. This interpretation is feasible but mic stratigraphy. This creates a definition problem where
unsatisfactory on two counts: the sequence boundary is drowning is not preceded by exposure (examples in Wendte
seismically nearly invisible and a major unconformity and et al., 1992; Erlich et al.,1990; Moldovanyi et al., 1995; Saller
stratigraphic turning point lies within a sequence. et al., 1993). The type-3 boundary, i.e. an unconformity
between a highstand tract and an overlying transgressive
To understand the origin of type-3 boundaries, we need to tract without intervening exposure but with intensive ma-
briefly discuss two processes: platform flooding or drown- rine erosion, avoids this problem. It also acknowledges the
ing, and current-amplification by sharp topography. The status of drowning unconformities as stratigraphic turning
sediment record of drowning is variable but always repre- points of the first order. In areas where the rate of sea-level
sents a major change in sediment input and dispersal. The fall in a eustatic cycle never matches the rate of subsidence,