Geology Weekly 5050: Dr. John Holbrook, Texas Christian University

DateFebruary 8, 2013
Time3:00 - 5:15 pm
LocationBSB E231
DescriptionIntroduction for a Familiar Stranger: Rethinking Sequence Boundaries, Bypass, and Time in the Rock

The sequence-bounding unconformity bears the key defining traits of being “a surface separating younger
from older strata, along which there is evidence of subaerial erosional truncation … or subaerial exposure,
with a significant hiatus indicated (Van Wagoner et al., 1988).” This subaerial component of sequence
boundaries (subaerial unconformity—SU) is also broadly considered to form as a topographic surface of
sediment bypass, carved during relative sea level fall and buried by backfilling during relative sea level rise.
Accordingly, the SU is commonly presumed to record an approximate time barrier, which separates older
from younger strata along its full length. In this paper we show that regional composite scour (RCS) surfaces
that are traditionally mapped as an integral component of the SU were never a single subaerial topographic
surface characterized by sediment bypass, are not unconformities, do not record an effective time barrier, and
form diachronously at the channel-belt scale over the entire fall to rise of a base-level cycle. These RCS
surfaces, and by inference the SU surfaces they comprise, thus do not fully fit key defining characteristics
embodied in the conceptual sequence boundary.

Flume observations and field data show that the RCS is buried by fluvial sediment simultaneously as it is
scoured. Accordingly, the RCS is perennially covered with stored sediment during formation, is only exposed
as a subaerial topographic surface at the local place and time where it is undergoing active growth, and forms
over the duration of local marine drainage during a relative sea-level cycle. This “cut-and-cover” model
differs greatly from more established “bypass” models, which assume that the RCS was roughly sediment free
and subaerially exposed for long durations of incision during regression and thus preserves a significant
depositional hiatus upon later burial. Instead, the RCS may commonly and locally record a hiatus more typical
of a facies-bounding diastem without a lacuna significantly greater than that of surfaces within the strata it
binds. Fragments of fluvial strata may commonly and sporadically be preserved above the RCS that are older
than underlying marine units overrun by this surface. Consequently, the RCS is not an effective time barrier.
Lateral planation by migrating and avulsing channels as the RCS expands laterally after nucleation can place
younger fluvial strata over much older units, which means that this surface is also composite and highly
diachronous laterally at the scale of channel belts.

The cut-and-cover model has additional implications not captured by the bypass model. First, significant
sediments may be stored within fluvial strata above the RCS during regression that are not available for
contemporary falling stage and lowstand marine shorelines. This can result in marine sediment starvation,
particularly of the sand fraction, and in extreme cases can result in sand autodetachment and an absence of
regressive marine reservoir sandstones. Second, cutting of the RCS co-generates a suprafluvial surface above
the covering fluvial strata during regression that may be used as a mappable proxy for the conceptual
maximum regressive surface (MRS). The MRS may be raised above this surface locally by lowaccommodation
aggradation during lowstand normal regression, but in either case preserves an approximate
time line where not reworked during later transgression. Third, valley development across the RCS does not
exclusively form by landward knickpoint growth, and may include complexly formed and potentially crosscutting
buffer valleys. SU valley incision can be divided into four modes, which include denudation,
structural, buttress, and buffer valley components, which may work together locally and tend to have
variable importance along the shore-to-hinterland profile.

Although the RCS is not a good rock proxy for the conceptual sequence boundary it remains a very mappable
surface which may separate facies of potentially very different origin and reservoir quality. The RCS is also
inseparable from the SU and typically the only terrestrial erosional surface of extent in most short-duration
sequences. Its nullification as a sequence boundary would mean abandonment of depositional sequence
stratigraphy as a correlation and interpretive tool within these sections. An alternative to abandonment of
the SU as a sequence boundary is to loosen the definition of a sequence boundary to ‘a discrete surface of
erosional truncation carved landward of contemporary shorelines that is traceable beyond the scale of a
single valley or comparable local system, and its correlative surfaces of conformity and/or non-deposition’,
and continue its use as before.
PublisherDepartment of Geosciences
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