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DTSTAMP:20260609T120043Z
SUMMARY:Geology Weekly 5050: Dr. John Holbrook, Texas Christian University
DESCRIPTION;ENCODING=QUOTED-PRINTABLE:Introduction 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=0D=0A=
from older strata, along which there is evidence of subaerial erosional truncation … or subaerial exposure,=0D=0A=
with a significant hiatus indicated (Van Wagoner et al., 1988).” This subaerial component of sequence=0D=0A=
boundaries (subaerial unconformity—SU) is also broadly considered to form as a topographic surface of=0D=0A=
sediment bypass, carved during relative sea level fall and buried by backfilling during relative sea level rise.=0D=0A=
Accordingly, the SU is commonly presumed to record an approximate time barrier, which separates older=0D=0A=
from younger strata along its full length. In this paper we show that regional composite scour (RCS) surfaces=0D=0A=
that are traditionally mapped as an integral component of the SU were never a single subaerial topographic=0D=0A=
surface characterized by sediment bypass, are not unconformities, do not record an effective time barrier, and=0D=0A=
form diachronously at the channel-belt scale over the entire fall to rise of a base-level cycle. These RCS=0D=0A=
surfaces, and by inference the SU surfaces they comprise, thus do not fully fit key defining characteristics=0D=0A=
embodied in the conceptual sequence boundary.=0D=0A=
Flume observations and field data show that the RCS is buried by fluvial sediment simultaneously as it is=0D=0A=
scoured. Accordingly, the RCS is perennially covered with stored sediment during formation, is only exposed=0D=0A=
as a subaerial topographic surface at the local place and time where it is undergoing active growth, and forms=0D=0A=
over the duration of local marine drainage during a relative sea-level cycle. This “cut-and-cover” model=0D=0A=
differs greatly from more established “bypass” models, which assume that the RCS was roughly sediment free=0D=0A=
and subaerially exposed for long durations of incision during regression and thus preserves a significant=0D=0A=
depositional hiatus upon later burial. Instead, the RCS may commonly and locally record a hiatus more typical=0D=0A=
of a facies-bounding diastem without a lacuna significantly greater than that of surfaces within the strata it=0D=0A=
binds. Fragments of fluvial strata may commonly and sporadically be preserved above the RCS that are older=0D=0A=
than underlying marine units overrun by this surface. Consequently, the RCS is not an effective time barrier.=0D=0A=
Lateral planation by migrating and avulsing channels as the RCS expands laterally after nucleation can place=0D=0A=
younger fluvial strata over much older units, which means that this surface is also composite and highly=0D=0A=
diachronous laterally at the scale of channel belts.=0D=0A=
The cut-and-cover model has additional implications not captured by the bypass model. First, significant=0D=0A=
sediments may be stored within fluvial strata above the RCS during regression that are not available for=0D=0A=
contemporary falling stage and lowstand marine shorelines. This can result in marine sediment starvation,=0D=0A=
particularly of the sand fraction, and in extreme cases can result in sand autodetachment and an absence of=0D=0A=
regressive marine reservoir sandstones. Second, cutting of the RCS co-generates a suprafluvial surface above=0D=0A=
the covering fluvial strata during regression that may be used as a mappable proxy for the conceptual=0D=0A=
maximum regressive surface (MRS). The MRS may be raised above this surface locally by lowaccommodation=0D=0A=
aggradation during lowstand normal regression, but in either case preserves an approximate=0D=0A=
time line where not reworked during later transgression. Third, valley development across the RCS does not=0D=0A=
exclusively form by landward knickpoint growth, and may include complexly formed and potentially crosscutting=0D=0A=
buffer valleys. SU valley incision can be divided into four modes, which include denudation,=0D=0A=
structural, buttress, and buffer valley components, which may work together locally and tend to have=0D=0A=
variable importance along the shore-to-hinterland profile.=0D=0A=
Although the RCS is not a good rock proxy for the conceptual sequence boundary it remains a very mappable=0D=0A=
surface which may separate facies of potentially very different origin and reservoir quality. The RCS is also=0D=0A=
inseparable from the SU and typically the only terrestrial erosional surface of extent in most short-duration=0D=0A=
sequences. Its nullification as a sequence boundary would mean abandonment of depositional sequence=0D=0A=
stratigraphy as a correlation and interpretive tool within these sections. An alternative to abandonment of=0D=0A=
the SU as a sequence boundary is to loosen the definition of a sequence boundary to ‘a discrete surface of=0D=0A=
erosional truncation carved landward of contemporary shorelines that is traceable beyond the scale of a=0D=0A=
single valley or comparable local system, and its correlative surfaces of conformity and/or non-deposition’,=0D=0A=
and continue its use as before.=0D=0A=

LOCATION:BSB E231
DTSTART;TZID=US_Central:20130208T150000
DTEND;TZID=US_Central:20130208T171500
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