The Cretaceous–Paleogene (K/Pg) boundary marks one of the major crises in the
history of life on Earth. The cause is widely regarded as a large bolide impact at
Chicxulub, Mexico, coincident with a major series of volcanic eruptions on the Deccan
Plateau, India. Fieldwork in the Brazos River area of Texas has involved an
investigation of the sections on the Brazos River and its tributaries. A previously
overlooked K/Pg section (RBS) on the Brazos River was found and contains the most
accessible and complete K/Pg boundary succession in the area. The RBS succession
provides a clear exposure of the various lithological units within the Paleocene and was
used to correlate to the successions in the nearby creeks. The K/Pg boundary is also
well-exposed and records an erosional relief of ~1 m, cut into the Maastrichtian
mudstone succession, creating a mounded topography. The overlying ‘Event Bed’,
containing reworked impact spherules at its base, is shown to infill troughs on this
irregular surface. The same features were recorded in tributary creeks, with all previous
descriptions of these locations clearly failing to recognise the various sedimentary
relationships.
Distinct, thin, yellow clay horizons within the uppermost Maastrichtian mudstones are
present in some sections. Geochemical analysis and radiometric dating have confirmed
these as volcanic ashes, with extracted zircons giving a date of 65.95+0.04 Ma. These
ash bands are located in Maastrichtian mudstones just below the K/Pg boundary and the
recorded date is, within error, that of the K/Pg boundary. This identification of this
latest Maastrichtian volcanic ash negates the suggestion of a pre-K/Pg boundary impact,
a pre-extinction impact or multiple impacts.
The benthic foraminiferal data generated indicates significant changes in palaeoecology
of the benthic foraminifera across the K/Pg boundary. The benthic foraminifera do not
experience a mass extinction, unlike the planktic foraminifera, which were significantly
affected by the end-Maastrichtian extinction event. The benthic foraminferal
assemblage appears to only experience transient, short-lived changes with pulses of
agglutinated, elongate and large species in the early Paleocene.
Mono-specific samples of Lenticulina rotulata have been analysed for stable isotopes
and the data may indicate the presence cyclicity across the K/Pg boundary interval. In
the earliest Paleocene significant negative δ18O excursions near the Pα/P1a and
NP1/NP2 boundary represents a potential hyperthermal event that may be coeval with
the DAN-C2 and Lower C29n events respectively recorded at Gubbio, and in the
Atlantic Ocean.
A sequence stratigraphy package is determined based on the micropaleontology and
sedimentology in the Brazos River area. The latest Maastrichtian is marked by a sealevel
rise immediately before the K/Pg boundary. Immediately after the K/Pg event, sealevel
fell and is recorded as a change from mid to inner shelf. The condensed unit of the
Middle Sandstone Bed (MSB) represents a Transgressive systems tract, with increasing
diversity and abundance of benthic foraminifera to the top of the MSB, where
maximum abundance and diversity is marks a Maximum Flooding Surface. The interval
above indicates sea-level continuing to rise to a mid to outer-shelf setting. These sealevel
changes are also recorded at the same stratigraphic level in Alabama, and at more
distal K/Pg boundary sections (e.g., Denmark and Tunisia) suggesting that these sealevel
changes are eustatically controlled.
Date of Award | 2014 |
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Original language | English |
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Awarding Institution | |
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Supervisor | Christopher Smart (Other Supervisor) |
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- Cretaceous
- Paleogene
- K/Pg
- Brazos River
- Texas
- Foraminifera
Benthic foraminiferal change and depositional history across the Cretaceous – Paleogene (K/Pg) boundary in the Brazos River area, Texas
Leighton, A. D. (Author). 2014
Student thesis: PhD