@misc{10481/81328,
year = {2023},
month = {1},
url = {https://hdl.handle.net/10481/81328},
abstract = {Glaucony occurrences have been reported both from exposed transgressive and overlying
highstand system tracts. However, its occurrences within highstand deposits are often invoked as the
result of underlying condensed section reworking. Detailed textural, mineralogical and geochemical
reports of glaucony grains in highstand deposits remain elusive. The northern Gulf of Cadiz shelf
(SW Iberia) offers a unique opportunity to investigate late Holocene glaucony authigenesis in a
well-documented time-stratigraphic context, where transgressive deposits are locally exposed on
the seafloor and are laterally draped by highstand muddy deposits. In this study, glaucony grains
extracted from a core retrieved from a highstand muddy depocenter off the Guadiana River were
investigated by means of digital microscopy, X-ray diffraction (XRD), and electron microscopic
methods (FESEM-EDX and TEM-HRTEM). To better constrain the glaucony origin (autochthonous vs.
allochthonous) in highstand muddy deposits, glaucony grains from surficial samples—taken from
exposed transgressive deposits—were also investigated. Glauconitization in the studied core can
be largely attributed to the replacement of faecal pellets from c. ~4.2–1.0 cal. ka BP. Both XRD and
TEM-HRTEM analyses indicate that glaucony consists mainly of an R1, with a minor presence of R0,
smectite-rich (nontronite) glauconite-smectite mixed-layer silicate, made up of 35–75% glauconitic
layers and 65–25% of interstratified smectite layers. At the mineral lattice level, minor individual 7Å
layers (berthierine) were also identified by HRTEM. Shallow radial cracks at the pellet surface, along
with globular and vermiform-like biomorphic to low packing density lamellar-flaky nanostructures,
mineralogical properties, and K-poor content (average 0.4 atoms p.f.u.) indicate a scarcely mature
glauconitization process, attesting to formation of the grains in situ (autochthonous). Glaucony grains
from exposed transgressive deposits, i.e., in the tests of calcareous benthic foraminifera, do not share
a genetic relationship with the grains investigated in the highstand deposits, thus supporting the
autochthonous origin of glaucony within the highstand deposits. Our combined dataset provides
evidence of a multiphase history for autochthonous glaucony formation in the Guadiana shelf, as its
genesis is traced to both transgressive and highstand conditions. While eustatic sea-level changes
favoured glaucony formation under transgressive conditions, factors such as protracted low sediment
supply and the establishment of a strong nutrient-rich upwelling system in the study area promoted
glaucony development during late Holocene highstand conditions.},
organization = {Fundacao para a Ciencia e a Tecnologia (FCT)	PDCTM/P/MAR/15289/1999},
organization = {Spanish Government	CGL2011-30302-C02-02},
organization = {MCIN/AEI	FJC2021-047046-I},
publisher = {MDPI},
keywords = {Glaucony},
keywords = {Sequence-stratigraphy},
keywords = {HST},
keywords = {Continental shel},
keywords = {Gulf of Cádiz},
keywords = {Holocene},
title = {Holocene Glaucony from the Guadiana Shelf, Northern Gulf of Cadiz (SW Iberia): New Genetic Insights in a Sequence Stratigraphy Context},
author = {López Quirós, Adrián and Lobo, Francisco José and Mendes, Isabel and Nieto García, Fernando},
}