New GSA Bulletin Articles Published Ahead of Print in November
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics studied this
month include the nature and dynamics of China and Tibet; major
reorganization of the Snake River modulated by passage of the Yellowstone
Hotspot; and Weddell Sea Embayment ice streams, Antarctica. You can find
these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Devonian to Triassic tectonic evolution and basin transition in the
East Kunlun−Qaidam area, northern Tibetan Plateau: Constraints from
stratigraphy and detrital zircon U−Pb geochronology
Jiaopeng Sun; Yunpeng Dong; Licheng Ma; Shiyue Chen; Wan Jiang
Abstract:
The late Paleozoic to Triassic was an important interval for the East
Kunlun−Qaidam area, northern Tibet, as it witnessed prolonged subduction of
the South Kunlun Ocean, a major branch of the Paleo-Tethys Ocean whose
closure led to the formation of Pangea. However, the geologic history of
this stage is poorly constrained due to the paucity of tectonothermal
signatures preserved during a magmatic lull. This article presents a set of
new provenance data incorporating stratigraphic correlation, sandstone
petrology, and zircon U−Pb dating to depict changes in provenance that
record multiple stages of topographic and tectonic transition in the East
Kunlun−Qaidam area over time in response to the evolution of the South
Kunlun Ocean. Devonian intra-arc rifting is recorded by bimodal volcanism
and rapid alluvial-lacustrine sedimentation in the North Qaidam Ultra
High/High Pressure Belt, whose sources include the Olongbuluke Terrane and
southern North Qaidam Ultra High/High Pressure Belt. Southward
transgression submerged the East Kunlun−Qaidam area during the
Carboniferous prior to the rapid uplift of the Kunlun arc, which changed
the provenance during the Early Permian. This shift in provenance for the
western Olongbuluke Terrane and thick carbonate deposition throughout the
North Qaidam Ultra High/High Pressure Belt in the late Early Carboniferous
indicate that the North Qaidam Ultra High/High Pressure Belt should have
been inundated, terminating an ∼95 m.y. erosion history. The closure of the
South Kunlun Ocean in the late Triassic generated a retroarc foreland along
the Zongwulong Tectonic Belt, which is represented by the development of a
deep-water, northward-tapering flysch deposystem that was supplied by the
widely elevated Kunlun−Qaidam−Olongbuluke Terrane highland. This new
scenario allows us to evaluate current models concerning the assembly of
northern Tibet and the tectonic evolution of the Paleo-Tethys Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36147.1/610043/Devonian-to-Triassic-tectonic-evolution-and-basin
Transition from slab roll-back to slab break-off in the central
Apennines, Italy: Constraints from the stratigraphic and
thermochronologic record
Maria Giuditta Fellin; Malwina San Jose; Claudio Faccenna; Sean D. Willett;
Domenico Cosentino ...
Abstract:
Stratigraphic and thermochronologic data are used to study the processes
that shaped the topography of the central Apennines of Italy. These are
part of a major, active mountain belt in the center of the Mediterranean
area, where several subduction zones control a complex topography. The
Apennines were shaped by contraction at the front of the accretionary wedge
overlying the subducting Adria microplate followed by extension at the
wedge rear in response to eastward slab roll-back. In the central
Apennines, intermontane extensional basins on the western flank rise
eastward toward the summit. We contribute with new data consisting of 28
(U-Th-Sm)/He and 10 fission track ages on apatites to resolve a complex
pattern of thermal histories in time and space, which we interpret as
reflecting the transitional state of the orogen, undergoing a two-phase
evolution related to initial slab retreat, followed by slab detachment.
Along the Tyrrhenian coast, we document cooling from depths ≥3−4 km
occurring between 8 and 5 Ma and related to the opening of marine
extensional basins. Post−5 Ma, a broader region of the central Apennines
exhibits cooling from variable depths, between <2 km in most areas and
≥3−4 km in the northeast, and with different onset times: at ca. 4 Ma in
the west, at ca. 2.5 Ma in the center and northeast, and at ca. 1 Ma in the
southeast. Between 5 and 2.5 Ma, exhumation is associated with modest
topographic growth during the late stages of thrusting. Since 2.5 Ma,
exhumation has concurred with the opening of intermontane basins in the
west and in the east, with regional topographic growth and erosion, that we
interpret to be associated with the locally detaching slab.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36123.1/609918/Transition-from-slab-roll-back-to-slab-break-off
Recalibrating the Devonian time scale: A new method for integrating
radioisotopic and astrochronologic ages in a Bayesian framework
Claire O. Harrigan; Mark D. Schmitz; D. Jeffrey Over; Robin B. Trayler;
Vladimir I. Davydov
Abstract:
The numerous biotic, climatic, and tectonic events of the Devonian cannot
be correlated and investigated without a well-calibrated time scale. Here,
we updated the calibration of the Devonian time scale using a Bayesian
age-depth model that incorporates radioisotopic ages and astrochronology
durations. We used existing radioisotopic ages collected and harmonized in
the last two geologic time scale compilations, as well as new U-Pb zircon
ages from Emsian {Hercules I K-bentonite, Wetteldorf, Germany: 394.290 ±
0.097(0.21)(0.47) Ma} and Eifelian K-bentonites {Tioga B and Tioga F
K-bentonites, Fayette, New York, USA: 390.82 ± 0.18(0.26)(0.48) Ma and
390.14 ± 0.14(0.23)(0.47) Ma, respectively}. We anchored floating
astrochronology stage durations on radioisotopic ages and chained
astrochronologic constraints and uncertainty together to extrapolate
conditioning age likelihoods up or down the geologic time scale, which is a
new method for integrating astrochronology into age-depth modeling. The
modeling results in similar ages and durations for Devonian stages
regardless of starting biostratigraphic scaling assumptions. We produced a
set of rescaled biostratigraphic zonations, and a new numerical calibration
of Devonian stage boundary ages with robust uncertainty estimates, which
allow us to evaluate future targets for Devonian time scale research. These
methods are broadly applicable for time scale work and provide a template
for an integrated stratigraphic approach to time scale modeling.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36128.1/609662/Recalibrating-the-Devonian-time-scale-A-new-method
The protoliths of central Himalayan eclogites
Guibin Zhang; Jiaxing Wang; A. Alexander G. Webb; Lifei Zhang; Shuaiqi Liu
...
Abstract:
Eclogites represent the highest pressure conditions yet observed from rocks
thrust to the surface in the central Himalaya. A detailed investigation of
the protolith nature of these eclogites is needed to better understand
pre-Himalayan geological history. Retrogressed eclogites were collected
from Thongmön (Dingri County) and Riwu (Dinggye County), central Himalaya,
China. We investigated the bulk rock major and trace elements, Sr-Nd
isotopes, zircon geochronology, and Hf-O isotopes. These retrogressed
eclogites experienced five stages of metamorphic evolution from prograde
amphibolite-facies to peak eclogite-facies, and high pressure
granulites-facies, granulites-facies then final amphibolite-facies
overprinting during exhumation. Geochemically, they are subalkaline basalts
with high FeO contents and a tholeiitic affinity; trace elements show
similarities with enriched mid-ocean ridge basalts. Bulk rocks have a wide
range of εNd(t) values from −0.24 to +7.08, and an unusually
wide range of initial 87Sr/86Sr ratios of
0.705961−0.821182. Zircon relict magmatic cores from both Thongmön and Riwu
eclogites yield a consistent protolith age of ca. 1850 Ma, with enriched
heavy rare earth element patterns and significant negative Eu anomalies.
These relict cores have oxygen isotopes signatures of δ18O =
5.8−8.1‰, εHf(t) values of −4.85 to +9.59, and two-stage model
ages (TDM2) of 1.91−2.81 Ga. Metamorphic overgrowth zircons
yield much younger ages of ca. 14 Ma. Integration of all of the above data
suggests that the protolith of these central Himalayan retrogressed
eclogites might be Proterozoic continental flood basalts of the North
Indian Plate, generated under a post-collisional extension setting during
the assembly of the Columbia Supercontinent. Occurrence of both
Neoproterozoic−early Paleozoic rocks and ca. 1.85 Ga rocks in the regional
crystalline rocks may reflect either unrecognized sub-horizontal Main
Central Thrust exposure(s) or exhumation of a deeply cut part of the
Greater Himalayan Crystalline complex. In combination with previous reports
of Late Cretaceous, Neoproterozoic, and similar but younger
Paleoproterozoic protolith, it is clear that the central Himalayan
eclogites originate from multiple sources of protolith.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36080.1/609663/The-protoliths-of-central-Himalayan-eclogites
Detrital geochronology and lithologic signatures of Weddell Sea
Embayment ice streams, Antarctica—Implications for subglacial geology
and ice sheet history
Liana M. Agrios; Kathy J. Licht; Trevor Williams; Sidney R. Hemming; Lauren
Welch ...
Abstract:
Tills from moraines adjacent to major ice streams of the Weddell Sea
Embayment contain distinct detrital zircon (n = 5304) and K-bearing mineral
age populations (n = 323) that, when combined with pebble composition data,
can be used to better understand Antarctica’s subglacial geology and ice
sheet history. Till representing the Institute, Foundation, Academy,
Recovery and Slessor Ice Streams each have distinct detrital zircon age
populations. Detrital Ar-Ar ages are mostly younger than zircon ages, and
distinctive populations include 270−300 Ma (Institute), 170−190 Ma
(Foundation), and 1200−1400 Ma (Recovery), which are not easily explained
by known outcrops. Pebble fractions of the Foundation and Academy tills are
most diverse with up to >40% exotic erratics. The southern side of the
Recovery Glacier has fossiliferous limestone erratics. Mixing models
created using a nonlinear squares curve-fitting approach were developed to
evaluate contributors of zircons to Foundation Ice Stream till. These model
results and pebble lithology data both indicate that unexposed (subglacial)
bedrock is mixed with exposed rocks to produce the observed till. Notably,
the model required limited local Patuxent Formation input to the Foundation
till’s zircon population. Our data suggest that sandstones underlie the
Foundation Ice Stream and Recovery Glacier troughs, which has a bearing on
basal ice flow conditions and results in geological controls on ice stream
location. This geo- and thermo-chronological characterization of the ice
streams will enable ice-rafted debris in Weddell Sea marine sediments to be
traced back to its sources and interpreted in terms of ice stream dynamics.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36117.1/609664/Detrital-geochronology-and-lithologic-signatures
Magmatism and related metamorphism as a response to mountain-root
collapse of the Dabie orogen: Constraints from geochronology and
petrogeochemistry of metadiorites
Yang Yang; Yi-Can Liu; Yang Li; C. Groppo; F. Rolfo
Abstract:
Post-collisional mountain-root collapse and subsequent massive partial
melting occurred in the high-temperature (HT) ultrahigh-pressure (UHP)
metamorphic terrane of the North Dabie complex zone (NDZ), central China.
The NDZ was deeply subducted in the Triassic, producing widespread
migmatites and various magmatic intrusions in the Cretaceous.
Post-collisional metadiorites with distinctive large K-feldspar augen
porphyroblasts, locally reported but rarely exposed in the NDZ, underwent a
complex evolutional history. In this contribution, integrated studies
including field investigation, petrographic observation and mineral
analysis, zircon U-Pb geochronological and Hf isotopic analyses, and
whole-rock elemental and Sr-Nd-Pb isotopic analyses of the metadiorites
were carried out. Our results provide new constraints on the mountain-root
collapse in the Dabie orogen. The metadiorites are enriched in large ion
lithophile elements and light rare earth elements, whereas they are
depleted in high field strength elements and heavy rare earth elements with
significant Ba positive anomalies, a composition consistent with the lower
continental crust. All the studied samples have moderately enriched initial 87Sr/86Sr ratios (0.707582−0.708099), low εNd(t)
values (−15.3 to −20.4), and low initial Pb isotopic ratios
(16.0978−16.8452, 15.3167−15.4544, and 37.1778−37.8397 for 206
Pb/204Pb, 207Pb/204Pb, and 208
Pb/204Pb, respectively). However, they have highly negative
εHf(t) values and Paleoproterozoic two-stage Hf model ages, which are only
partially consistent with data from the associated UHP metamorphic rocks.
Such features suggest the metadiorites resulted from a magma produced by
mixing of Triassic UHP mafic lithologies and minor amounts of
mantle-derived materials. Zircon morphological analysis and U-Pb sensitive
high-resolution ion microprobe dating combined with conventional
thermobarometry indicate that these upwelling melts crystallized at
pressure-temperature (P-T) conditions of 5.4−5.7 kbar and 750−768 °C at ca.
130 Ma and subsequently suffered HT metamorphism at ca. 125 Ma. We conclude
that the metadiorites’ precursors were derived from partial melting of the
Triassic subducted Neoproterozoic mafic lower-crustal rocks, with addition
of minor amounts of mantle-derived materials in the Early Cretaceous, in
response to mountain-root collapse of the orogen. Based on petrographic
textures and mineral compositions, it is moreover inferred that formation
of the distinctive K-feldspar porphyroblasts is likely related to a
two-stage process, i.e., crystallization derived from biotite breakdown
after the formation of the metadiorite at T = 640−703 °C and P < 4.5
kbar and coarsening related to shear deformation.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36194.1/609626/Magmatism-and-related-metamorphism-as-a-response
Major reorganization of the Snake River modulated by passage of the
Yellowstone Hotspot
Lydia M. Staisch; Jim E. O’Connor; Charles M. Cannon; Chris Holm-Denoma;
Paul K. Link ...
Abstract:
The details and mechanisms for Neogene river reorganization in the U.S.
Pacific Northwest and northern Rocky Mountains have been debated for over a
century with key implications for how tectonic and volcanic systems
modulate topographic development. To evaluate paleo-drainage networks, we
produced an expansive data set and provenance analysis of detrital zircon
U-Pb ages from Miocene to Pleistocene fluvial strata along proposed
proto-Snake and Columbia River pathways. Statistical comparisons of
Miocene-Pliocene detrital zircon spectra do not support previously
hypothesized drainage routes of the Snake River. We use detrital zircon
unmixing models to test prior Snake River routes against a newly
hypothesized route, in which the Snake River circumnavigated the northern
Rocky Mountains and entered the Columbia Basin from the northeast prior to
incision of Hells Canyon. Our proposed ancestral Snake River route best
matches detrital zircon age spectra throughout the region. Furthermore,
this northerly Snake River route satisfies and provides context for shifts
in the sedimentology and fish faunal assemblages of the western Snake River
Plain and Columbia Basin through Miocene−Pliocene time. We posit that
eastward migration of the Yellowstone Hotspot and its effect on thermally
induced buoyancy and topographic uplift, coupled with volcanic
densification of the eastern Snake River Plain lithosphere, are the primary
mechanisms for drainage reorganization and that the eastern and western
Snake River Plain were isolated from one another until the early Pliocene.
Following this basin integration, the substantial increase in drainage area
to the western Snake River Plain likely overtopped a bedrock threshold that
previously contained Lake Idaho, which led to incision of Hells Canyon and
establishment of the modern Snake and Columbia River drainage network.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36174.1/609554/Major-reorganization-of-the-Snake-River-modulated
Apatite fingerprints on the magmatic-hydrothermal evolution of the
Daheishan giant porphyry Mo deposit, NE China
Pan Qu; Wubin Yang; Hecai Niu; Ningbo Li; Dan Wu
Abstract:
Porphyry deposits are the main source for global Cu and Mo production. The
generation of hydrous silicate magmas and subsequent separation of
volatile-rich magmatic fluids with hydrothermal alteration are significant
processes leading to the formation of porphyry deposits. However, a
specific understanding of these processes has been limited by a lack of
direct mineralogical records in the evolving magmatic-hydrothermal system.
In this paper, we present an integrated textural and geochemical
investigation on apatite from the giant Daheishan porphyry Mo deposit in NE
China, illustrating that apatite can be a potential recorder of the
magmatic-hydrothermal evolution of porphyry systems. Apatite from the
ore-forming porphyry displays distinctive core-rim textures, with melt
inclusions in the resorption cores (Type-A1) and co-existing of melt and
fluid inclusions in the euhedral rims (Type-A2), indicating a
magmatic-hydrothermal origin of apatite. This is also supported by both
chemical and isotopic compositions obtained by in situ analyses using laser
ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and
LA-multi collector-ICP-MS. The late Type-A2 apatite is relatively enriched
in incompatible elements, such as rare earth elements (REE) and Th, but
slightly depleted in fluid-mobile elements such as Na and S, compared to
the early Type-A1 apatite. Relatively homogeneous (87Sr/ 86Sr)i ratios (0.70436−0.70504) of the Type-A1 and
Type-A2 apatites indicate that they were formed in a relatively closed
system without detectable contamination. Meanwhile, some apatite in the
wall rock (biotite granodiorite) shows characteristics of secondary altered
textures, resulting from the intensive alteration by hydrothermal fluids
exsolved from the porphyry system. Apatite trapped in mineral phenocrysts
of the wall rock is usually unaltered (Type-B1 apatite), with clear
oscillatory growth zones in cathodoluminescence (CL) images. In contrast,
the intergranular apatite is commonly altered (Type-B2 apatite), with
chaotic zoning in CL images, abundant micro-fractures and secondary fluid
inclusions. Compositionally, the Type-B2 apatite shows notable tetrad REE
patterns, relatively lower light-REE and S contents, and elevated 147Sm/144Nd ratios compared to the Type-B1 apatite.
LA-ICP-MS U-Pb dating yields a lower intercept age of 171.4 ± 2.3 Ma for
Type-B2, which is consistent with the age of 171.5 ± 2.4 Ma for Type-A2,
but is notably younger than the Type-B1 apatite (175.5 ± 1.3 Ma). It is
suggested that the Type-B2 apatite has been significantly reset by
hydrothermal fluids exsolved from the porphyry system. Therefore, we
conclude that the textures and geochemistry of apatite in porphyry systems
can be used as a potential proxy for recording fluid exsolution and
hydrothermal alteration processes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36093.1/609555/Apatite-fingerprints-on-the-magmatic-hydrothermal
Rapid eruption of the Emeishan continental flood basalts: New
paleomagnetic and geochronologic constraints
Yingchao Xu; Liandi Zheng; Zhenyu Yang; Ya-Bo Tong; Bin Wang ...
Abstract:
The duration of the eruption of the Emeishan large igneous province is
hotly debated. We conducted a magnetostratigraphic and geochronological
study of the core area of the large igneous province in the Binchuan area
of Yunnan Province, southwestern China, in order to constrain the duration
of the eruption. The results of detailed thermal demagnetization
experiments revealed two remanent magnetic components from the volcanic
rocks of 11 composite sections. A low-temperature component separated below
300 °C is interpreted as a recent viscous remanence. Additionally, reliable
characteristic remanent magnetizations were revealed above 400 °C, with
unblocking temperatures up to 580−680 °C, which passed the fold test and
record three magnetozones. Zircons from the felsic ignimbrites exposed in
the final stage of the mafic volcanism are dated to 258.2 ± 0.7 Ma ( n = 15; mean square of weighted deviates = 1.3) by sensitive
high-resolution ion microprobe. Stratigraphic and magnetostratigraphic
correlations of the Emeishan basalts in the Binchuan sections indicate that
the eruption of the mafic rocks of the Emeishan large igneous province can
be clearly divided into early (reverse polarity subzone), middle (normal
polarity subzone), and late (reverse polarity subzone) stages, with a total
duration of less than 1.7 m.y. (260.8−259.1 Ma). However, by combining this
chronology with previously reported conodont biostratigraphic results from
locations around the Emeishan large igneous province, and comparing the
dominant normal-reverse polarity sequence in the Emeishan large igneous
province with the geomagnetic polarity time scale, we obtain a much shorter
duration of the main eruptive stage of <0.8 m.y. (260.4−259.6 Ma). About
three quarters of the basalts of the Emeishan large igneous province record
have a normal polarity and erupted within 0.4 m.y., while the other
quarter, mainly distributed in the central zone, shows a reverse polarity
and much shorter duration. Given the short duration of the eruption, gas
volatiles would have been released into the atmosphere at high rates, which
might provide a causal link between the rapid eruption and the
end-Guadalupian mass extinction. Before the mantle plume eruption,
localized eruptions probably occurred. After eruption of the mafic Emeishan
flood basalts, an acid volcanic eruption occurred in the early
Wuchiapingian, which was sporadically distributed in the Emeishan large
igneous province.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36132.1/609535/Rapid-eruption-of-the-Emeishan-continental-flood
Constraining the timing of Arabia-Eurasia collision in the Zagros
orogen by sandstone provenance (Neyriz, Iran)
Parisa Gholami Zadeh; Xiumian Hu; Eduardo Garzanti; Mohammad Hossein Adabi
Abstract:
The Zagros orogen, formed by the collision of the Arabian and Eurasian
continental margins, represents one of the largest and richest oil and gas
provinces in the world. The Zagros fold-thrust belt records collision and
convergence along the Neotethys suture zone. By coupling field
observations, sandstone modal analysis, U-Pb zircon dating, and Hf isotopic
data from the Upper Cretaceous to Pliocene sedimentary succession of the
Neyriz region, this paper documents several major provenance changes that
allow us to propose a refined scenario for the Zagros orogeny. An
ophiolitic complex dated by detrital-zircon U-Pb geochronology as ca. 95 Ma
provided detritus to Upper Cretaceous-Paleocene strata deposited along the
northeastern margin of the Arabian lower plate (ophiolite provenance). Yet,
on the southwestern margin of the Eurasian upper plate, upper
Paleocene-lower Eocene strata indicate provenance from Mesozoic magmatic
rocks yielding zircons dated as ca. 240 Ma and 170 Ma as well as the
recycling of clastic rocks. Since the early Miocene, the sedimentary basin
located on the Arabian plate received both ophiolitic detritus and
magmatic-arc, recycled clastic, and axial-belt metamorphic detritus from
Eurasia. U-Pb ages of detrital zircons reflect polyphase magmatism at 170
Ma, 95 Ma, and 40 Ma on the Eurasian active margin. Our results indicate
that progressive accretion, uplift, and exhumation of the Zagros orogen was
well under way by the beginning of the Miocene in the Neyriz region.
Literature data from adjacent regions suggest that the Arabia/Eurasia
collision may have occurred diachronously and later in the Kermanshah and
Lurestan areas to the north.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35950.1/609485/Constraining-the-timing-of-Arabia-Eurasia
Age, material source, and formation mechanism of bedding-parallel
calcite beef veins: Case from the mature Eocene lacustrine shales in
the Biyang Sag, Nanxiang Basin, China
Ao Su; Paul D. Bons; Honghan Chen; Yue-xing Feng; Jian-xin Zhao ...
Abstract:
The mechanisms leading to the formation of bedding-parallel calcite beef
veins have been widely debated, with discussions centered on timing or
burial depth of vein generation, source of the vein material, driving
forces for vein initiation and widening, and growth direction and
mechanism. To address these issues, a comprehensive study of drill core
samples containing beef veins in the mature Eocene lacustrine Hetaoyuan
Formation in the Biyang Sag, Nanxiang Basin, China was undertaken through a
combination of microstructural observation, isotopic geochronological,
geochemical, and fluid inclusion analyses, as well as basin modeling. X-ray
diffraction and total organic carbon content analyses indicate that most of
the beef veins accumulated in calcite-rich laminated shales with high
organic matter contents. These beef veins yielded an absolute laser
ablation−multi-collector−inductively coupled plasma−mass spectrometry U-Pb
age of ca. 41.02 ± 0.44 Ma, which corresponds to a burial depth of 500−800
m. Such a shallow burial depth suggests that the full compaction and
consolidation of sediments would not yet have been achieved, which is
compatible with the following observations: (1) plastic deformation of
shale laminae adjacent to the veins, and (2) a beef vein formation
temperature of ∼59 °C derived from fluid-inclusion microthermometry. The
radio-isotopic age of the beef veins is ∼1−3 m.y. younger than the
stratigraphic age of the host rock (ca. 43.1 Ma) but earlier than the
model-derived timing of oil generation (ca. 35.8 Ma) and tectonic extrusion
(ca. 23.0−13.0 Ma). The beef vein formation predated bacterial sulfate
reduction, as evidenced by crosscutting relationships with carbonate
concretions, pyrite framboids, and apatite pellets. A two-stage formation
model for these beef veins is proposed. When burial depth of laminated
shales rich in organic matter and calcite reaches the methanogenic zone,
overpressure triggered by biogenetic gas generation results in horizontal
hydrofracturing, initiating cracks that act as gas expulsion pathways. Once
all the generated gas has migrated, the opened fractures close again due to
overburden load. The materials fed by pressure solution of host-rock
calcite fractions then mobilized into the unhealed horizontal fractures by
diffusion. Subsequently, by a force of crystallization, antitaxial,
displacive growth of calcite fibers commenced, contemporaneous with
fracture dilation, eventually leading to the formation of bedding-parallel
beef veins.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35866.1/609486/Age-material-source-and-formation-mechanism-of
Linking sediment supply variations and tectonic evolution in deep time,
source-to-sink systems—The Triassic Greater Barents Sea Basin
Albina Gilmullina; Tore Grane Klausen; Anthony George Doré; Valentina
Marzia Rossi; Anna Suslova ...
Abstract:
Triassic strata in the Greater Barents Sea Basin are important records of
geodynamic activity in the surrounding catchments and sediment transport in
the Arctic basins. This study is the first attempt to investigate the
evolution of these source areas through time. Our analysis of sediment
budgets from subsurface data in the Greater Barents Sea Basin and
application of the BQART approach to estimate catchment properties shows
that (1) during the Lower Triassic, sediment supply was at its peak in the
basin and comparable to that of the biggest modern-day river systems, which
are supplied by tectonically active orogens; (2) the Middle Triassic
sediment load was significantly lower but still comparable to that of the
top 10 largest modern rivers; (3) during the Upper Triassic, sediment load
increased again in the Carnian; and (4) there is a large mismatch (70%)
between the modeled and estimated sediment load of the Carnian. These
results are consistent with the Triassic Greater Barents Sea Basin
succession being deposited under the influence of the largest volcanic
event ever at the Permian-Triassic boundary (Siberian Traps) and concurrent
with the climatic changes of the Carnian Pluvial Event and the final stages
of the Northern Ural orogeny. They also provide a better understanding of
geodynamic impacts on sedimentary systems and improve our knowledge of
continental-scale sediment transport. Finally, the study demonstrates
bypass of sediment from the Ural Mountains and West Siberia into the
adjacent Arctic Sverdrup, Chukotka, and Alaska Basins in Late Carnian and
Late Norian time.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36090.1/609416/Linking-sediment-supply-variations-and-tectonic
Effects of sea-level variation and sedimentary noise variation on the
development of biogenic reefs since the Pliocene among the Xisha
Islands, South China Sea
Ke Zhao; Xuebin Du; Jixin Jia; Xinong Xie; Fang Hao ...
Abstract:
Carbonate strata are a vital and favorable reservoir for global oil and gas
exploration, and carbonate sedimentary systems record ancient oceanic and
paleoclimatic conditions, including paleoenvironmental variations
throughout geologic periods. Carbonate platforms are widely distributed
among the Xisha Islands in the South China Sea and contain large amounts of
oil and gas resources. Biogenic reefs are the dominant parts of the
carbonate platforms in the Xisha Islands; however, research on the factors
that control and affect biogenic reef development is lacking. In this
study, a core from well XK-1, which is located on Yongxing Island in the
Xisha Islands, a sedimentary noise model, and time-series analyses were
used to determine the effects of sea-level fluctuations from 5.3 Ma to
present. The results show that coral reefs in the Xisha Islands are
sensitive to eustatic fluctuations and that a decrease in sea level
essentially corresponds to an increase in sedimentation rate. Indexes of
the East Asian monsoon and other environmental indexes show that the
Pleistocene and Holocene were suitable for coral growth; however, the
trends shown by these indexes and the sea-level variation indicate that the
future growth of coral reefs will be at a disadvantage. Research on the
controlling factors of biogenic reefs is of significance for understanding
reef growth, performing global reef comparisons, and encouraging the future
protection of coral reefs.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36138.1/609371/Effects-of-sea-level-variation-and-sedimentary
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