TY - JOUR
T1 - Depositional environment, organic matter sources, and variable 17α(H)-diahopane distribution in Early Permian samples, southern Sydney Basin, Australia
AU - Baydjanova, Shirin
AU - George, Simon C.
PY - 2019/5
Y1 - 2019/5
N2 - Early Permian strata exposed in the southern part of the Sydney Basin,
NSW, Australia, include the Pebbley Beach Formation, the Snapper Point
Formation and the Wandrawandian Siltstone. These formations were
deposited during the Late Paleozoic Ice Age, when Australia was close to
the South Pole. The Pebbley Beach and Snapper Point formations were
deposited in shallow marine to coastal environments, while the
Wandrawandian Siltstone was deposited in deeper water following a marine
transgression. However, opinions differ on the precise environmental
and climatic conditions that prevailed during their deposition. Sixteen
outcrop samples from these formations were collected and analysed to
determine their depositional conditions and the source(s) of their
organic matter.The biomarker and aromatic hydrocarbon distributions show that the rocks
are in the early to mid-oil window. Some aromatic parameters show that
the stratigraphically younger Wandrawandian Siltstone is more thermally
mature than the older Pebbley Beach Formation, which is interpreted to
be due to the geographic position of these outcrops in the Sydney Basin.
The samples have low to intermediate pristane/phytane ratios (from 1.4
to 3.4), showing variation from oxic to suboxic depositional conditions.
Most of the mudstones and siltstones sampled were deposited in a marine
environment, with some fluvial-deltaic or estuarine influence, and
contain mixed type II and type III organic matter. The Pebbley Beach
formation is characterised by terpane distributions dominated by the C24 tetracyclic terpane, and to a lesser extent the C19 tricyclic terpane. The Snapper Point Formation has a mix of C19 and C23 tricyclic terpanes and C24 tetracyclic terpane, while the Wandrawandian Siltstone is dominated by C19, C21, and C23 tricyclic terpanes. The sterane distribution follows a C29 > C28 > C27
pattern in the majority of the samples, including in the marine
deposited Wandrawandian Siltstone. The three formations have very
variable relative amounts of diahopanes and other rearranged hopanes,
with the highest C30*/C30 αβ hopane ratio (6.9) in
the Wandrawandian Siltstone. This formation also has high amounts of
diasteranes compared to the other two formations, but the relative
amounts of diasteranes are not as high as for the diahopanes.
Classically, this suggests a clay-rich sediment in an oxic or suboxic,
acid-catalysed depositional environment, with enhanced diagenetic
alteration of the biomarkers. The Wandrawandian Siltstone was deposited
in suboxic conditions, not unusual to many rocks that contain much less
rearranged hopanes. Unusual characteristics of the Wandrawandian
Siltstone include significant slumping and soft sediment deformation,
and deposition under periglacial-glacial conditions. The occurrence of
the slumps on the continental slope led to significant sediment
overturn, and this may have resulted in enhanced diagenetic and
catalytic rearrangement reactions, leading to the elevated levels of
diahopanes and other rearranged hopanes. The disequilibrium between the
relative abundances of diahopanes and diasteranes suggests different
formation mechanisms for these biomarkers.
AB - Early Permian strata exposed in the southern part of the Sydney Basin,
NSW, Australia, include the Pebbley Beach Formation, the Snapper Point
Formation and the Wandrawandian Siltstone. These formations were
deposited during the Late Paleozoic Ice Age, when Australia was close to
the South Pole. The Pebbley Beach and Snapper Point formations were
deposited in shallow marine to coastal environments, while the
Wandrawandian Siltstone was deposited in deeper water following a marine
transgression. However, opinions differ on the precise environmental
and climatic conditions that prevailed during their deposition. Sixteen
outcrop samples from these formations were collected and analysed to
determine their depositional conditions and the source(s) of their
organic matter.The biomarker and aromatic hydrocarbon distributions show that the rocks
are in the early to mid-oil window. Some aromatic parameters show that
the stratigraphically younger Wandrawandian Siltstone is more thermally
mature than the older Pebbley Beach Formation, which is interpreted to
be due to the geographic position of these outcrops in the Sydney Basin.
The samples have low to intermediate pristane/phytane ratios (from 1.4
to 3.4), showing variation from oxic to suboxic depositional conditions.
Most of the mudstones and siltstones sampled were deposited in a marine
environment, with some fluvial-deltaic or estuarine influence, and
contain mixed type II and type III organic matter. The Pebbley Beach
formation is characterised by terpane distributions dominated by the C24 tetracyclic terpane, and to a lesser extent the C19 tricyclic terpane. The Snapper Point Formation has a mix of C19 and C23 tricyclic terpanes and C24 tetracyclic terpane, while the Wandrawandian Siltstone is dominated by C19, C21, and C23 tricyclic terpanes. The sterane distribution follows a C29 > C28 > C27
pattern in the majority of the samples, including in the marine
deposited Wandrawandian Siltstone. The three formations have very
variable relative amounts of diahopanes and other rearranged hopanes,
with the highest C30*/C30 αβ hopane ratio (6.9) in
the Wandrawandian Siltstone. This formation also has high amounts of
diasteranes compared to the other two formations, but the relative
amounts of diasteranes are not as high as for the diahopanes.
Classically, this suggests a clay-rich sediment in an oxic or suboxic,
acid-catalysed depositional environment, with enhanced diagenetic
alteration of the biomarkers. The Wandrawandian Siltstone was deposited
in suboxic conditions, not unusual to many rocks that contain much less
rearranged hopanes. Unusual characteristics of the Wandrawandian
Siltstone include significant slumping and soft sediment deformation,
and deposition under periglacial-glacial conditions. The occurrence of
the slumps on the continental slope led to significant sediment
overturn, and this may have resulted in enhanced diagenetic and
catalytic rearrangement reactions, leading to the elevated levels of
diahopanes and other rearranged hopanes. The disequilibrium between the
relative abundances of diahopanes and diasteranes suggests different
formation mechanisms for these biomarkers.
KW - Permian
KW - Australia
KW - Biomarker
KW - Diahopane
KW - Rearranged hopane
KW - Diasterane
UR - http://www.scopus.com/inward/record.url?scp=85064192872&partnerID=8YFLogxK
U2 - 10.1016/j.orggeochem.2019.03.008
DO - 10.1016/j.orggeochem.2019.03.008
M3 - Article
VL - 131
SP - 60
EP - 75
JO - Organic Geochemistry
JF - Organic Geochemistry
SN - 0146-6380
ER -