Early Cretaceous syn- to post-rift evolution of the Mentelle Basin on the southwest Australian rifted margin (IODP Expedition 369 Sites U1513–U1516)

<p>The Mentelle Basin is a large and deep-water sedimentary basin located on the southwest Australian rifted margin. The basin lies west of the Perth Basin, east of the Naturaliste Plateau and south of the Perth Abyssal Plain. The rifted margin formed when the Greater Indian plate separated from the Australian-Antarctic plate during the Jurassic to early Cretaceous. Based on seismic reflection data, several km thick sediments infilling the basin have been interpreted. However, due to lack of geological and geophysical data, the basin has not been studied enough to understand its evolution. In 2017, International Ocean Discovery Program (IODP) Expedition 369 drilled four sites, U1513–U1516, in the Mentelle Basin and recovered important cores including late Jurassic to Early Cretaceous sections. At Site U1515 on the eastern margin of the basin, drilling penetrated below the seismically imaged breakup unconformity into the middle Jurassic to earliest Cretaceous syn-rift strata. Holes at Site U1513 on the western margin cored the syn-rift volcanic sequence, the Hauterivian to early Aptian volcaniclastic-rich sandstone sequence spanning the syn- to post-rift phase, and the Aptian to Albian post-rift claystone sequence. Drilling at Sites U1514 and U1516 in the central part reached the Albian post-rift sequence. Using a combination of shipboard and post-expedition data, we interpret the lithological, paleontological and geochemical characteristics of the syn- to post-rift sequences. The results allowed us to reconstruct the Early Cretaceous stratigraphy, tectonics, paleo-environment, and basin evolution of the Mentelle Basin. During the syn-rift phase, the middle Jurassic to lower Cretaceous non-marine sediments were deposited in the eastern Mentelle Basin, while volcanic rocks were emplaced in the western part. The 82 m thick volcanic sequence consists of alternating basalt flows and volcaniclastics with dolerite dikes, which indicate multiple volcanic eruption events in subaerial to shallow water environments. It was overlain by the 235 m thick volcaniclastic-rich sequence consisting of massive or laminated sandstone layers, deposited in shelf to upper bathyal depths. The deposition period spans the syn- to post-rift phase of the basin but decreasing sedimentation rate and shallow marine setting suggest that the post-rift thermal subsidence did not immediately follow the final continental breakup. We interpret that the delayed thermal subsidence was likely to be induced by adjacent mantle plume activities. Deep marine claystone sequences blanketing most of the basin indicate Aptian to Albian post-rift thermal subsidence.</p>

LATE EOCENE BIOSTRATIGRAPHIC AGE FOR ANDESITIC VOLCANIC HOST ROCKS FORMED IN AN ISLAND ENVIRONMENT AT THE COBRE PANAMA PORPHYRY Cu-Mo-Au-Ag DEPOSIT, PANAMA

The giant Cobre Panama porphyry Cu-Mo-Au-Ag deposit in western Panama is hosted by an undated andesitic volcanic sequence, the Petaquilla batholith (32.20 ± 0.76–28.28 ± 0.61 Ma), and porphyry stocks (28.96 ± 0.62–27.48 ± 0.68 Ma). Here we present a biostratigraphic age for the volcanic sequence based on stratigraphically diagnostic large foraminifera from thin limestone beds within kilometer-thick andesitic rocks. These yield a late middle to late Eocene biostratigraphic age (41.2–33.9 Ma), with a probable late Eocene age (Priabonian stage, 37.8–33.9 Ma), which is slightly older than the age of the batholith and porphyry intrusions. The volcanic sequence is dominated by fine-grained, massive basalt to andesite lavas with subordinate volcaniclastic deposits. A preliminary description of volcanic textures based on macroscopic observation of drill core and quarry/road exposures supports the occurrence of lavas, fallout tuffs, volcanic breccias, and possible pyroclastic density current deposits. Rare polymictic conglomerates with well-rounded clasts of igneous rocks attest to minor sedimentary reworking from a nearby subaerial volcanic environment. The dated limestone that is interbedded with the submarine volcanic sequence was deposited in an estimated water depth of 50 to 80 m, probably in a middle- to outer-shelf large foraminiferal shoal. These results support deposition on the flank of an active volcanic island during early shallowing of the Isthmus of Panama. The Cobre Panama volcanic center is interpreted to have formed in the final stages of the latest Cretaceous-Eocene volcanic arc before, or possibly during, the 175-km sinistral offset of the Panama volcanic front in the late Eocene-Oligocene. However, it remains unclear whether the volcanic center formed on the western continuation of the San Blas-Chagres arc segment or the eastern termination of the Azuero-Soná arc segment and whether it was emplaced during broadening of the pre-Oligocene volcanic front or in a back-arc setting.

Stratigraphy and age of Karoo basalts of Lesotho and implications for correlations within the Karoo igneous province. Springbok Flats.xls

The Lesotho remnant contains the type succession for Karoo low-Ti basalts of central southern Africa. The ⁴⁰Ar/³⁹Ar dating indicates that the sequence was emplaced within a very short period at about 180 Ma and consists of a monotonous pile of compound basalt lava flows which lacks significant palaeosols and persistent sedimentary intercalations. We have used geochemistry to establish a stratigraphic subdivision of the lava pile. Thin units of basalt flows, the Moshesh's Ford, Golden Gate, Sani, Roma, Letele, and Wonderkop units, with diverse geochemical character and restricted geographical distribution, are present at the base of the succession. These are overlain by extensive units of compositionally more uniform basalt, the Mafika Lisiu, Maloti, Senqu and Mothae units, which build the bulk of the sequence.<p>Location of this section is described in Marsh et al. (1997) AGU Geophysical Monograph, 100, 247-272.</p> <p>Title of data set: Springbok Flats</p> <p>Location of Borehole RL1 (SF samples) – S24.9367 deg; E 28.3750 deg</p> <p>Location of Borehole RTL1 – S 24.4400 deg; E 29.1767 deg</p> <p>Location of Borehole WD4 – S 24.6483 deg; E 28.7450 deg</p> <p>Location of Borehole LB1 – S 24.8817 deg; E 28.5833 deg</p> <p>Borehole TF2 – base of volcanic sequence – 768m</p> <p>Borehole TF1 – base of volcanic sequence – 357m</p> <div>All Sr-, Nd- and Pb-isotope values are MEASURED values. </div>

Stratigraphy and age of Karoo basalts of Lesotho and implications for correlations within the Karoo igneous province. Springbok Flats.xls

The Lesotho remnant contains the type succession for Karoo low-Ti basalts of central southern Africa. The ⁴⁰Ar/³⁹Ar dating indicates that the sequence was emplaced within a very short period at about 180 Ma and consists of a monotonous pile of compound basalt lava flows which lacks significant palaeosols and persistent sedimentary intercalations. We have used geochemistry to establish a stratigraphic subdivision of the lava pile. Thin units of basalt flows, the Moshesh's Ford, Golden Gate, Sani, Roma, Letele, and Wonderkop units, with diverse geochemical character and restricted geographical distribution, are present at the base of the succession. These are overlain by extensive units of compositionally more uniform basalt, the Mafika Lisiu, Maloti, Senqu and Mothae units, which build the bulk of the sequence.<p>Location of this section is described in Marsh et al. (1997) AGU Geophysical Monograph, 100, 247-272.</p> <p>Title of data set: Springbok Flats</p> <p>Location of Borehole RL1 (SF samples) – S24.9367 deg; E 28.3750 deg</p> <p>Location of Borehole RTL1 – S 24.4400 deg; E 29.1767 deg</p> <p>Location of Borehole WD4 – S 24.6483 deg; E 28.7450 deg</p> <p>Location of Borehole LB1 – S 24.8817 deg; E 28.5833 deg</p> <p>Borehole TF2 – base of volcanic sequence – 768m</p> <p>Borehole TF1 – base of volcanic sequence – 357m</p> <div>All Sr-, Nd- and Pb-isotope values are MEASURED values. </div>

Volcanism in The Pre-Semilir Formation at Giriloyo Region; Allegedly as Source of Kebo-Butak Formation in the Western Southern Mountains

Kebo-Butak Formation was known to be the oldest volcanic rocks limited in regional terms in the lower Baturagung Hills, Gedangsari area, Gunungkidul Regency. The main constituents of the Kebo-Butak Formation consist of intersection of volcanic-clastic rocks and calcareous sediments, locally also found basalt lava with pillow structures; which distinguished it from other volcanic rock formations in the Southern Mountains. This study aims to determine the relationship of volcanic rocks exposed in Giriloyo with the Kebo-Butak Formation in the Baturagung Hills; the chronostratigraphy and the history of volcanic activities that produced the volcanic rocks of Giriloyo. This research was approached by volcanic geological mapping using surface mapping suported by gravity anayses. From the bottom to the top of the frontier areas result volcaniclastic rocks consisting of black tuffs with several fragments of volcanic bombs with basalt composition intersecting with thin basaltic lava inserted by calcareous claystone having an age of N5-7 (Early Miocene); pyroxene-rich basalt volcanic sequence consists of thick layers of tuff with creamy-brown color intersecting with lava and breccia inserted by calcareous sandstone aged N7-8; dikes, lava and agglomerates with basaltic composition and lava and agglomerates with andesitic composition. Stratigraphically, the volcanic rocks exposed at Giriloyo correlated with the volcanic rocks exposed at Karangtalun (Wukirsari) were under the Semilir Formation, bordered with normal fault N210oE/77o, the hanging wall composed by light grey tuff of Semilir Formation. Gravity analyses found high anomalies below the Semilir Formation exposed at Karangtalun-Munthuk (east of study area) continued to below the Giriloyo area. The high anomalies were identified as the igneous/ignimbrite volcanic sequence. Descriptively and stratigraphically, the Giriloyo volcanic sequence are a part of Kebo-Butak Formation. The petrogenesis of the volcanic rocks will be discussed in further research to interpret magmatological properties, the evolving paleo-volcano, and the absolute age of the rocks.