Nils Lenhardt

Palaeoproterozoic sedimentation on the Singhbhum Craton: global context and comparison with Kaapvaal

Abstract The Singhbhum Craton has a limited Palaeoproterozoic supracrustal record, which suggests a three-part history, comprising: a long period of high freeboard and palaeosol formation on granitoids; subsequent rift-related mafic–ultramafic volcanism and subordinate sedimentation (c. 2.25–2.1 Ga: Dhanjori and Jagannathpur basin-fills; possibly also Simlipal, Malangtoli and Ongarbira basin-fills), which overlapped locally with mafic soil formation; and a major regression at around 2.0 Ga. Following a long hiatus, the approximately 1.6 Ga Dhalbhum–Dalma succession was laid down, probably under continental conditions. This rather truncated record stands in contrast to the chronologically and geographically much more widespread supracrustal basin-fills of the Kaapvaal Craton, and there appears to be an overall poor comparison between these two early Precambrian crustal blocks. However, on Kaapvaal, three analogous events are identified: widespread approximately 2.2 Ga mafic volcanism, followed by a well-developed palaeosol and a major transgression prior to 2.05 Ga. The three shared events between the two cratons are compatible with the postulate of a global, approximate 2.45–2.2 Ga shutdown of magmatic and tectonic geodynamics, with the origin of the triumvirate directly reflecting its resumption again after about 2.2 Ga. We recognize here that a large diversity of views on Singhbhums geodynamic history exists, predicated on a lack of precise geochronology and commonly poor outcrops, and the current hypotheses are presented with these factors in mind.

Palynomorph preservation in volcaniclastic rocks of the Miocene Tepoztlan formation (Central Mexico) and implications for paleoenvironmental reconstruction

ABSTRACT Palynomorph preservation in sedimentary rocks is strongly affected by various taphonomic factors related to transport, deposition, diagenesis, and preservation potential. The palynological record may help distinguish different taphonomic factors and also displays changes in paleoenvironment, especially in volcanic settings where a very complex interaction of eruptive, gravitational, and fluvial processes in time and space can be observed. Herein, we report on new palynological data from the Miocene Tepoztlán Formation. The 800-m-thick formation mainly consists of pyroclastic rocks, mass flow units (lahars) and fluvial deposits. It is part of the southern Transmexican Volcanic Belt, cropping out south of the Valley of Mexico and within the two states of Morelos and Mexico State. The volcaniclastic succession records various stages of recovery of vegetation related to a wide variety of disturbance factors and mechanisms. During the entire period of deposition, mixed mesophytic forests appear to have been widespread in the lowlands along streams and midaltitude uplands surrounding the valley. Pollen assemblages were repeatedly reset by volcanic eruptions or their secondary effects (lahars) to more limited assemblages with gradual recoveries to the initial stages before the eruption. A clear distinction can be made between samples taken from different transport regimes (fluvial, lahar and pyroclastic flow transport). The highest percentages of well-preserved, amorphous, and crumpled palynomorphs can be found in fluvial sediments while the highest percentage of fragmented palynomorphs is characteristic of lahar deposits. In contrast, the highest percentage of corroded palynomorphs can be found in deposits originating from pyroclastic flows.

Goldilocks or Plain Jane? The PGE metallogeny of the Bushveld LIP as recorded by the Rooiberg lavas

The bulk rock geochemistry of lavas (as ‘liquid’ compositions) has been used to assess the Ni-Cu-PGE prospectivity of LIPs globally, with geochemical tools (often including chalcophile element ratios) being used to trace whether the magmas have equilibrated with sulphides prior to or during emplacement (e.g., [1]). According to these criteria, LIPs that contain both metal-depleted and undepleted magmas are deemed particularly prospective. In addition, the abundance of PGE, Cu and Au in lavas is also used as an indicator of fertility for the region.

The Ituwa Surge deposits of the Holocene Ngozi caldera, Mbeya Region, Tanzania

Fig. 1 a Panoramic view of the Ituwa quarry and its medial Ituwa Surge deposits with its source area, the Ngozi caldera, in the background (picture taken from coordinates: S8 56025.500, E33 30022.800), b the surge deposits showing an upward change from thinly bedded planar bed forms at the base to abundant dune forms at the top (coordinates: S8 56037.000, E33 30025.900), c typical smallscale feature as it can be seen in the Ituwa Surge deposits with symmetric ripples growing into an antidune