Secondary welding of submarine,pumice-lithic breccia at Mount Chalmers,Queensland, Australia

Very thick units of massive pumice and lithic clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensland, are deposits from cold, water-supported, volcaniclastic mass flows emplaced in a below-wave base submarine setting. Adjacent to syn-volcanic andesitic and rhyolitic sills and dykes, the pumice-lithic breccia shows a well-developed eutaxitic texture. The eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact. At these sites, pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening. Some lenticular pumice clasts contain small (2 mm), round, quartz-filled amygdales and spherulites. Further away from the sills and dykes, the pumice clasts have randomly oriented, delicate tube vesicle structure and are blocky or lensoid in shape. Round amygdales were generated by re-vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases involving sills, secondary welding along the contacts formed extensive, conformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit.     

Unsupervised separation of seismic waves using the watershed algorithm on time-scale images

This paper illustrates the use of image processing techniques for separating seismic waves. Because of the non‐stationarity of seismic signals, the continuous wavelet transform is more suitable than the conventional Fourier transforms for the representation, and thus the analysis, of seismic processes. It provides a 2D representation, called a scalogram, of a 1D signal where the seismic events are well localized and isolated. Supervised methods based on this time‐scale representation have already been used to separate seismic events, but they require strong interactions with the geophysicist. This paper focuses on the use of the watershed algorithm to segment time‐scale representations of seismic signals, which leads to an automatic estimation of the wavelet representation of each wave separately. The computation of the inverse wavelet transform then leads to the reconstruction of the different waves. This segmentation, tracked over the different traces of the seismic profile, enables an accurate separation of the different wavefields. This method has been successfully validated on several real data sets.     

Calcite uranium–lead geochronology applied to hardground lithification and sequence boundary dating

Hardground discontinuities within carbonate platforms form important stratigraphic surfaces which can be used at basin scale to correlate sequence boundaries. Although these surfaces are commonly used in sequence strati‐graphy, the timing and duration of hardground lithification and the crystallization of early cements remain unexplored. Here, early calcite cements were dated by U‐Pb geochronology for five Jurassic hardgrounds, interpreted as third‐order sequence boundaries, situated within a well‐constrained petrographic, sedimentological and stratigraphic framework. The consistency or the slight deviation between the age of the cements and the stratigraphic age of deposition of the formations illustrate that cementation occurred early in the diagenetic history. The ages obtained on dogtooth cements, replacing aragonite in gastropod shells and pendant cements in intergranular spaces, match those of the standard Jurassic biostratigraphic ammonite Zones, making calcite U‐Pb geochronology a promising method for dating third‐order sequence boundaries of depositional sequences and refining the Jurassic time scale in the future.