Volume 25 (2019): Issue 1 (April 2019)

Sedimentary structures discussed in the present study are genetically linked to ripples that consist of pure sand or alternating sand and mud layers. All types of ripple-related structures, such as climbing-ripple cross-lamination and heterolithic bedding, i.e., flaser, wavy and lenticular (nodular), have been identified for the first time in fluvial strata that have been characterised previously as commonly massive. These small-scale bedforms, produced by migrating ripples, have been documented in a fluvial channel of late Neogene age in central Poland. The abundance and co-occurrence of the structures discussed and their spatial distribution provide evidence of their formation under very low-energy conditions, when flow velocity changed markedly, but was often significantly less than 0.5 m/s. Therefore, these ripple-derived sedimentary structures are here recognised as typical of channel fills of an anastomosing river.

The silty top parts of graded turbidites of the Late Ordovician Pingliang Formation, which accumulated along the southern margin of the Ordos Basin (central China), have been reworked by contour currents. The reworking of the turbidites can be proven on the basis of paleocurrent directions in individual layers: the ripple-cross-bedded sandy divisions of some turbidites show transport directions consistently into the downslope direction (consistent with the direction of other gravity flows), but in the upper, silty fine-grained division they show another direction, viz. alongslope (consistent with the direction that a contour current must have taken at the same time). Both directions are roughly perpendicular to each other. Moreover, the sediment of the reworked turbidites is better sorted and has better rounded grains than the non-reworked turbidites.

Although such type of reworking is well known from modern deep-sea environments, this has rarely been found before in ancient deep-sea deposits. The reworking could take place because the upper divisions of the turbidites involved are silty and consequently relatively easily erodible, while the contour current had locally a relatively high velocity – and consequently a relatively large erosional capability – because of confinement within a relatively narrow trough.

Synrift basin margin successions are greatly influenced by eustatic sea level changes, tectonics and accommodation space filled in by sediments. The Middle Jurassic (Bajocian–Callovian) of Chorar Island (western India) comprises a ~109-m-thick synrift basin margin succession of clastic, non-clastic and mixed siliciclastic-carbonate rocks which are here analysed and categorised into nine lithofacies. The succession is bioturbated to varying intensities; 16 identified ichnogenera can be assigned to environmentally related groups of five trace fossil assemblages, which include Gyrochorte, Hillichnus, Rhizocorallium, Skolithos and Thalassinoides. These ichnoassemblages document the Skolithos and Cruziana Ichnofacies which marks a change in energy conditions, sedimentation dispersal patterns and bathymetry in a shallow-marine environment. The Bajocian–Callovian succession is further analysed on the basis of sedimentological and ichnological data that show two genetic sequences consisting of Transgressive Systems Tract and Highstand Systems Tract bounded by Maximum Flooding Surface. The synrift basin margin succession of the Middle Jurassic of Chorar Island shows cyclicity in deposition; the Bajocian–Bathonian succession represents progradational to retrogradational coastlines, while the Callovian succession documents an aggrading progradational coastline.

Available reconstructions of Jurassic global sea level changes differ and are in need of an update. New stratigraphical charts and palaeogeographical developments for a number of large continental blocks or their portions of these (e.g., Germany, India, northeast Africa, northwest Australia, the Russian Platform and western Siberia) reveal regional long-term shoreline shifts (i.e., transgressions and regressions) during the Bajocian Stage (168.3–170.3 Ma). A comparison of these allows to document only a single coherent feature, namely the earliest Bajocian transgression, in the majority of the continental blocks considered. Undoubtedly, this event was triggered by a eustatic rise. However, long-term shoreline shifts were either weak to absent or differed between the blocks during almost the entire Bajocian, thus providing evidence of the apparent stability of global sea level and the importance of regional tectonic activity as a control mechanism on particular transgressions and regressions. Interestingly, it appears that the earliest Bajocian eustatic rise was a constituent of a long-term eustatic pattern; the nature of this event has yet to be fully understood. Generally, the findings presented here are in better agreement with Anthony Hallam’s view of Jurassic eustasy and question some other global sea level reconstructions.

Grain size analysis is an important sedimentological tool used to unravel hydrodynamic conditions, mode of transportation and deposition of detrital sediments. For the present study, detailed grain size analysis was carried out in order to decipher the palaeodepositional environment of Middle–Upper Jurassic rocks of the Ler Dome (Kachchh, western India), which is further reinforced by facies analysis. Microtextures were identified as grooves, straight steps and V-shaped pits, curved steps and solution pits suggesting the predominance of chemical solution activity. Grain size statistical parameters (Graphic and Moment parameters) were used to document depositional processes, sedimentation mechanisms and conditions of hydrodynamic energy, as well as to discriminate between various depositional environments. The grain size parameters show that most of the sandstones are medium- to coarse-grained, moderately to well sorted, strongly fine skewed to fine skewed and mesokurtic to platykurtic in nature. The abundance of medium- to coarse-grained sandstones indicates fluctuating energy levels of the deposition medium and sediment type of the source area. The bivariate plots show that the samples are mostly grouped, except for some samples that show a scattered trend, which is either due to a mixture of two modes in equal proportion in bimodal sediments or good sorting in unimodal sediments. The linear discriminant function analysis is predominantly indicative of turbidity current deposits under shallow-marine conditions. The C-M plots indicate that the sediments formed mainly by rolling to bottom suspension and rolling condition in a beach subenvironment. Log probability curves show that the mixing between the suspension and saltation populations is related to variable energy conditions.

Borehole K-1 is an exploratory well that was drilled in the North Makassar Basin (West Sulawesi) in 2011. Gas chromatography (GC) and gas chromatography-mass chromatography (GC-MS) analyses have been conducted on extracts from well cuttings from the Paleogene to Neogene interval in order to investigate the characteristics of biomarkers present. Although the well was drilled with oil-based mud and gas chromatographic analysis reveals that the alkane fractions are heavily contaminated, detailed investigation of biomarkers in these rock extracts and comparison with biomarkers in the oil-based mud has revealed that, while there are hopane and sterane biomarkers in the mud, there are also a discrete set of biomarkers that are indigenous to the rocks. These include oleanane, bicadinanes, taraxastane and other higher-plant-derived triterpanes. The presence of these compounds in environments that range from bathyal to marginal marine and even to lacustrine, shows the extent of reworking of terrestrial material into aquatic settings in this region during the Paleogene and Neogene and provides further evidence of a predominance of terrestrial material, even in deep-marine settings, with little ‘in-situ’ material noted. These findings have important implications for the use of biomarkers as indicators of palaeoenvironment in both source rocks and oils.

In discussions on loess, two types are often demarcated: glacial loess and desert loess. The origin of the idea of desert loess appears to lie with V.A. Obruchev who observed wind-carried silt on the Potanin expedition to Central Asia in 1895. It might be considered that desert loess would be defined as loess associated with deserts but it came to be thought of as loess produced in deserts. This led to some controversy as no mechanism for producing silt particles in deserts was readily available. Bruce Butler in Australia in particular cast doubt on the existence of desert-made loess.

Butler indicated loess-like deposits in Australia which he called Parna; these are very like loess but the silt sized particles are actually clay mineral agglomerates of silt size- formed in dry lake regions.

At the heart of the desert loess discussion is the problem of producing loess material in deserts. It has been suggested that there are no realistic mechanisms for forming large amounts of loess dust but there is a possibility that sand grain impact may produce particle shattering and lead to the formation of quartz silt. This would appear to be a reasonable mechanism for the African deposits of desert loess, but possibly inadequate for the huge deposits in China and Central Asia. The desert loess in China and Central Asia is loess associated with a desert. The material is formed in cold, high country and carried by rivers to the vicinity of deserts. It progresses then from deserts to loess deposit.

Adobe ground may be defined as desert loess. Adobe occurs on the fringe of deserts, notably in the Sahelian region of Africa, and in SW USA. The use of adobe in construction represents the major utilization of desert loess in a social context. More understanding of adobe is required, in particular with respect to the adobe reaction, the low order chemical reaction which provides modest cementitious properties, and can be likened to the pozzolanic reactions in hydrating cement systems.

The location of loess and loess-like ground on the peripheries of deserts is aided by the observation of the nesting sites of bee-eater birds. These birds have a determined preference for loess ground to dig their nesting tunnels; the presence of nest tunnels suggests the occurrence of desert loess, in desert fringe regions.

We seek amalgamation and contrast: ten main topics are considered: words and terms, particles, parna, geotechnical, adobe, people, birds, Africa, Central Asia, Mars. The aim is some large generalizations which will benefit all aspects of desert loess investigation.