Martin Gibling

The rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone, siltstone and mudstone bedding planes. In recent years, an increasing number of these textures have been attributed to surfi-cial microbial mats at the time of deposition, resulting in their classification as microbially induced sedimentary structures, or MISS. Research into MISS has developed at a rapid rate, resulting in a number of misconceptions in the literature. Here, we attempt to rectify these MISS misunderstandings. The first part of this paper surveys the stratigraphic and environmental range of reported MISS, revealing that contrary to popular belief there are more reported MISS-bearing rock units of Phanerozoic than Precambrian age. Furthermore, MISS exhibit a pan-environmental and almost continuous record since the Archean. Claims for the stratigraphic restriction of MISS to intervals prior to the evolution of grazing organisms or after mass extinction events, as well as claims for the environmental restriction of MISS, appear to result from sampling bias. In the second part of the paper we suggest that raised awareness of MISS has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features. By introducing the umbrella term 'sedimentary surface tex-tures', of which MISS are one subset, we suggest a practical methodology for classifying such structures in the geological record. We illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata. Resultant interpretations, microbial or non-microbial, should be couched within a reasonable degree of uncertainty. This approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin, and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current MISS lexicon. It is hoped that this new terminology will reduce the number of overly sensational and misleading claims of MISS occurrence, and permit the means to practically separate initial observation from interpretation. Furthermore, this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone, informing the search for true MISS in Earth's geological record and potentially on other planetary bodies such as Mars.

... recognized in stacked fluvial channel bodies or valley fills on the basis of changes in petrography and paleoflow direction (Miall and Arush 2001 ... accord well with anecdotal evidence from villagers that floods do not cover interfluves along parts of the Yamuna River (Tandon et al. ...

ABSTRACT Although some advances have been made towards understanding the interfluve stratigrahy in the southern Indo-Gangetic plains, the dynamics of valley filling and its forcing mechanisms remain poorly understood. We describe here sedimentological and chronological evolution of the subsurface alluvial stratigraphy developed in a section of river Ganga (width~13km; area:26°30'-27°00'N and 80°00'-80°30'E) The sedimentary succession was studied using litho- and electro-logs from six boreholes up to 33 m deep. These boreholes lie on two parallel transects (separated by 40km), across the valley, and well beyond the influence of the active channel. The sediment cores from these boreholes were used for facies analysis, magnetic susceptibility and optically stimulated luminescence (OSL) dating. These investigations suggest an upper about 13km wide and 12-15m deep continuous sandy channel deposits containing about 3-4 depositional cycles (unit1). The unit 1 is underlain by alternating layers of channel, aeolian and floodplain facies (unit2). The flood plain and aeolian facies show weak to moderate peodogenic alteration, terminating at prominent kankar layers, markers of discontinuity in deposition. About eighteen samples were used for OSL dating of the borehole sediments. A single-aliquot regenerative- dose protocol was applied to quartz obtained from medium to very fine sand and coarse silt. A good agreement between OSL and C-14 ages has been observed in the adjacent interfluve stratigraphy. The OSL chronology from the boreholes suggest that the upper channel filling activity (unit1) started around 10 ka. The base of the channel body in different boreholes gives ages from 10 to 1 ka, thus constraining the rate of progressive movement of the channel across the valley. The quartz derived from unit 2 was in OSL saturation suggesting an age of >40ka for these units. These results indicate a major hiatus in the youngest channel sand filling episodes in the area; this is supported by greater pedogenic alteration in the upper part of unit2. It is possible that unit 2 represents a period of relative fluvial dormancy during OIS 3 and 2, followed by rejuvenation during OIS 1. The details of these investigations and their implications for understanding the role of climate in shaping valley fill architecture will be discussed.

ABSTRACT The Ganges rises in the Himalaya and supplies water for agriculture to hundreds of millions of people, as it has since the beginnings of agriculture in the Neolithic. The river derives most of its discharge from the Southwest Indian Monsoon with some local snowmelt, and proxy records from the Arabian Sea indicate strong fluctuations in past monsoon intensity, driven largely by changes in solar insolation. OSL dating of cores and river cliffs along the Ganges Valley near Kanpur demonstrates a first-order link between monsoon intensity and phases of incision and aggradation in the Ganges system. The river has adjusted its equilibrium profile over relatively short periods (centuries to thousands of years), and is sensitive to the changing balance between sediment and water discharge imposed by monsoonal fluctuations. This has important implications for the agricultural plains over the next century. In more detail, the Ganges experienced reduced discharge and was underfit during the Last Glacial Maximum (LGM) of Marine Isotope Stage 2. Eolian and lacustrine strata accumulated on the interfluves adjacent to the Ganges Valley until about 26 ka B.P., after which gully erosion set in. Following the LGM, the monsoon intensified. Clusters of dates indicate that Ganges channels aggraded between about 16 and 11 ka, with dates especially focused around the Younger Dryas period of low monsoon intensity. Channels also aggraded late in the Holocene (after 2.5 ka) as the monsoon declined through to the present following its mid-Holocene peak. Although more difficult to constrain, phases of incision correspond broadly with periods of monsoonal intensification, especially following the LGM. In general, the river had sufficient energy for both sediment transport and incision as the monsoon waxed, but its transport capability was reduced as the monsoon waned, leading to aggradation. The Ganges appears to have shifted course in its valley mainly during monsoon highs. Archeological sites along the tributary Belan River testify to the links between early human settlements and the monsoon. Reworked gravels with Upper Paleolithic artefacts are dated at 21-31 ka, and may represent floodplain gully erosion during the LGM monsoonal low. Mesolithic settlements appear to have been established during a period of climatic instability that includes the Younger Dryas and early Holocene, based on eolian sediments dated at 7-14 ka. Neolithic settlements were probably established in the mid Holocene under stronger monsoon conditions suitable for the development of agriculture, as indicated by overlying floodplain muds.