Provenance of the Incipient Passive Margin of NW Laurentia (Neoproterozoic): Detrital Zircon from Continental Slope and Basin Floor Deposits of the Windermere Supergroup, Southern Canadian Cordillera

The origin of the passive margin forming the paleo-Pacific western edge of the ancestral North American continent (Laurentia) constrains the breakup of Rodinia and sets the stage for the Phanerozoic evolution of Laurentia. The Windermere Supergroup in the southern Canadian Cordillera records rift-to-drift sedimentation in the form of a prograding continental margin deposited between ~730 and 570 Ma. New U-Pb detrital zircon analysis from samples of the post-rift deposits shows that the ultimate source area was the shield of NW Laurentia and the near uniformity of age spectra are consistent with a stable continental drainage system. No western sediment source area was detected. Detrital zircon from postrift continental slope deposits are a proxy for ca. 676-656 Ma igneous activity in the Windermere basin, likely related to continental breakup, and set a maximum depositional age for slope deposits on the eastern side of the basin at 652±9 Ma. These results are consistent with previous interpretations. The St. Mary-Moyie fault zone near the Canada-U.S. border was most likely a major transform boundary separating a rifted continental margin to the north from intracratonic rift basins to the south, resolving north-south variations along western Laurentia in the late Neoproterozoic at approximately 650-600 Ma. For Rodinia reconstructions, the conjugate margin to the southern Canadian Cordillera would have a record of rifting between ~730 and 650 Ma followed by passive margin sedimentation.

The Paleomorpholigic Conditions of the Formation of the Settlement of Shitkino (Northwest Russia)

Для многих раннесредневековых памятников Северо-Запада Европейской России палеогеографические условия реконструированы недостаточно. При этом географический контекст развития поселений играет важную роль в исторической и экономической жизни людей Средневековья на локальном уровне. Археологический памятник поселение Шниткино расположен в бассейне Западной Двины на западном макросклоне Валдайской возвышенности. Изучаемое поселение находится на берегу озера Шнидкино, через которое насквозь протекает река Торопа. Используя геолого-геоморфологические методы, авторы статьи описали строение озерной террасы и озерно-речной поймы, в которых был найден культурный слой IX–X веков и отдельные археологические артефакты. Седиментационные архивы торфа, озерных, аллювиальных и склоновых отложений, продатированные радиоуглеродным методом, позволили оценить, насколько рельеф раннего Средневековья отличался от современного, и определить возможную функциональность территории во время существования поселения. Выявленные стабильность берега и аккумулятивная динамика озерно-речной системы свидетельствуют о том, что за последние несколько тысяч лет форма берега почти не изменилась. Речные и озерные террасы с похожей динамикой, обнаруживаемые в бассейне реки Торопы, считаются наиболее привлекательными позициями для поселенцев раннего Средневековья. The palaeomorphology of many early medieval monuments in Northwest Russia has not been reconstructed yet. In historical and economic perspective, the geographical location has a great impact on settlement formation and affects people’s lives significantly. The settlement of Shitkino is an archaeological monument situated in the basin of the Western Dvina at the western slope of the Valdai Hills. The settlement of Shitkino is located at the shore of Shnidkino lake, which is traversed by the Toropa River. The authors of the article employ geological and geomorphological analyses to explore the floodplain and its fluvial terraces, to make archaeological finds and to gain evidence of the events of the 9th–10th centuries. The method of radiocarbon dating enabled the scientists to estimate the age of peat sediments, fluvial and slope deposits, to estimate the degree of relief transformation and to tentatively assess the extent of the functionality of the land in medieval times. The accumulated data, such as the structural stability of the shore and the dynamics of the river-lake system show that during the recent thousands of years the shores of the lake have been only insignificantly transformed. It is believed that in the early middle ages people preferred to settle in the basin of the Toropa River.

Identificação dos leques aluviais dissecados do Quadrilátero Ferrífero (MG) através de critérios espaciais e sedimentológicos

O objetivo desse estudo foi identificar os depósitos de leques aluviais colmatados ao longo da base das escarpas do Quadrilátero Ferrífero. A referida identificação ocorreu primeiramente em gabinete, onde foram realizadas leituras de trabalhos relacionados a geomorfologia da região bem como a interpretação dos mapas geológicos disponíveis. Os dados obtidos em gabinete serviram de base para norteamento dos trabalhos de campo, onde foi possível analisar características espaciais, sedimentológicas e morfométricas dos depósitos de encosta e inferir sobre seu possível ambiente deposicional. Os trabalhos de campo foram direcionados a cinco depósitos localizados nas bordas Sul, Oeste e Noroeste do Quadrilátero Ferrífero cujos sedimentos estão posicionados de forma oblíqua ao vale principal com características faciológicas que remetem a fluxos de detritos desconfinados. Nos depósitos 1 e 2 foram verificadas diminuição granulométrica de montante para jusante, indicando brusca perda de energia. Essas características somadas as elevadas espessuras dos depósitos de fluxos de detritos permitiram inferir que tais acumulações compreendem leques aluviais inativos, atualmente em fase de dissecação pelos elementos erosivos naturais e antrópicos. Tais leques aluviais podem conter importantes informações a respeito do quadro paleoclimático e paleomorfológico da região, tendo potencial para contribuir com novos dados e aumentar o horizonte de interpretações a respeito da dinâmica paleoambiental quaternária regional. Identification of alluvial fans dissected from the Quadrilátero Ferrífero (MG) through spatial and sedimentological criteriaA B S T R A C TThis study aims to identify deposits of alluvial fans that were filled in along the base of the Iron Quadrangle escarpments. This identification occurred first in the office, where readings of works related to the geomorphology of the region were carried out, as well as the interpretation of the available geological maps. The data obtained in the office was the basis for guiding fieldwork, through which it was possible to analyze spatial, sedimentologic and morphometric characteristics of the slope deposits and to infer their possible depositional environment. The fieldwork was directed to five deposits located on the South, West and Northwest edges of the Iron Quadrangle whose sediments are placed obliquely to the main valley with faciological characteristics denoting unfinished debris flows. In deposits 1 and 2, there was a granulometric decrease from downstream to upstream, indicating an abrupt loss of energy. These characteristics added to the high thickness of the deposits of debris flows allowed to infer that such accumulations comprise inactive alluvial fans, currently in the dissection phase by natural and antropic erosive elements. Such alluvial fans may contain important information about the region's paleoclimatic and paleomorphological framework, with the potential of contributing with new data and increasing the range of interpretations regarding the regional quaternary paleoenvironmental dynamics.Keywords: Quaternary, sedimentary deposits, debris flow, paleogeography.

Regional-scale susceptibility modelling of shallow landslides involving the weathered and fractured sub-surface bedrock

<p>Landsliding is a complex phenomenon and its modelling aimed at predicting where the processes are most likely to occur is a tricky issue to be performed. Apart the chosen modelling approach, for both data-driven and physically-based models, paying adequate attention to the predisposing and triggering factors, as well as the input parameters is no less important. Generally, shallow landslides mobilize relatively small volumes of material sliding along a nearly planar rupture surface which is assumed to be roughly parallel to the ground surface. In the literature it is also widely accepted that shallow landslides involve only unconsolidated slope deposits (i.e., the colluvium), then the rupture surface corresponds to the discontinuity between the bedrock and the overlying loose soil. In this work, based on systematic field observations, we highlight that shallow landslides often involve also portions of the sub-surface bedrock showing different levels of weathering and fracturing. Then, we show that the engineering geological properties of slope deposits, as well as those related to the underlying bedrock, must be considered to obtain more reliable shallow landslides susceptibility assessment. As a first task, a multi-temporal shallow landslide inventory was built by photointerpretation of aerial orthoimages. Then, a new fieldwork-based method is proposed and implemented to acquire, process and spatialize the engineering geological properties of both slope deposits and bedrock. To support the regional scale approach, field observations were collected within, in the neighbour and far from the shallow landslide areas. Finally, both physically-based and data-driven methods were implemented to assess and compare shallow landslide susceptibility at regional scale, as well as to analyse the role of spatial distribution of rock mass quality for shallow slope failure development. The results highlight that, according to geology, structural setting and morphometric conditions, bedrock properties spatially change, defining clusters influencing both the distribution and characters of shallow landslides. As a consequence, the physically-based modelling provides better prediction accuracy when two possible rupture surfaces are analysed, the shallower one located at the slope deposit / bedrock discontinuity, and the deeper one located at the bottom of the fractured and weathered bedrock horizon. Even though the physically-based and data-driven models provide similar results in terms of ROC curves, the resulting susceptibility maps highlight quite substantial differences.</p>