Avulsion cycles and their stratigraphic signature on an experimental backwater‐controlled delta

River deltas grow in large part through repeated cycles of lobe construction and channel avulsion. Understanding avulsion cycles is important for coastal restoration and ecology, land management, and flood hazard mitigation. Emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics; however, our knowledge of backwater-controlled avulsion cycles is limited. Here, we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges, shallow bed-slopes, and subcritical flows. The experimental avulsion cycles consisted of an initial phase of avulsion setup, an avulsion trigger, selection of a new flow path, and abandonment of the parent channel. Avulsions were triggered during the largest floods (78% of avulsions) after the channel was filled by a fraction (0.3 ± 0.13) of its characteristic flow depth at the avulsion site, which occurred in the upstream part of the backwater zone. The new flow path following avulsion was consistently one of the shortest paths to the shoreline, and channel abandonment occurred through temporal decline in water flow and sediment delivery to the parent channel. Experimental synthetic stratigraphy indicates that the bed thicknesses were maximum at the avulsion sites, consistent with our morphologic measurements of avulsion setup and the idea that there is a record of avulsion locations and thresholds in sedimentary rocks. Finally, we discuss the implications of our findings within the context of sustainable management of deltas, their stratigraphic record, and predicting avulsions on deltas

The accumulation of organic-matter-rich rocks within an earth systems framework: the integrated roles of plate tectonics, atmosphere, ocean, and biota through the Phanerozoic

Kevin M. Bohacs, Ian O. Norton, Debbie Gilbert, Jack E. Neal, Martin Kennedy, Walter Borkowski, Marcia Rottman, Tracy Burk

LAMINATED MUDSTONES IN THE CLAY-SULFATE TRANSITION INTERVAL AT MT. SHARP -SIMILARITIES TO EVAPORITIC MUDSTONES FROM PAHRUMP HILLS AND POSSIBLE

International audienceMudstone-rich lacustrine strata at Gale Crater, Mars, dominate the Mt. Sharp sedimentary succession. The Gale crater strata provide the oppor-tunity to test hypotheses regarding past climate shifts early in the planets history [1, 2], a key motivator for selecting Gale Crater as a landing site for the Curiosity rover.At the base of the Mt. Sharp succession, at Pahrump Hills, mudstones with sedimentary features and geo-chemical attributes occur that suggest that these rocks accumulated in an underfilled lake basin. Lake waters were saline to hypersaline, and lake levels, shorelines, and salinities fluctuated greatly at various temporal scales [3, 4, 5].Laminated mudstones, characterized by alternating softer and harder (cemented) layers and apparent evaporite pseudomorphs [4, 5] associated with the lat-ter, dominate the Pahrump succession. They show multiple textural similarities to evaporitic lacustrine strata on Earth [3]. Although indications of evaporitic conditions have been reported from higher in the sec-tion [6], laminated evaporitic mudstones of the type seen at Pahrump Hills were not been observed for the next 400 m of Mt. Sharp stratigraphy since leaving Pahrump Hills.During the recent ascent from the clay trough to-wards the Sulfate bearing unit, rocks resembling the Pahrump Hills evaporitic mudstones were again en-countered at elevations above -4030 m. Examples of these laminated mudstones are compared to textural equivalents from Pahrump Hills

LAMINATED MUDSTONES IN THE CLAY-SULFATE TRANSITION INTERVAL AT MT. SHARP -SIMILARITIES TO EVAPORITIC MUDSTONES FROM PAHRUMP HILLS AND POSSIBLE

International audienceMudstone-rich lacustrine strata at Gale Crater, Mars, dominate the Mt. Sharp sedimentary succession. The Gale crater strata provide the oppor-tunity to test hypotheses regarding past climate shifts early in the planets history [1, 2], a key motivator for selecting Gale Crater as a landing site for the Curiosity rover.At the base of the Mt. Sharp succession, at Pahrump Hills, mudstones with sedimentary features and geo-chemical attributes occur that suggest that these rocks accumulated in an underfilled lake basin. Lake waters were saline to hypersaline, and lake levels, shorelines, and salinities fluctuated greatly at various temporal scales [3, 4, 5].Laminated mudstones, characterized by alternating softer and harder (cemented) layers and apparent evaporite pseudomorphs [4, 5] associated with the lat-ter, dominate the Pahrump succession. They show multiple textural similarities to evaporitic lacustrine strata on Earth [3]. Although indications of evaporitic conditions have been reported from higher in the sec-tion [6], laminated evaporitic mudstones of the type seen at Pahrump Hills were not been observed for the next 400 m of Mt. Sharp stratigraphy since leaving Pahrump Hills.During the recent ascent from the clay trough to-wards the Sulfate bearing unit, rocks resembling the Pahrump Hills evaporitic mudstones were again en-countered at elevations above -4030 m. Examples of these laminated mudstones are compared to textural equivalents from Pahrump Hills

Three-fold nature of coastal progradation during the Holocene eustatic highstand, Po Plain, Italy – close correspondence of stratal character with distribution patterns

Although general trends in transgressive to highstand sedimentary evolution of river-mouth coastlines are well-known, the details of the turnaround from retrogradational (typically estuarine) to aggradational–progradational (typically coastal/deltaic) stacking patterns are not fully resolved. This paper examines the middle to late Holocene eustatic highstand succession of the Po Delta: its stratigraphic architecture records a complex pattern of delta outbuilding and coastal progradation that followed eustatic stabilization, since around 7·7 cal kyr bp. Sedimentological, palaeoecological (benthic foraminifera, ostracods and molluscs) and compositional criteria were used to characterize depositional conditions and sediment-dispersal pathways within a radiocarbon-dated chronological framework. A three-stage progradation history was reconstructed. First, as soon as eustasy stabilized (7·7 to 7·0 cal kyr bp), rapid bay-head delta progradation (ca 5 m year−1), fed mostly by the Po River, took place in a mixed, freshwater and brackish estuarine environment. Second, a dominantly aggradational parasequence set of beach-barrier deposits in the lower highstand systems tract (7·0 to 2·0 cal kyr bp) records the development of a shallow, wave-dominated coastal system fed alongshore, with elongated, modestly crescent beaches (ca 2·5 m year−1). Third, in the last 2000 years, the development of faster accreting and more rapidly prograding (up to ca 15 m year−1) Po delta lobes occurred into 30 m deep waters (upper highstand systems tract). This study documents the close correspondence of sediment character with stratal distribution patterns within the highstand systems tract. Remarkable changes in sediment characteristics, palaeoenvironments and direction of sediment transport occur across a surface named the ‘A–P surface’. This surface demarcates a major shift from dominantly aggradational (lower highstand systems tract) to fully progradational (upper highstand systems tract) parasequence stacking. In the Po system, this surface also reflects evolution from a wave-dominated to river-dominated deltaic system. Identifying the A–P surface through detailed palaeoecological and compositional data can help guide interpretation of highstand systems tracts in the rock record, especially where facies assemblages and their characteristic geometries are difficult to discern from physical sedimentary structures alone