Interval Flood Research Articles

Environmental and biotic changes across the Permian–Triassic boundary in western Tethys: The Bulla parastratotype, Italy

The sedimentary and biotic evolution of a 190 m interval of shallow marine and lagoonal facies in the Bellerophon and Werfen formations in the Southern Alps has allowed comparison of western with eastern Tethys: Meishan D section (southern China), Salt Range (Pakistan) and Abadeh (Iran). Results are as follows: (1) The upper part of the Bellerophon Fm. (Changhsingian changxingensis–deflecta Zone) shows only modest biotic variation connected with tectonically driven local variation and perhaps to more general climatic variation. The δ 13C decrease starting in the uppermost 30 m of the Bellerophon Fm. is correlated with decrease in global organic productivity starting about 1 m below the PTB in Chinese sequences and 20 m below in the Abadeh section. This interval culminated in a regression truncated by an unconformity–paraconformity (Unconformity 1). (2) The uppermost Bellerophon Fm. is a ca. 1 m transgressive-regressive sedimentary cycle, the informally named Bulla Mbr (Changhsingian: Early praeparvus Zone). The maximum flooding interval of this unit possibly had a slight increase in biodiversity, mainly in foraminifers, algae and brachiopods. The high increase in biodiversity previously reported may, in part, reflect abundance of biota and organic matter reworked into transgressive and regressive intervals. We suggest partial correlation of the basal unconformity of the Bulla Mbr (Unconformity 1) with the regressive uppermost Bed 24e of the Meishan D section marking the disappearance of foraminifers and algae in the eastern Tethys. We also suggest diachronous disappearance of benthic taxa in Tethys, with the Southern Alps acting like a refugium. (3) The main extinction (first extinction phase, mainly regarding foraminifers) in the Southern Alps occurred in a thin ca. 25 cm interval including the uppermost regressive Bulla Mbr, Unconformity 2, and possibly, the basal transgressive bed of the Tesero Mbr of the Werfen Fm. This interval is correlated in part with regressive Bed 26 of Meishan D section. The main decrease in abundance and biodiversity in the Southern Alps coincides with appearance of small oolites with crystalline outer cortex near the basal transgressive tract of the lower Tesero Mbr, a ca. 220 cm sedimentary cycle, that is followed by extension of microbialitic layers alternating with veritable biostromes with brachiopods and byssate bivalves as salient components among the algae. Vacated niches favored evolution of conodonts. Hindeodid conodont biodiversity increased with species developing characteristics of Isarcicella. This interval has been referred to the Changhsingian Late praeparvus Zone and correlated with Bed 27a–b of Meishan D section. The PTB has been identified in the Bulla section at 1.30 m from the base of the Werfen Fm. – with the first appearance of Hindeodus parvus defining the base of the parvus Zone – in a microbialitic layer correlated with Bed 27c of Meishan D section. Around the PTB the major part of the remaining Permian biota disappeared. Gymnocodiacean algae were the last group to undergo extinction within the Triassic layers. (4) Biodiversity dropped severely in the succeeding bioturbated microbialitic interval from which conodonts are absent. More humid climate is reflected in a more sandy marine substrate inhabited by Lingula, Unionites and Claraia. Conodonts recurred in the schizohaline upper part. The succeeding entry of the biozonal markers Isarcicella lobata, Isarcicella staeschei and Isarcicella isarcica allow discrimination of three conodont biozones. The layer with entry of I. staeschei has been aligned with Bed 28 of Meishan D section. The main extinction phase in the western Tethys seems to correspond to a gradual but swift transition from acid-bath to alkaline-bath according to the Deev Jahi model of Heydari and Hassanzadeh [Heydari, E. and Hassanzadeh, J., 2003. Deev Jahi model of the Permian–Triassic boundary mass extinction: a case for gas hydrates as the main cause of biological crisis on Earth. Sed. Geol., 163, 147–163.]. The Bulla section with abundant data on biotic and depositional variation is here considered as the PTB parastratotype for the shallow marine western Tethys.

First record of Permian conodont “jinogondolella” cf. altaduensis from the Midhnab Member, Khuff Formation, Saudi Arabia

ABSTRACT A single specimen of conodont is described for the first time from outcrops of the Khuff Formation in central Saudi Arabia. The specimen was recovered from 22 samples that were located in the maximum flooding intervals of the Khuff Formation and specifically processed for conodont research. The sample originated from the maximum flooding interval located at the lower part of the Midhnab Member of the Khuff Formation, at Jabal al Murayrah in the Ad Darma’ quadrangle. The conodont occurs in reworked lithoclastic and bioclastic calcarenites, secondary sparitized, as a single broken and corroded specimen, which belongs to the genus Mesogondolella (Jinogondolella) and is tentatively conferred to the species “Jinogondolella” cf. altaduensis. The conodont is associated with broken pieces of fauna including bivalves, gastropods, echinoids, brachiopods and bryozoans, as well as foraminifers and dasycladacean algae. This genus is rarely encountered in the open-marine deposits of the Tethyan platforms, where it appeared preferentially in semi-restrictive (saline) basins. A Late Capitanian age is interpreted for some species of the genus Jinogondolella in America (Texas), China and Oman, but this age interpretation is not firmly established for the Midhnab Member of the Khuff Formation. Also due to the reworked nature of the horizon that yielded this condont, the specimen is not considered to be age-indicative.

The late Hauterivian Faraoni oceanic anoxic event in the western Tethys: Evidence from phosphorus burial rates

In the uppermost Hauterivian sediments of the western Tethys, a short-lived anoxic event (Faraoni event) is documented both in the form of an interval enriched in organic matter (pelagic realm) and in a condensed interval enriched in glauconite and phosphate (shelf realm). This latter interval represents the onset of a drowning episode on the Helvetic carbonate platform along the northern tethyan margin that lasted throughout the early Barremian. This drowning episode marks a turning point in the way the platform carbonate factory functioned: during the Hauterivian carbonate production was dominated by heterozoans, whereas during the late Barremian a photozoan assemblage developed that is preserved in the so-called Urgonian limestone. The late Hauterivian Faraoni oceanic anoxic event is of particular interest because it is not accompanied by a major positive shift in δ 13 C unlike other oceanic anoxic events during the Cretaceous (Valanginian, early Aptian, Cenomanian–Turonian boundary). We have analyzed four (hemi-)pelagic sections with regards to their phosphorus content to better understand the palaeoceanographic conditions related to this anoxic event and the associated changes in the shallow-water carbonate factory. The sections are located in Angles (SE France), Fiume-Bosso and Gorgo a Cerbara (central Italy), and Veveyse de Châtel-St. Denis (west Switzerland). We calculated phosphorus mass accumulation rates by using a cyclostratigraphic approach in order to obtain an adequate age model. We observe a comparable and correlatable long-term trend for the four sections, which suggests that the phosphorus mass accumulation rates and temporal changes therein are representative for the western tethyan pelagic realm. The Faraoni event is marked by a minimum in phosphorus accumulation and a positive shift in the C org /P tot ratios, which is interpreted as a reflection of the decreased capacity of storing and preserving phosphorus in oxygen-depleted sediments. Moreover, the onset in the decrease in phosphorus accumulation coincides with a sea level rise, while the Faraoni level itself corresponds to a maximum flooding interval. This phase of sea-level rise may have been important in the establishment of marine connections between the boreal and tethyan realms and, as such, in the exchange of nutrient-enriched waters. The model for the origin of the Faraoni oceanic anoxic event proposed here incorporates these aspects together with a positive feedback loop generated by phosphorus regeneration and a negative feedback loop related to changes in the ocean oxygen cycle. The subsequent long-term changes in phosphorus burial rates during the Barremian suggest that the Faraoni event marks the onset of a long period of environmental instability with regards to platform growth, leading to periodic phases of eutrophication and drowning of the northern tethyan carbonate platform. This environmental crisis ended during the late Barremian with the onset of the deposition of the Urgonian limestone under oligotrophic conditions.

Geomorphological modelling of entrance improvement options - Lake Illawarra

The Delft3D Online Sediment modelling system has been used to investigate six preliminary designs for a northern entrance-training wall at Lake Illawarra on the NSW south coast. The designs were modelled under tidal conditions to investigate whether or not the ‘design’ entrance channels were self-scouring, and in a 100 Years Average Recurrence Interval (ARI) flood event to investigate the extent of any scour. The entrance to the lake has historically changed position due to the influence of tides, wave action, catchment runoff and longshore sediment transport, or been closed. This modelling study identified that some of these options did not provide a self-scouring channel under tidal flow, and others caused significant scour along the training walls during flood flows. Subsequently, two additional options were developed. They were designed to reduce the risk of entrance closure and to limit scour along the training walls during flood flows.

Formation, distribution and ecological consequences of flood-related wood debris piles in a bedrock confined river in semi-arid South Africa

The Sabie River, located within the semi-arid savanna of South Africa, experienced a ∼100-year return interval flood in 2000, providing a rare opportunity to quantify the resultant pattern and consequence of woody debris piles for riparian areas. Biophysical attributes of rivers in arid and semi-arid regions differ strongly from those in mesic regions, but differences in the role and dynamics of woody debris remain largely unknown. Our primary goal was to determine the basic ecology and distribution of wood piles formed by a large infrequent flood in a bedrock-confined river. In particular, we sought to determine the mechanism of pile formation, their position, size, and structural attributes of piles as well as regeneration from resprouting and seedling recruitment within piles in different channel types. We observed relatively distinct patterns of distribution and mechanisms of formation in 421 piles and provide a description of the distribution, abundance and short-term fate of each pile type. Patterns of wood accumulation were largely controlled by characteristics of the trapping sites, particularly toppled trees that remain rooted in their growth location. In general, wood was primarily strewn against toppled, transported and standing trees, though several large piles formed on bedrock outcrops. Buried piles were also discovered upon excavation of depositional features (alluvial mounds). Resprouting of toppled trees with intact roots and living tissues was the dominant mode of vegetation reestablishment in the piles. We observed resprouting in 36% of piles surveyed. Resprouting piles were most common (48%) in the seasonally inundated portion of the macro-channel. Tree seedlings were recorded in 28% of the piles. The flood appeared to create heterogeneity in the establishment of new woody vegetation by forming distinct wood piles from the ruins of the previous riparian forest. Even though the input of wood to semi-arid rivers appears to be a fairly rare occurrence and wood piles are rapidly depleted by decay and fire, we expect their influence will be long-lived, preserved in the pattern of vegetation establishment and alluvial mounds within the macro-channel. Copyright © 2006 John Wiley & Sons, Ltd.

Hydrologic modeling of flood conveyance and impacts of historic overbank sedimentation on West Fork Black's Fork, Uinta Mountains, northeastern Utah, USA

This study assesses historic overbank alluvial sedimentation along a low-gradient reach of West Fork Black's Fork in the northern Uinta Mountains, Utah. In this previously glaciated setting, an alluvial floodplain that is approximately 400 m wide by 1500 m long has been modified by the combined effects of valley morphometry and the recent history of clear-cut logging during the late 19th and early 20th Centuries. To quantify the effects on sedimentation and flow conveyance, three natural streambank exposures were sampled and analyzed for nuclear bomb fallout 137Cs. The distribution of 137Cs within the three profiles suggests that a remnant outwash terrace exerts a first-order control over the deposition of overbank alluvium. Upstream from a constriction in the floodplain caused by the terrace remnant, as much as 40 cm of overbank alluvium has been deposited since the beginning of clear-cut logging. Immediately downstream of that constriction, no evidence exists for any overbank sedimentation during that same period. Vibracore samples and Oakfield soil probe sampling throughout the study reach quantified the geographic extent and thicknesses of the historic alluvial package. Flood conveyance through the study area was modeled using the U.S. Army Corps of Engineers HEC-RAS modeling program. Model simulations were run for modern conditions (using surveyed topography) and for prehistoric conditions (using the modern topography less the historic alluvial package determined by 137Cs analyses). Model results indicate that the floodplain constriction caused a significant impediment to flood conveyance at even modest discharges during prehistoric conditions. This promoted ponding of floodwaters upstream of the constriction and deposition of alluvium. This has increased bank heights upstream of the constriction, to the point that under modern conditions 1- to 5-year recurrence interval floods are largely confined within the channel. These results confirm the validity of this new approach of combining 137Cs dating of alluvial sediments with HEC-RAS flow modeling to compare flood conveyance along a single stream reach prior to and since an abrupt change in alluvial sedimentation patterns.

Sequence biostratigraphy and paleoenvironmental reconstruction in the Early Eocene Figols Group of the Tremp–Graus Basin (south-central Pyrenees, Spain)

The dinoflagellate cyst, palynofacies and foraminiferal records from the Early Eocene Figols Group of the Spanish Pyrenees exhibit major changes associated with relative sea-level fluctuations and variations in efficiency of the fluvial systems. The mixed siliciclastic–carbonate marine succession exposed in the Merli–Esdolomada area was deposited on the shelf in a tectonically active basin, under oxic conditions, in shallow to moderately deep marine waters (close to the lower boundary of the photic zone) influenced by river discharge from adjacent emerged lands. Sedimentary cycles are mainly recorded by fine-grained deposits. The paleoenvironmental interpretation of palynological and calcareous microfaunal records allows the definition of depositional sequences and enables to trace basinward surfaces identified in more proximal settings by means of sedimentological facies analysis. At the same time, the solid regional stratigraphic framework already available for the Figols Group allows new insights on the paleoecology of extinct taxa. The Merli–Esdolomada section (ME) spans two 3rd order sequences. The upper sequence reflects more marginal marine conditions than the lower one, thus pointing to an overall shallowing trend with time. In the lower sequence the maximum flooding interval is characterised by a rich and diverse microfauna, highest relative abundances of marine phytoplankton and typically neritic dinoflagellate cyst (dinocyst) assemblages dominated by Spiniferites and Cordosphaeridium. The final phase of the highstand systems tract, in proximity of the main sequence boundary, is characterised by a decrease in abundance and diversity of dinocysts, with the dominance of the lagoonal genus Polysphaeridium, and by decreased microfaunal diversity (discorbids, miliolids, larger foraminifers). Palynological records permit the reconstruction of activation and deactivation phases of the fluvial systems, variations in runoff from the hinterland, nutrient delivery to the sea and productivity in surface waters of the Tremp–Graus Basin. The overall abundance of Spiniferites and the scarcity of peridinioid cysts, Lingulodinium, Pediastrum and Botryococcus algae are interpreted to indicate oligotrophic conditions during the deposition of the investigated succession. Recurrent intervals where the temporary disappearance of Spiniferites corresponds to highest relative abundances of Operculodinium and herbaceous debris are the main evidence for river plumes and denote periods of enhanced fluvial discharge reflecting a Milankovitch-type cyclicity. The Rotalia group, which shows abundance peaks of Cuvillierina spp. in the shallower intervals, is often associated with these episodes, thus suggesting for this taxon more tolerance to turbidity than larger foraminifera. Within the maximum flooding zone, a Thalassiphora patula acme corresponding to the local disappearance of Homotryblium is deemed to record the distal expression of a fluvial activation which triggered water salinity stratification. Despite broad morphological similarities, Homotryblium exhibits more cosmopolitan preferences than Polysphaeridium, which is instead confirmed as a lagoonal euryhaline taxon. The increase of Spinizonocolpites pollen toward the top of the Figols Group records the northwestward migration of Nypa mangrove-palms from southern Europe during the mid Early Eocene.

The Permian-Triassic Khuff Formation of central Saudi Arabia

ABSTRACTThe Permian-Triassic Khuff Formation crops out in central Saudi Arabia along a N-S belt, some 1,200 km in length. It is 171.4 m (562.2 ft) thick in the type section and divided into five members; from oldest to youngest: Ash Shiqqah (formerly Unayzah member of the Khuff Formation), Huqayl, Duhaysan, Midhnab and Khartam. The base of the Khuff Formation is recognised as a regional unconformity (Pre-Khuff Unconformity), and the top of the formation is defined by the conformable contact with the overlying Sudair Shale. The Ash Shiqqah Member is tentatively dated as ?Middle Permian ?Capitanian (?Midian) based on the presence of the fusulinids Monodiexodina kattaensis and Reichelina sp. The Huqayl Member is tentatively dated as ?Late Permian ?Wuchiapingian (?Dzhulfian) based on an assemblage of smaller foraminifers dominated by Pseudomidiella cf. labensis, Earlandia? spp. and Neodiscus aff. qinglongensis. The Duhaysan Member is dated as Late Permian (Wuchiapingian-Changhsingian) based on Hemigordius baoqingensis, Graecodiscus cf. kotlyarae, “Dentalina” hoi, and Colaniella cf. minuta. The sMidhnab Member is dated as Changhsingian (Dorashamian) based on Paradagmarita sp. and “Glomospirella spirillinoides” and the disappearance of the genera Nankinella and Globivalvulina. Within the continental facies in the upper part of the Midhnab Member, incised channels facies yielded the Late Permian Midhnab Flora. The Lower Khartam Member is also dated as latest Permian (Dorashamian) based on the presence of several species of Paradagmarita and “Nodosaria” dzhulfensis in the lowest beds of the member, and the ostracods Paraparchites spp., Knoxiella spp. and Kloedenellitina sp. throughout the Lower Khartam Member. The Upper Khartam Member is assigned to an Early Triassic (‘Scythian’) age based on the occurrence of the characteristic annelid Spirorbis phlyctaena.The Khuff Formation, in central Saudi Arabian outcrops, consists of four regional depositional sequences (DS PKh, DS PKm, DS PKk and DS TrS), each containing a maximum flooding interval (MFI, with the same designation). The oldest sequence DS PKh (named after Permian-Khuff-Huqayl) consists of the Ash Shiqqah and overlying Huqayl members. The basal sequence boundary corresponds to the Pre-Khuff Unconformity (PKU) and represents the onset of the first Permian flooding event recorded in the outcrops of Saudi Arabia. The massive gypsum, gypsiferous claystone, and solution breccias in the Ash Shiqqah Member are correlated to the subsurface Khuff-D Anhydrite. The MFI PKh is positioned in the basal part of the Huqayl Member, and is followed by the regressive evaporitic palaeoenvironments of the Huqayl Member.The DS PKm (named after Permian-Khuff-Midhnab) starts with the subtidal to littoral deposits of the Duhaysan Member, over an erosive surface upon DS PKh; and ends with the regressive supratidal to continental deposits at the upper part of the Midhnab Member. The MFI PKm is located at the base of the Midhnab Member, where limestones yielded an abundant marine fauna including cephalopods and brachiopods. The terminal Permian DS PKk (Permian-Khuff-Khartam) corresponds to the Lower Khartam Member. The basal SB PKk is marked by a return to marine subtidal conditions after the continental break at the end of DS PKm. The MFI PKk is characterised by marine fauna including abundant Permian ostracods, bactritids, and locally cephalopods. The DS TrS (named after the Sudair Shale Formation) starts with the littoral, tidal to intertidal deposits of the Early Triassic Upper Khartam Member of the Khuff Formation, and ends with the closed basin, clayey to evaporitic rocks of the Early Triassic Sudair Shale.

Flood-deposited wood debris and its contribution to heterogeneity and regeneration in a semi-arid riparian landscape

We investigated whether large woody debris (LWD) piles create nodes of environmental resources that contribute to the recovery of riparian vegetation and that also augment the heterogeneity and resilience of the riverine system. River and riparian systems are typified by a large degree of heterogeneity and complex interactions between abiotic and biotic elements. Disturbance such as floods re-distribute the resources, such as LWD, and thereby add greater complexity to the system. We examined this issue on a semi-arid savanna river where approximately a 100-year return interval flood in 2000 uprooted vegetation and deposited substantial LWD. We investigated the micro-environment within the newly established LWD piles and compared this with conditions at adjacent reference sites containing no LWD. We found soil nutrient concentrations to be significantly higher in LWD piles compared with the reference plots (total N +19%, available P +51%, and total C +36%). Environmental variables within LWD piles and reference sites varied with landscape position in the river-riparian landscape and with LWD pile characteristics. Observed differences were generally between piles located in the terrestrial and riparian areas as compared to piles located on the macro-channel floor. After 3 years the number and cover of woody species were significantly higher when associated with LWD piles, regardless of landscape position or pile type. We conclude that LWD piles formed after large floods act as resource nodes by accumulating fine sediments and by retaining soil nutrients and soil moisture. The subsequent influence of LWD deposition on riparian heterogeneity is discerned at several spatial scales including within and between LWD piles, across landscape positions and between channel types. LWD piles substantially influence the initial developmental of riparian vegetation as the system regenerates following large destructive floods.

A foraminiferal assemblage as a bioevent marker of the main Ladinian transgressive stage in the Betic Cordillera, southern Spain

The Betic Cordillera comprises the mountain range in the southern Iberian Peninsula that extends from Cadiz to Alicante, which displays typical features of alpine cordilleras. The Betic Cordillera includes two large geological tectonic domains, namely an External Zone, and an Internal Zone. The External Zone is essentially composed of epicontinental Triassic rocks that consist of the Buntsandstein, Muschelkalk and Keuper facies. The Buntsandstein facies rarely occur in outcrops, whereas the Keuper stratigraphic successions are truncated. The Muschelkalk facies frequently displays complete successions that are very useful for interpreting the Ladinian stage palaeogeography. The Muschelkalk facies consists of a lower carbonate member formed by three thick, massive limestone beds with intervals of thin-bedded marly limestone, and an upper marly member with intercalation of distinct beds of limestones and marls, where tempestites predominate. The first member has been interpreted as a transgressive systems track in which subtidal and ramp deposits can be identified. The second member is related to a highstand systems track, and consists of marine-platform and coastal deposits, which correspond to a shallowing-upward sequence. We describe and interpret a distinct bioclastic limestone bed with high concentrations of Involutinidae benthic foraminifers, that has been found in the Muschelkalk of several distant sections. This unusual bed is just above the third bed of massive Muschelkalk limestone, and consists of a bioclastic wackestone–packstone, up to 8 cm thick, with many Involutinidae of Ladinian age. The most abundant species are: Lamelliconus gr. biconvexus– ventroplanus, Lamelliconus cordevolicus and Triadodiscus eomesozoicus. These foraminiferal concentrations form a well-defined marker bed that may be related to a bioevent. The appearance of these foraminiferal taxa, which are also very common in the Triassic Alpine realm, coincides with the maximum flooding interval of a transgressive sequence. We surmise that these Tethyan foraminifers moved into newly developed coastal zones during maximum transgression and, at times, produced prolific shallow-water communities, which were reworked by storm currents that caused their unusual concentrations in deeper water. These concentrations thus generated the marker beds of foraminiferal packstones that can be found over distant outcrops in the region. Similar involutinid foraminiferal assemblages occur in the upper Muschelkalk of the Iberian Ranges and Catalan Coastal Ranges, as well as in the upper part of the transgressive systems track of the UAA-2.2 third-order cycle identified in the Muschelkalk of the Pyrenees Basin. Thus, this foraminiferal assemblage can be considered as an extensive bioevent marker related to an important transgressive stage of Ladinian age.

Depositional architecture and transgressive–regressive cycles within Liassic backstepping carbonate ramps in the Basque–Cantabrian basin, northern Spain

An integrated stratigraphical, sedimentological, geochemical and sequential analysis has been carried out on the Liassic rocks of the Basque–Cantabrian basin in northern Spain using outcrop and subsurface data. The study documents the progressive drowning and backstepping of an epeiric carbonate ramp that originated during the worldwide late Triassic–early Jurassic transgression. Two major carbonate depositional systems have been recognized within the drowning succession: (1) a late Triassic–early Sinemurian shallow-marine ramp, which reflects a progressive marine transgression over Triassic continental deposits; (2) a late Sinemurian–Toarcian set of hemipelagic ramps, which recorded the progressive drowning and backstepping of the ramp system during a second major event of marine flooding. Sequence stratigraphic analysis of the Liassic succession has revealed that transgressive–regressive trends constitute the most evident cyclicity. A major transgressive–regressive cycle and six subsidiary transgressive–regressive facies cycles have been defined in the basin and compared with those defined in other European basins. Although a good correlation exists for the major transgressive–regressive cycle, significant discrepancies have been found for the number, limits and extent of the transgressive–regressive facies cycles reflecting the effects of local tectonics. The overall backstepping within the Basque–Cantabrian basin resulted in a dominance of transgressive episodes across a range of different scales. Accordingly, regressive intervals are less well developed except during limited periods of active tectonics as occurred in the mid-Sinemurian. These progressively longer and deeper marine flooding intervals, followed by relative stillstand or minor sea-level falls, are interpreted to reflect high thermal subsidence rates coupled with a Liassic eustatic transgression.

English

Anthropogenic accommodation space, or that space in the Delta that lies below sea level and is filled neither with sediment nor water, serves as a useful measure of the regional consequences of Delta subsidence and sea level rise. Microbial oxidation and compaction of organic-rich soils due to farming activity is the primary cause of Delta subsidence. During the period 1900-2000, subsidence created approximately 2.5 billion cubic meters of anthropogenic accommodation space in the Delta. From 2000-2050, subsidence rates will slow due to depletion of organic material and better land use practices. However, by 2050 the Delta will contain more than 3 billion cubic meters of anthropogenic accommodation space due to continued subsidence and sea level rise. An Accommodation Space Index, which relates subaqueous accommodation space to anthropogenic accommodation space, provides an indicator of past and projected Delta conditions. While subsidence and sea level rise create increasing anthropogenic accommodation space in the Delta, they also lead to a regional increase in the forces that can cause levee failure. Although these forces take many forms, a Levee Force Index can be calculated that is a proxy for the cumulative forces acting on levees. The Levee Force Index increases significantly over the next 50 years demonstrating regional increases in the potential for island flooding. Based on continuing increases in the Levee Force Index and the Accommodation Space Index, and limited support for Delta levee upgrades, there will be a tendency for increases in and impacts of island flooding, with escalating costs for repairs. Additionally, there is a two-in-three chance that 100-year recurrence interval floods or earthquakes will cause catastrophic flooding and significant change in the Delta by 2050. Currently, the California Bay-Delta Authority has no overarching policy that addresses the consequences of, and potential responses to, gradual or abrupt landscape change in the Delta.

Hydro-geomorphic hazards and impact of man-made structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)

The Guil River Valley (Queyras, Southern French Alps) is prone to catastrophic floods, as the long historical archives and Holocene sedimentary records demonstrate. In June 2000, the upper part of this valley was affected by a “30-year” recurrence interval (R.I.) flood. Although of lower magnitude and somewhat different nature from that of 1957 (>100-year R.I. flood), the 2000 event induced serious damage to infrastructure and buildings on the valley floor. Use of methods including high-resolution aerial photography, multi-date mapping, hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the Guil River and its tributaries. The total rainfall (260 mm in four days) and maximum hourly intensity (17.3 mm h −1), aggravated by pre-existing saturated soils, explain the immediate response of the fluvial system and the subsequent destabilisation of slopes. Abundant water and sediment supply (landsliding, bank erosion), particularly from small catchment basins cut into slaty, schist bedrock, resulted in destructive pulses of debris flow and hyperconcentrated flows. The specific stream power of the Guil and its tributaries was greater than the critical stream power, thus explaining the abundant sediment transport. The Guil discharge was estimated as 180 m 3 s −1 at Aiguilles, compared to the annual mean discharge of 6 m 3 s −1 and a June mean discharge of 18 m 3 s −1. The impacts on the Guil valley floor (flooding, aggradation, generalised bank erosion and changes in the river pattern) were widespread and locally influenced by variations in the floodplain slope and/or channel geometry. The stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades, as exemplified by the Aiguilles case study, where the worst damage took place. A comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that, despite their poor maintenance, the flood control structures built after the 1957 event were relatively efficient, in contrast to unprotected places. The comparison also demonstrates the role of land-use changes (conversion from traditional agro-pastoral life to a ski/hiking-based economy, construction of various structures) in reducing the Guil channel capacity and, more generally, in increasing the vulnerability of the human installations. The efficiency of the measures taken after the 2000 flood (narrowing and digging out of the channel) is also assessed. Final evaluation suggests that, in such high mountainous environments, there is a need to keep most of the 1957 flooded zone clear of buildings and other structures (aside from the existing villages and structures of particular economic interest), in order to enable the river to migrate freely and to adjust to exceptional hydro-geomorphic conditions without causing major damage.

The importance of changing oceanography in controlling late Quaternary carbonate sedimentation on a high‐energy, tropical, oceanic ramp: north‐western Australia

AbstractThe North West Shelf is an ocean‐facing carbonate ramp that lies in a warm‐water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long‐period swells and large internal tides, resulting in preferentially accumulating coarse‐grained sediments. Circulation is dominated by the south‐flowing, low‐salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward‐flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea‐level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid‐ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15·4–12·7 Ka, in somewhat saline waters during initial stages of post‐Last Glacial Maximum (LGM) sea‐level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner‐ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine‐derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid‐ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer‐ramp sediment is mainly pelagic, veneering shallow‐water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at ≈ 200 m water depth suggest periodic upwelling or Fe‐redox pumping, whereas enhanced near‐surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at ≈ 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner‐ramp sediments can contain important heterozoan elements, mid‐ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid‐ramp sedimentation mimicking maximum flooding intervals, and outer‐ramp planktic productivity can generate locally important fine‐grained carbonate sediment bodies. Changing oceanography during sea‐level rise can profoundly affect sediment composition, sedimentation rate and packaging.

Regional to global correlation of lower Cretaceous (Hauterivian–Barremian) shallow-water carbonates of the southern Apennines (Italy) and Dinarides (Montenegro), southern Tethyan Margin

We propose a high-resolution regional to global scale correlation of two Hauterivian/Barremian shallow water carbonate platform sections belonging to different palaeogeographic domains of the Southern margin of the Tethys. These successions are now located more than 500 km apart and are represented by 137 m of core drilled at Monte Raggeto (southern Apennines, Italy) and by 80 m of well stratified rocks cropping out in the Podgorica Mountains (Dinarides, Montenegro). They display three superimposed orders of stratigraphic cyclicity defined by elementary cycles grouped into bundles and superbundles and attributed to orbitally induced composite sea-level fluctuations. Elementary cycles are interpreted as the precession and the obliquity periodicities (≅20–40 ky), while bundles and superbundles correspond to the short (≅100 ky) and long (≅400 ky) eccentricity cycles, respectively. As both the ratios between the elementary cycles and the bundles and between the bundles and the superbundles do not always reflect the classical 5:1 and 4:1 relationship, the sedimentary record of both the studied sections is likely to include omission of bundles or their portion. The general evolution of depositional and diagenetic facies, the vertical change in thickness of the cyclic units, and the interpretation of superbundles as small-scale depositional sequences, suggest that elementary cycles and their groups are superimposed on lower-frequency environmental oscillations. These display an upward Transgressive/Regressive Trend (T/RT) of the facies and testify to larger-scale variations of accommodation potential on the shelf. T/RTs, sequence stratigraphic interpretation of the superbundles as well as the common occurrence of the biostratigraphic dated Hauterivian/Barremian boundary in the studied sections, support the long-distance regional correlation between the successions to be carried out on the 100 ky scale. Moreover, by assuming the maximum flooding intervals of time-equivalent superbundles as virtually isochronous intervals, we have detected stratigraphic gaps of 1200 and 600 ky in the Monte Raggeto and in the Podgorica Mountains sections, respectively. Both sections are formed of 10 complete superbundles each of them testifying to the 400-ky-long eccentricity. Hence, the Monte Raggeto and Podgorica Mountains successions represent a duration of at least 4.0 My in terms of elapsed time, regardless of gaps within the stratigraphic record. Finally, a correlation between the orbitally induced compared sections and both the 3rd order Tethyan stratigraphic cycles and the global cycle chart is suggested. Our study confirms that cyclically organized, shallow marine carbonate platform deposits controlled by orbitally induced sea-level fluctuations represent a powerful tool for high-precision regional and global correlations. This can be of fundamental importance for quantification of sedimentary, diagenetic, and evolutionary processes as well as for the understanding of autocyclic versus allocyclic control on sequence development.

Lower Paleozoic stratigraphic and biostratigraphic correlations in the Canadian Cordillera: implications for the tectonic evolution of the Laurentian margin

The ancient Laurentian margin rifted in the latest Neoproterozoic to early Cambrian but appears not to have developed as a simple passive margin through a long, post-rift, drift phase. Stratigraphic and conodont biostratigraphic information from four platform-to-basin transects across the margin has advanced our knowledge of the early Paleozoic evolution of the margin. In northeastern British Columbia, two northern transects span the Macdonald Platform to Kechika Trough and Ospika Embayment, and a third transect spans the parautochthonous Cassiar Terrane. In the southern Rocky Mountains, new conodont biostratigraphic data for the Ordovician succession of the Bow Platform is correlated to coeval basinal facies of the White River Trough. In total, from 26 stratigraphic sections, over 25 km of strata were measured and > 1200 conodont samples were collected that yielded over 100 000 conodont elements. Key zonal species were used for regional correlation of uppermost Cambrian to Middle Devonian strata along the Cordillera. The biostratigraphy temporally constrains at least two periods of renewed extension along the margin, in the latest Cambrian and late Early Ordovician. Alkalic volcanics associated with abrupt facies changes across the ancient shelf break, intervals of slope debris breccia deposits, and distal turbidite flows suggest the margin was characterized by intervals of volcanism, basin foundering, and platform flooding. Siliciclastics in the succession were sourced by a reactivation of tectonic highs, such as the Peace River Arch. Prominent hiatuses punctuate the succession, including unconformities of early Late Ordovician, sub-Llandovery, possibly Early to Middle Silurian and Early Devonian ages.

The environment of Venice area in the past two million years

A ca. 950 m thick succession that was continuously cored in 1971 in Venice has been revisited, in order to reconstruct the environmental history of the Venice area since about 2.15 Ma. Magnetic polarity stratigraphy, integrated with refined calcareous nannofossil biostratigraphy, provides a firm chronostratigraphic framing of the succession. In order to improve the chronological resolution, we derived astrochronological refinements in the lower Pleistocene sapropel-bearing interval by a comparison with other time-correlative sections in the Mediterranean. The pollen record is used as a proxy of climatic changes and as an indirect tool in the chronological reconstruction in the upper part of the succession. The following history has been inferred: (1) in the late Gelasian (late Pliocene), the depositional area was a strongly subsiding shelf which shoaled to near sea level; (2) following a hiatus of a minimum duration of 0.2 Myr, encompassing most of the Olduvai Subchron, the shelf rapidly drowned to bathyal depths over the early Pleistocene (biozones MNN 19a to 19e: from 1.947 Ma to 0.96 Ma). This interval was characterized by starved sedimentation (less than 10 cm/kyr), represented by hemipelagic muds interbedded with sapropel layers; (3) during most of biozone MNN19f ( Pseudoemiliania lacunosa Zone, 0.96–0.42 Ma) a thick package of turbidites was laid down as a result of massive terrigenous input from the eastern Southern Alps; (4) later, in the middle part of Chron 1n (Brunhes), deltaic sedimentation, primarily related to the progradation of the paleo-Po system, led to the progressive infill of the basin. This progradational episode was a major building phase, and ended with the first appearance of continental sediments, tentatively correlated with marine oxygen isotope substage 8.4; (5) the upper part of the succession shows a cyclic organization, with an upward increasing amount of marginal-marine and subaerial deposits. In this interval the Venice area was below sea level during glacioeustatic highstands but became emergent during subsequent major glacioeustatic lowstands. Pollen data support an overall good correspondence of continental sediment packages of sequences with glacial conditions and of maximum flooding intervals with interglacial conditions.

Progradation along a deeply submerged Oligocene–Miocene heterozoan carbonate shelf: How sensitive are clinoforms to sea level variations?

We combine two- and three-dimensional seismic stratigraphic interpretation with paleobathymetric analysis from benthic foraminifera to understand the genetic significance of prominent seismic discontinuity surfaces typically mapped as sequence boundaries and flooding surfaces in the late Paleogene–early Neogene northern Carnarvon Basin.The progradational succession, dominated by heterozoan carbonate sediments, is divided into 5 northwest-prograding clinoformal sequences and 19 subsequences. Clinoform fronts progress from smooth to highly dissected, with intense gullying apparent only after the middle Miocene optimum. Once initiated, gullies become the focus for sediment distribution across the front. Bottomsets remain relatively sediment starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through the slope conduits. Along-strike sediment transport superimposed on progradation changes from southwest directed in the late Oligocene to northeast directed in the late middle Miocene, suggesting a major reorganization of circulation in the southeastern Indian Ocean.Prominent seismic discontinuity surfaces represent both intervals of shallow paleowater depth and flooding of the shelf. Partial exposure of the shelf indicated by karst morphology is coeval, with middle to outer neritic paleowater depths on the outer shelf. Instead of building to sea level, progradation occurs with shelf paleowater depths at the clinoform rollover greater than 100 m. Therefore, in the northern Carnarvon Basin, onlap onto the clinoform front is not coastal, and the sensitivity of the clinoforms to sea level changes is muted.

Mio–Pliocene to Pleistocene paleotopographic evolution of Brittany (France) from a sequence stratigraphic analysis: relative influence of tectonics and climate

The Mio–Pliocene in Western Europe is a period of major climatic and tectonic change with important topographic consequences. The aim of this paper is to reconstruct these topographic changes (based on sedimentological analysis and sequence stratigraphy) for the Armorican Massif (western France) and to discuss their significance. The Mio–Pliocene sands of the Armorican Massif (Red Sands) are mainly preserved in paleovalleys and are characterized by extensive fluvial sheetflood deposits with low-preservation and by-pass facies. This sedimentological study shows that the Red Sands correspond to three main sedimentary environments: fluvial (alluvial fan, low-sinuosity rivers and braided rivers), estuarine and some rare open marine deposits (marine bioclastic sands: “faluns” of French authors). Two orders of sequences have been correlated across Brittany with one or two minor A/ S cycles comprised within the retrogradational trend of a major cycle. The unconformity at the base of the lower cycle is more marked than the unconformity observed at the top, which corresponds to a re-incision of the paleovalley network. A comparison of the results of the sequence stratigraphy analysis with eustatic variations and tectonic events during the Mio–Pliocene allows (1) to discuss their influence on the evolution of the Armorican Massif and (2) to compare the stratigraphic record with other west-European basins. The unconformity observed at the base of the first minor cycle may be attributed to Serravallian–Tortonian tectonic activity and/or eustatic fall, and the unconformity of the second minor cycle may be attributed to Late Tortonian–Early Messinian tectonic activity. The earlier unconformity is coeval with the development of a “smooth” paleovalley network compared to the jagged present-day relief. A single episode of Mio–Pliocene deformation recorded in Brittany may be dated as Zanclean, thus explaining the lack of the maximum flooding surface except in isolated areas. From this study, five paleogeographic maps were drawn up also indicating paleocurrent directions: three maps for the lower cycle (Tortonian retrogradational trend, Late Tortonian to Early Messinian maximum flooding surface and Messinian progradational trend) and two for the upper cycle (Pliocene retrogradational trend and Piacenzian maximum flooding surface). These maps show (1) the variations of paleocurrent directions during the Mio–Pliocene, (2) the extent of estuarine environments during the maximum flooding intervals and (3) a paleodrainage watershed oriented NNW–SSE following the regional Quessoy/Nort-sur-Erdre Fault during the retrogradational trend of the upper cycle and possibly during the progradational trend of the lower cycle. The present-day morphology of the Armorican Massif is characterized by (1) incised valleys and jagged topography, in contrast with the “smooth” morphology described for Mio–Pliocene times and (2) a main East–West drainage watershed, located to the north, separating rivers flowing towards the English Channel from rivers flowing towards the Atlantic Ocean. The Mio–Pliocene/Pleistocene paleotopographic changes seem to be controlled by climatic effects. These can be related to the change in runoff associated with warmer and wetter conditions during the Mio–Pliocene, which control the river discharge and lead to the development of extensive fluvial sheetflood deposits. Tectonic or eustatic factors exert a second-order control.

The influence of land use, soils and topography on the delivery of hillslope runoff to channels in SE Spain

AbstractIt is generally accepted that within particular physiographic and climatic regions catchments exhibit differences in their hydrological response. These differences result from the interaction of spatial variability in catchment characteristics, variability of rainfall inputs and surface and subsurface hydrological processes. These interactions are complex and difficult to unravel. Hydrologically similar surfaces (HYSS) have been used to identify catchment areas that have a similar response to rainfall and have been identified at a number of scales. HYSS have been identified at the subcatchment scale for the Rambla de Nogalte in SE Spain. Areas with similar at‐a‐point hydrological storages were distinguished by using a combination of geology, land use and topography. This mapping was compared with discharge estimates made throughout the catchment following a seven‐year return interval flood in September 1997. From this significant flood source areas were identified from reaches showing rapidly increasing channel discharge, and associated with HYSS that combined suitable internal characteristics with good connectivity to the main channel. This paper presents a simulation model that has been developed to investigate the way in which the hydrological response of areas within a HYSS respond to changes in source area, gradient, connectivity to the channel, storm size and intensity profile. This is one of the first studies using a hillslope model to investigate spatial patterns of runoff‐response in semi‐arid areas and results have implications for scaling up hydrological response, and on how the dynamics of runoff producing areas vary both under changing storm conditions and over time. It is implicit in our results that the nature of stream–slope coupling differs substantively between semi‐arid and humid areas. Copyright © 2002 John Wiley & Sons, Ltd.