Wave-cut Terraces Research Articles

High frequency sea level fluctuations recorded in the Black Sea since the LGM

This paper presents a 3D geometric interpretation of very high resolution seismic Chirp profiles acquired on the Romanian shelf during ASSEMBLAGE European Project. The results provide a solid record of the Black Sea Last Glacial Maximum (LGM) water level fluctuations. This pseudo-3D seismic interpretation shows that the Black Sea lacustrine shelf deposits form a significant basinward-prograding wedge system. On top of these prograding sequences is a set of sand dunes that delineates a wave cut-terrace like feature around the isobath − 100 m. Landward of this dune field are small depressions containing barkhan-like bodies. The upper part of the last prograding sequence is incised by anastomosed channels which end in the Danube (Viteaz) canyon which are also built on the lacustrine prograding wedge. Ten stratigraphic units were distinguished and correlated with analyses of cores retrieved from this area. The results demonstrate that the first eight sequences represent lacustrine prograding wedges, the ninth sequence is the dune system itself and the tenth is a marine mud drape which covers the entire continental shelf. The lacustrine prograding wedges represent a lowstand deposit characterised by forced regression-like reflectors mapped from the pseudo-3D seismic data. Their hinge point corresponds to the wave erosion surface mapped around − 100 m isobath on the multibeam mosaic. Dated cores give age control on this lowstand period, which lasted from 11 to 8.5 kyr 14C BP as implied by: (1) the continuously dry climatic conditions in the region inferred by high percentages of herbs and steppe elements determined from ASSEMBLAGE cores and, (2) the formation of dunes between 10 and 8.5 kyr 14C BP on the desiccated north-western Black Sea shelf at − 100 m and (3) all of these covered by a marine mud drape confirming that the dune system is no longer active. The buried, anastomosed fluvial channels that suddenly disappear below 90 m depth, and a unique wave-cut terrace between 95 and 100 mbsl on the outer shelf are also consistent with a major lowstand base-level at around − 100 m water depth. Preservation of sand dunes and the occurrence of small, buried incised valleys mark a rapid transgression within less than a century during which ravinement processes related to the water level rise had no time to significantly erode the seafloor.

Application of Terrestrial Laser Scanning in determining the pattern of late Pleistocene and Holocene fault displacement from the offset of pluvial lake shorelines in the Alvord extensional basin, northern Great Basin, USA

The northwardtrending Alvord extensional basin of southeastern Oregon lies along the northern margin of the Great Basin. The basin is nearly 200 km long and 15 km wide and is bound and internally dissected by a complex system of active normal faults. The faults cut Pleistocene wave-cut terraces that formed along successive shorelines of pluvial Lake Alvord. The wave-cut terraces are incised into late Tertiary volcanic and sedimentary rocks and unconsolidated Quaternary sediments. The terraces formed during stillstands of the ancient lake, during at least two cycles of lake fill and desiccation in the Pleistocene. Wave-cut terraces are divided into two sets, with the topographically higher and older Serrano terrace series consisting of three shorelines and the Alvord terrace series composed of the five topographically lower and younger shorelines. Shorelines are dated locally, and together with regional correlation to other pluvial lakes, the Serrano highstand is estimated as 200–130 ka and the Alvord highstand as 20–15 ka. High-resolution topographic images of the terrace morphology acquired by Terrestrial Laser Scanning georeferenced with the Global Positioning System allowed detailed analysis of shoreline altitudes where they are crosscut by faults. Variation in shoreline altitude measured across faults and on opposing sides of the basin indicates that fault slip occurred during and following periods of lake-level recession. The Serrano terrace highstand records a cumulative vertical displacement of 137.5 ± 3.6 m and the vertical offset of the younger Alvord terrace series highstand is 72.5 ± 2.8 m. Fault displacement is heterogeneously distributed across the basin. Faults along the western margin of the basin accommodated nearly 50% of the total displacement, with one fault accommodating over 20% of the total displacement budget. The residual displacement is distributed across the basin. As much as 30% of the cumulative offset is taken up by structures concealed beneath basin cover and 20% by structures exposed in the highlands along the eastern flank of the basin. Since formation of the Alvord shorelines, the rate of fault displacement was nonperiodic and the basin underwent elevated activity in the late Pleistocene and early Holocene. Vertical displacement rates vary through time, and offset of Alvord terraces occurred at 3.6–7.3 mm/yr, whereas displacement of the Serrano terraces ranged from 0.7 to 1.1 mm/yr. When the horizontal component of motion is calculated by using fault dips of 60°, the 104 yr time-scale rate determined from the Alvord terraces is as much as 2.4–4.2 mm/yr and exceeds the contemporary horizontal displacement across the basin of 1.75 mm/yr determined geodetically. The 105 yr horizontal displacement rate calculated for the Serrano highstand is substantially lower, 0.4–0.6 mm/yr. Spatial and temporal pattern of faulting within the Alvord basin illustrates the complexity of strain release within the basin and highlights the length- and time-scale dependence of deformation rate estimation within extensional basins.

Shoreface migrations at the shelf edge and sea-level changes around the Last Glacial Maximum (Gulf of Lions, NW Mediterranean)

The Bourcart–Hérault canyon interfluve in the Gulf of Lions (NW Mediterranean) was the site of very high sedimentation (up to 2.5 m/kyr) around the Last Glacial Maximum, due to the vicinity of major fluvial systems that fed the shelf edge. Shoreface deposits and offshore muds deposited during each glacial/interglacial 100 kyr-cycle, created a repeated motif with high-angle and low-angle clinoforms on seismic profiles. New detailed morphological, sedimentological and paleo-environmental data, constrained by 14C dating, allow us to propose a scenario for the evolution of this critical area between 46 and 15 cal. kyr BP. The major seismic sequence (S5), formed as a forced regression during the overall sea-level fall between MIS-3 and MIS-2, can be sub-divided into several prograding units, which indicate that relative sea-level changes were punctuated by intervals of increased fall or slow-down, or even stillstand. Similarly, the onset of sea-level rise was marked by steps, during which wave-cut terraces formed, and can be tracked all around the Gulf of Lions. Three stillstands or slow-downs of sea-level change are identified and dated by deposits found at 90 m relative sea-level (21 cal. kyr BP), 110–115 m rsl (18–17 cal. kyr BP), and 98–105 m rsl (before 15.9 cal. kyr BP). Cementation of beach rocks, that now form eroded pinnacles culminating at 90 m, occurred during the 21 kyr stillstand. The position of relative sea-level during the maximum lowstand is not known, but should be deeper than 115 m. Periods of increased northwesterly winds favoured transport of coarser sediment at the shelf edge, in relation with deep water cascading: this created a distinct sandy interval during the period of Heinrich event 2 (around 24 cal. kyr BP), at the origin of amplitude anomaly on very high-resolution seismic data. The increased sea-level rise, around 15 cal. kyr BP, resulted in an abrupt decrease of sedimentation.

The application of lidar-derived digital elevation model analysis to geological mapping: an example from the Fundy Basin, Nova Scotia, Canada

High-resolution laser altimetry (lidar) is applied to geological problems such as bedrock and surficial mapping and the relationship to earth surface processes in the Fundy Basin of Nova Scotia. A "bald earth" lidar digital elevation model (DEM) is used in conjunction with field observations to map three flow units of the Jurassic North Mountain Basalt (NMB) based on contrasting terrain characteristics (slope, smoothness, and relief). The variable resistance of the flow units to erosion, documented by shatterbox experiments and down-core fracture density data, has a measurable control on incision by post-glacial consequent streams. In catchments where till cover is thick, greater surface runoff and weaker infiltration increase incision by as much as 43% for a given flow unit. Interpretation of field, petrologic, and digital topography data indicates previously unrecognized craters in the lower flow unit are the result of interaction between partially solidified lava and surface water or groundwater. Two new sets of surficial landforms have been identified that indicate ice was directed northwestward into the Bay of Fundy during the late stages of glaciation. Twice as many wave-cut terraces have been identified in the lidar DEM than previously mapped, and elevations have been extracted and compared with published values that are related to higher sea-levels at 12–14 ka. This paper demonstrates through a range of examples that the precision and enhanced resolution of lidar can improve our understanding of how landscapes form and evolve.

Aeolian and marine deposits of the Tabakbaai Quarry area, western Cape, South Africa

The Prospect Hill Formation is well exposed in the Tabakbaai Quarry near Saldanha Bay and represents the southern portion of a 20km long fossilised dune cordon oriented north-northeastward. The Prospect Hill Formation consists of basal sandy beach deposits conformably overlain by up to 70m of aeolian sands. Carbonate shell fragments, foraminiferal tests, echinoderm spines and barnacle plates make up 90% of the sand-size fraction, well-rounded quartz grains make up the remaining 10%. Previously proposed ages for the Prospect Hill Formation range from middle Miocene (~10 to ~12 Ma), based on eggshell of an extinct ostrich species, to latest Pliocene/early Pleistocene based on foraminiferal biostratigraphy. Strontium isotope stratigraphy of various biogenic components from this study, however, indicates a latest Miocene/early Pliocene age for the Prospect Hill Formation, making these dunes the oldest coastal dune system in the Western Cape. Our strontium ages are consistent with the global sealevel highstand at the Mio/Pliocene boundary and appear to correlate with the terrestrial fauna at Langebaanweg Quarry. A reconstruction of the palaeoshoreline in the Saldanha Bay area demonstrates that coastal progradation was dominant in the area up until the latest Miocene/early Pliocene, when sea level was 25m above present day, and strong southerly winds formed the carbonate dune cordon. Evidence of a 23 to 24m highstand during the early Pleistocene is recorded by a coquina bed deposited on a wave cut terrace, similar to other marine terraces preserved along the west and east coasts of southern Africa.

The Danube submarine canyon (Black Sea): morphology and sedimentary processes

The Danube Canyon is a large shelf-indenting canyon that has developed seaward of the late Pleistocene paleo–Danube valley. Mechanisms of canyon evolution and factors that controlled it are revealed by analyzing the morphology and the sedimentary structure of the canyon, as well as the main features of the continental margin around the canyon. This is based on investigation by swath bathymetry in the canyon area combined with different types of seismic data. The canyon is a major erosional trough with a flat bottom cut by an entrenched axial thalweg. The thalweg path varies from highly meandering to fairly straight in relation to the local gradient. Segments of the canyon are characterized by specific morphology, orientation and gradient along the axial thalweg. We interpret these segments in terms of canyon maturity. The sedimentary structure of the canyon documents an older phase of erosion followed by partial infilling, and thus attests for repeated cycles of canyon development. Canyon morphology is interpreted as a result of erosive sediment flows along the entrenched axial thalweg that caused downcutting into the canyon bottom and instability of the canyon walls, and hence enlargement of the canyon and expansion by headward erosion. During the last lowstand level of the Black Sea the canyon was located in an area of high sediment supply close to the paleo-Danube River mouths. This is indicated by buried fluvial channels on the shelf and by a wave-cut terrace associated with a water level situated about −90 m below the present level. We infer that erosive flows in the canyon resulted from hyperpycnal currents at the river mouths, probably favored by the low salinity environment that characterized the Black Sea during lowstand times. Other mechanisms could have contributed to trigger sediment failure along the canyon, such as instability related to the presence of shallow gas, or the effect of a deep fault.

A terrestrial record of Last Interglacial climate preserved by voluminous debris avalanche inundation in Taranaki, New Zealand

AbstractAt Airedale Reef, western North Island, New Zealand, a ca. 4 m thick volcanogenic debris avalanche deposit has facilitated the preservation of an enveloping sequence of peats with interbedded andesitic tephras spanning marine isotope (MIS) 5. The sequence closely overlies a wave‐cut terrace correlated to MIS 5e and, in turn, is overlain by andic beds with tephra interbeds including the Rotoehu and Kawakawa tephras deposited during early MIS 3 and mid‐MIS 2, respectively. Pollen analysis of the organic sequence shows a coherent pattern of fluctuating climate for the Last Interglacial–Last Glacial transition that corresponds with marine isotope stratigraphy and supports the contention that orbital variations were a primary factor in late Quaternary southern mid‐latitude climate change. A five‐stage subdivision of MIS 5 is clearly recognised, with marine isotope substage (MISS) 5b drier than MISS 5d, and the cooling transition from 5a to MIS 4 also may have been comparatively dry and characterised by natural fire, perhaps associated with volcanism. Several other examples of volcanic impact on vegetation and the landscape are evident. The Airedale Reef sequence exhibits strong similarities with fragmentary MIS 5 pollen records preserved elsewhere in New Zealand and enables the proxy record of southern mid‐latitude climatic variability during the Last Interglacial–Glacial cycle to be extended. Copyright © 2004 John Wiley & Sons, Ltd.

The Offshore Palos Verdes Fault Zone near San Pedro, Southern California

High-resolution seismic-reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington graben, the Palos Verdes fault zone, various faults below the west part of the San Pedro shelf and slope, and the deep-water San Pedro basin. The structure of the Palos Verdes fault zone changes markedly along strike southeastward across the San Pedro shelf and slope. Under the north part of the shelf, this fault zone includes several strands, with the main strand dipping west. Under the slope, the main fault strands exhibit normal separation and mostly dip east. To the southeast near Lasuen Knoll, the Palos Verdes fault zone locally is low angle, but elsewhere near this knoll, the fault dips steeply. Fresh seafloor scarps near Lasuen Knoll indicate recent fault movement. We explain the observed structural variation along the Palos Verdes fault zone as the result of changes in strike and fault geometry along a master right-lateral strike-slip fault at depth. Complicated movement along this deep fault zone is suggested by the possible wave-cut terraces on Lasuen Knoll, which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll. Modeling of aeromagnetic data indicates a large magnetic body under the west part of the San Pedro shelf and upper slope. We interpret this body to be thick basalt of probable Miocene age. This basalt mass appears to have affected the pattern of rock deformation, perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt. West of the Palos Verdes fault zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because we lack age dates on deformed or offset sediment. Manuscript received 26 February 2003.

Modeling the development of marine terraces on tectonically mobile rock coasts

A mathematical model was used to study the effect of glacially induced fluctuations in sea level on the formation of wave-cut terraces on tectonically mobile rock coasts. Two deep water wave sets were used to calculate breaker height and depth, which, along with surf zone width, bottom roughness, and the gradient of the submarine slope, dictated the force exerted at the shoreline. An erosional force threshold was employed to represent the variable strength of the rocks. The model also considered tidal range, the time that the water level spent at each intertidal elevation, and the protective effect of debris accumulation at the cliff foot. Three hundred runs were made with constant sea level and with different rates of rising and falling relative sea level. Rates of erosion increased with the rate of rising and falling sea level, but eventually decreased in some runs with very rapid changes in relative sea level. Fifty-five longer runs were also made with a Quaternary sea level model that consisted of 26 glacial cycles representing the period from 2 million to 0.9 million years ago, and nine cycles, of approximately twice the amplitude and wavelength, in the last 0.9 million years. These runs were made on a landmass experiencing slow (0.11 mm yr −1) or fast (0.74 mm yr −1) positive or negative changes in the elevation of the land. It was found that on rising landmasses, erosional coastal terraces are formed during interglacial stages, and on subsiding landmasses during glacial stages. The number of terraces increased with the rate of uplift and subsidence, and with the slope of the hinterland. Terrace gradient increased with tidal range, and decreased with rock resistance. The was an inverse relationship between terrace width and the strength of the rock.

Rapid sea-level change and coastal evolution on the Pacific margin of Canada

Abstract A rapid regression occurred off the northern Pacific margin of Canada approximately between 14,600 and 12,500 14C years B.P., contemporary with the deglaciation of the continental shelf, primarily due to the rapid isostatic rebound. Sea level had reached a maximum lowering of greater than 150 m and remained low until approximately 12,400 14C years B.P., after which a rapid transgression occurred. At 12,500 14C years B.P., the shelf tilt across the northern Pacific margin of Canada ranged between 50 m of submergence at Prince Rupert on the British Columbia mainland to greater than 150 m of emergence on the western edge of the Queen Charlotte Islands. This regressive/transgressive cycle lasted only from 5200 to 5500 14C years, a result of the development and collapse of a glacioisostatic forebulge. Sediment supply, wave action and tidal-current energy are the primary factors that controlled the coastal response to these late Quaternary relative sea-level changes. Plate tectonics, on the other hand, played a secondary role in coastal evolution. During deglacial regression, extensive glacial outwash and glaciomarine deposition occurred except on the outer coast where sediment was transferred to the shelf break. During the transgression of the shelf, the sediment supply was primarily restricted to the erosion of the previously deposited deglacial deposits, resulting in the formation of drowned wave-cut terraces, spit platforms and modification of estuarine environments, as the sea level rose in steps. The present unconsolidated coastline is mostly erosional. During the most recent ENSO event, the sea-level rise of up to 0.4 m resulted in as much as 12 m of coastal retreat.

Sedimentary Architecture and Dynamic Stratigraphy of a Marine Ice-Contact System

ABSTRACT The Mona moraine in southern Norway is a sedimentary ridge, 3.5 km long, formed at the mouth of a shelf trough during a late Younger Dryas readvance of the Scandinavian ice sheet. The moraine architecture, sedimentation processes, and record of ice-front behavior have been reconstructed in a high-resolution allostratigraphic framework on the basis of ground-penetrating radar profiles and sedimentologic analysis of outcrop sections. The shelf trough was draining ice from adjacent platforms, which resulted in ice-front protrusion with a moraine offset relative to the coeval swarm of small moraines formed in the shallow-water zone. The grounding-line system at Mona commenced its development as an ice-contact submarine fan, while the glacier front was thinning and grinding to a halt on a bedrock threshold. The ice flux declined, but fluctuated, and the subglacial sediment supply by meltwater prevailed. The ice-front stillstand allowed the submarine fan to aggrade to the sea level and turn into a Gilbert-type delta with a short (160 m) distributary plain. The delta foreset, 50-60 m thick, consists of gravelly massflow deposits, and its sandy turbiditic toe has been extended on the threshold's steep lee slope to a distance of at least 2 km. The delta was abandoned by rapid calving of the ice front, and the ice-contact slope was covered with marine mud and resedimentation products. A radiocarbon date from the base of this thin cover indicates ice retreat at around 10,100 yr BP. The moraine was surrounded by the sea and subject to reworking by waves, tides, and longshore currents. A thick package of regressive foreshore deposits with downstepping wave-cut terraces on the moraine slopes reflects gradual emergence due to regional glacioisostatic uplift. The study demonstrates that the facies architecture of marine moraines bears an important record of glacier dynamics and ice-front behavior.

Paleoshoreline geomorphology of Böön Tsagaan Nuur, Tsagaan Nuur and Orog Nuur: the Valley of Lakes, Mongolia

We conducted a preliminary study of paleoshoreline features associated with Böön Tsagaan Nuur, Tsagaan Nuur, and Orog Nuur, lakes located in the Gobi–Altai transition zone of the Valley of Lakes (Dolina Ozor) which stretches from central to western Mongolia. The paleoshoreline features were first identified on RADARSAT satellite SAR imagery. We investigated the features during the 1998 field season of the Joint Mongolian–Russian–American Archaeological Expedition to the Gobi–Altai region. We identified paleoshorelines of multiple elevations in the field, which are considered to be relict beach ridges and wave-cut terraces. Other paleolake landforms include spits and Gilbert-type deltas. These landforms are complex, large and well established, implying that the paleolakes were stable for extended periods. The reconstructed paleolakes cover extensive areas of the valley floor, implying that hydrological and climatic conditions were very different in the past. Paleolake expansions may have occurred under a variety of circumstances. One hypothesis is that the high lake stands occurred during the wetter period corresponding to the Oxygen Isotope Stage 3 prior to the Last Glacial Maximum (LGM), during the warmest early Holocene and the late Holocene, or during all these periods. If low evaporation rates due to lower temperatures, glacier meltwater and possibly increased precipitation are important factors, then the expansions may have occurred during the terminal Late Glacial period after the Last Glacial Maximum. The greatly expanded lakes in the Gobi–Altai could have significantly affected the Quaternary human demography and migration in the region.

Cretaceous karst guyots: New evidence for inheritance of atoll morphology from subaerial erosional terrain

Data from recent surveys and drilling suggest that carbonate platforms over a wide region of the oceanic western Pacific became emergent by amounts of 100–200 m during late Albian time, ca. 100 Ma. The resulting erosional landscapes, now modified by differential compaction over the buried volcanic basement hills, included central basins surrounded by perimeter rims, sinkhole-like depressions, and perimeter benches interpreted as wave-cut terraces. Resubmergence resulted in the sealing of atoll-like erosional topography by pelagic sediments. These Cretaceous proto-atolls, now guyots, provide evidence that ancient as well as modern atolls inherit their form mainly from subaerial landscapes.

A double island arc between Taiwan and Luzon: consequence of ridge subduction

Analysis of geomorphological, geochronological, geochemical and geophysical features in the segment of the Taiwan-Luzon Arc between Taiwan and Luzon (the Bashi Segment) allows the recognition of a double arc structure. The two volcanic chains are separated by 50 km just north of Luzon (18°N), and converge near 20°N. Islets in the western chain are older and largely composed of volcanic rocks of Miocene to Pliocene age. They all show low relief, lateritic platforms and wave-cut terraces, and are covered by massive recrystallized limestone. In contrast, all active volcanoes in this segment of the Taiwan-Luzon Arc belong to the eastern chain, where most islets are Quaternary in age. The volcanoes have well-developed cone shapes, and well-preserved deposits of near-vent facies. Magmas of the eastern chain have higher K Si, (La) n,(La/Yb) n, and lowerε Nd than their counterparts at the same latitude in the western chain. Therefore, the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain. Moreover, the available seismological data seem to suggest an abrupt increase of the dip angle from 30° at 18°N to 80° at 20°N. Thus, the double arc structure is located in the region where the Benioff zone suddenly changes. In analogy with the Lesser Antilles Arc, we propose a geodynamic model in which the double arc in the Bashi Strait is the tectonic manifestation of the subduction of the aseismic Scarborough Seamount Chain, the extinct mid-ocean ridge of the South China Sea. Before that ridge reached the Manila Trench, the western chain was the volcanic front. When the ridge reached the subduction zone at 5–4 Ma, its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity. Furthermore, this ridge-arc collision was probably also responsible for regional uplift causing extensive sub-aerial weathering and erosion as well as massive reef formation in the western chain. When subduction started again, the dip angle became shallower in response to the extra buoyancy of the downgoing ridge. If the depth of magma generation remained constant, the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain. Moreover, we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest, thus imprinting a geochemically more enriched signature on the magmas of the eastern chain.

Forced Regressions and Lowstand Deltas: Holocene Canadian Examples

ABSTRACT Some Canadian Holocene deltas that formed during and after relative-sea-level falls of over 100 m are modern examples of forced regressions and lowstand deltas. The principal controls on deltaic morphology and stratigraphy are rate of sea-level change, sediment supply (timing, type), basin physiography, and sediment dispersal processes in the receiving basin. Our observations indicate important alongstrike variability in deposition and erosion that affect facies architecture and stratal geometry. During rapid relative-sea-level fall, fluvial channels incise rapidly and do not migrate. Lateral channel migration, which would produce a widespread fluvial unconformity, begins once relative-sea-level fall has slowed and continues through the early part of relative-sea-level rise. Parts of he emerging shoreline cut off from the fluvial sediment supply become sites of shoreface incision into prodelta deposits (regressive marine erosion), and wave-cut terraces are developed. Where sandy sediment is then supplied to the shoreface, by rivers or updrift sources, sharp-based littoral sandbodies directly overlie the shoreface erosion surface. Other parts of the deltas below wave base that become abandoned through lobe switching can also be eroded by tidal currents. Deltas that have prograded into deep water show evidence of submarine slope instability, such as submarine channel development, base-of-slope turbidites, and thick- and thin-skinned failures that disrupt clinoforms. Isopach trends reflect the existence of underlying structure, with Holocene sediments locally either thi ning or pinching over bedrock highs.

Morphology and sedimentology of two contemporary fan deltas on the southeastern Baja California Peninsula, Mexico

The San Juan de Los Planes drainage basin is located on the southeastern margin of the Baja California Peninsula, approximately 20 km southeast of the city of La Paz. Pleistocene alluvial fans have been built into the basin from the surrounding areas of high relief and are presently being eroded. At the fan apex, the dissection depth ranges from 20 to 40 m, and is 6 to 15 m at the coast.The sediments have been redeposited in several active modern fan deltas built out into the narrow shelf of La Ventana Bay. Shelf width ranges from 0 to 5 km (average 3 km). Wave energy is delivered from southeasterly storms and from the local sea breeze winds; the respective wave heights average 2 and 1 m.The Agua Caliente and Las Canoas fan deltas, the subjects of this paper, are located on the active El Sargento Fault Zone and are fed by channels that deeply dissect the older fan surfaces. The Agua Caliente fan delta is a classic Gilbert-type delta with a steeply inclined delta front. Wave reworking and redistribution of the sediments at the delta face are important and a wave-cut terrace has developed at about 6 m depth. The Las Canoas fan delta is a shoal-water-profile, Hjulstrom type with a gently inclined delta front. Wave energy is less important. The alignments of the coasts of both fans are controlled by the fault line. Sediment supply is largest for the Las Canoas fan delta. The fan deltas are examples of this depositional form as it has developed in an arid, tectonically active, high-relief and narrow neo-trailing edge margin.

Pleistocene shoreline ridges from tide-dominated and wave-dominated coasts: northern Gulf of California and western Baja California, Mexico

The arid climate of northwestern Mexico preserves Pleistocene beach deposits forming linear, shoreline-parallel ridges on the coastal plains of Sonora, northern Gulf of California, and the Vizcaíno Peninsula of Baja California. Beach deposits represent both cheniers (northern Gulf) and beach ridges (Vizcaíno Peninsula). They formed during Pleistocene sea-level highstands, most commonly during oxygen-isotope stage 5e (∼ 125 ka). The ridges are 0.5 to 5.0 m thick, < 10 to > 100 m wide, and < 100 to > 1000 m long, coarse-grained with common low- to high-angle cross stratification, and are dominated by bivalve shells. Ridges from the tide-dominated northern Gulf of California and the wave-dominated western Vizcaíno Peninsula differ in underlying lithology, size, profile, and shell fragmentation. Northern Gulf ridges progradationally overlie deltaic or marsh deposits, are smaller (median thickness 1.4 m; median width 25 m; median length 300 m), typically have a seaward-dipping upper surface and a linear crest on the landward side, and have low levels of fragmentation of large bivalve shells. In contrast, Vizcaíno ridges disconformably overlie bedrock on wave-cut terraces, are larger (median thickness 3.3 m; median width 100 m; median length 2000 m), typically have a landward-dipping upper surface and a linear crest on the seaward side, and contain beds of extensively fragmented large bivalve shells. These differences are interpreted to reflect contrasting processes of ridge formation in the two regions. The ridges of the tide-dominated northern Gulf are true cheniers—transgressive beach deposits produced during episodes of clastic starvation associated with shifts in Colorado River delta deposition. They are modified primarily through episodic washover processes during high tides and storms. The ridges of the wave-dominated Vizcaíno Peninsula are regressive beach ridges developed on highstand wave-cut terraces. They are modified primarily through continuous wave accretion to the beach face, and secondarily by washover processes.

95/01619 Conductive heat flow through the sediments in Lake Rotomahana, New Zealand

Present-day Lake Rotomahana is one of the two focal points of the most destructive eruption in New Zealand's historical record, i.e., that of Mt. Tarawera on 10 June 1886, with devastating loss of life and presumed destruction of the iconic Pink and White Terraces that adorned the margins of the lake. Basaltic dikes are considered to have ascended near surface in the area, intruding into hydrothermally altered and water-saturated ground beneath the existing lake. The consequential hydrothermal and phreatomagmatic eruptions ejected 0.5325 km3 of material from the lakefloor and below, plastering the nearby landscape for several kilometers with mud and other debris. The eruption buried the natural outlet of the lake, with the bottom of the craters becoming filled by water within months and completely concealed from view within years; today Lake Rotomahana has depths up to 118 m.High-resolution (0.5 m) bathymetric mapping, when combined with a 2-D seismic reflection survey, enables us to ‘see’ details of the maar craters on the lakefloor, including those parts subsequently buried by sediment. The large Rotomahana Crater described by workers immediately after the eruption measures ~ 2.5 km in diameter near its southwestern end, and excavated ground to 155 m below present-day lake level. The vent system, as revealed by the present study, forms an array of right-stepping (dextral) craters, with the main crater being host to two sub-craters Rotomahana West Crater and Rotomahana East Crater today buried beneath the lakefloor, and which are in-filled by 36 and 37 m of sediment, respectively. Subordinate craters along the same 057° Tarawera Rift trace include Hochstetter Crater (11 m of infill), Waingongongongo Crater (14 m) and Rotomakariri Crater (26 m). These craters host a total 0.0268 km3 of sediment. Other features highlighted by the bathymetric data include; craters not filled by sediment, sediment fan deltas, volcanic ridges and dikes, submerged wave-cut terraces formed during times of lower lake levels and gas pockmarks, all either related to the 1886 eruptive episode or post-eruption hydrothermal and erosional processes.Application of results from bubble plume, CO2 flux, magnetic and heat flux surveys of Lake Rotomahana to this study, when combined with regional earthquake relocation analysis and broadband magnetotelluric data, suggest an explanation in terms of a magmatic heat source located south of Waimangu and a corresponding convective water/heat transport system extending thence to beneath the western end of the lake. A holistic approach has provided a coherent context for the eruption and its effect on the historical Pink and White Terraces hydrothermal system that appears to have been the eastern-most extension of a larger system that lay beneath the Waimangu area before the eruption. The newly named Pink Terraces hydrothermal system (~ 1.5 km2) is a continuation of the historical hydrothermal activity that was concentrated on the western shores of the old lake, and together with the formation of the new, post-1886 Patiti hydrothermal system (~ 1 km2) located SW of Patiti Island, mark the two distinct areas of hydrothermal activity in the lake today.

95/01621 Deep resistivity surveys in the Waiotapu-Waikite- Reporoa region, New Zealand

Present-day Lake Rotomahana is one of the two focal points of the most destructive eruption in New Zealand's historical record, i.e., that of Mt. Tarawera on 10 June 1886, with devastating loss of life and presumed destruction of the iconic Pink and White Terraces that adorned the margins of the lake. Basaltic dikes are considered to have ascended near surface in the area, intruding into hydrothermally altered and water-saturated ground beneath the existing lake. The consequential hydrothermal and phreatomagmatic eruptions ejected 0.5325 km3 of material from the lakefloor and below, plastering the nearby landscape for several kilometers with mud and other debris. The eruption buried the natural outlet of the lake, with the bottom of the craters becoming filled by water within months and completely concealed from view within years; today Lake Rotomahana has depths up to 118 m.High-resolution (0.5 m) bathymetric mapping, when combined with a 2-D seismic reflection survey, enables us to ‘see’ details of the maar craters on the lakefloor, including those parts subsequently buried by sediment. The large Rotomahana Crater described by workers immediately after the eruption measures ~ 2.5 km in diameter near its southwestern end, and excavated ground to 155 m below present-day lake level. The vent system, as revealed by the present study, forms an array of right-stepping (dextral) craters, with the main crater being host to two sub-craters Rotomahana West Crater and Rotomahana East Crater today buried beneath the lakefloor, and which are in-filled by 36 and 37 m of sediment, respectively. Subordinate craters along the same 057° Tarawera Rift trace include Hochstetter Crater (11 m of infill), Waingongongongo Crater (14 m) and Rotomakariri Crater (26 m). These craters host a total 0.0268 km3 of sediment. Other features highlighted by the bathymetric data include; craters not filled by sediment, sediment fan deltas, volcanic ridges and dikes, submerged wave-cut terraces formed during times of lower lake levels and gas pockmarks, all either related to the 1886 eruptive episode or post-eruption hydrothermal and erosional processes.Application of results from bubble plume, CO2 flux, magnetic and heat flux surveys of Lake Rotomahana to this study, when combined with regional earthquake relocation analysis and broadband magnetotelluric data, suggest an explanation in terms of a magmatic heat source located south of Waimangu and a corresponding convective water/heat transport system extending thence to beneath the western end of the lake. A holistic approach has provided a coherent context for the eruption and its effect on the historical Pink and White Terraces hydrothermal system that appears to have been the eastern-most extension of a larger system that lay beneath the Waimangu area before the eruption. The newly named Pink Terraces hydrothermal system (~ 1.5 km2) is a continuation of the historical hydrothermal activity that was concentrated on the western shores of the old lake, and together with the formation of the new, post-1886 Patiti hydrothermal system (~ 1 km2) located SW of Patiti Island, mark the two distinct areas of hydrothermal activity in the lake today.

Paleoecologic, temporal, and spatial analysis of early Silurian Reefs of the Chicotte Formation, Anticosti Island, Quebec, Canada

Reefs of the Lower Silurian Chicotte Formation are the largest and most faunally diverse known on Anticosti Island, Quebec. They reach up to 25 m in thickness and 250 m in diameter and are present predominantly at two intervals, forming a lower and upper reef cluster. Remnants of bioherms are represented on the present-day wave-cut terrace as 60 to 100 m diameter, subcircular erosional depressions known as Philip structures or as outcrop.