Modeling seismic reflections from thin-bed dolomite layers in diatomite formation

Abstract The origin of dolomite and its diagenesis are still not fully understood. The timing and occurrence of dolomite beds remain key problems. Many thin‐bed layers of dolomite in marine sediments were discovered at drill sites in the forearc basin west of the Japan Trench during the Ocean Drilling Program Leg 186. The thickness of the dolomite layers ranges from 0.5 to 2 m, estimated from the well log. These thin‐bed dolomite layers were able to be identified on seismic data, through quantitative analysis and comparison of the data with synthetic seismograms. Amplitude‐preserved seismic data processing with broadband high‐resolution enhancement helps to estimate the reflectivity series dominated by the large impedance contrast between the dolomite layers and the host sediments. In this report, the first geophysical evidence of large‐scale dolomitization is provided. The occurrence of thin dolomitic layers embedded in diatomaceous sediments is on a basin scale rather than being an isolated event, which constrains the type of dolomitization and the timing of layered dolomitization. It was found that thin‐bed dolomite layers occur where there is an abrupt change in sedimentation rate. Layered dolomitization starts when sedimentation rate changes (from high to low or vice versa). Two dolomite layers coincide with the Miocene–Pliocene and Pliocene–Pleistocene age boundaries. Formation of dolomite beds could be a timing signal of a dramatic change in paleoceanographic and paleoclimatic environments.     

Bed scour by debris flows: experimental investigation of effects of debris‐flow composition

Debris flows can grow greatly in size by entrainment of bed material, enhancing their runout and hazardous impact. Here, we experimentally investigate the effects of debris‐flow composition on the amount and spatial patterns of bed scour and erosion downstream of a fixed to erodible bed transition. The experimental debris flows were observed to entrain bed particles both grain by grain and en masse, and the majority of entrainment was observed to occur during passage of the flow front. The spatial bed scour patterns are highly variable, but large‐scale patterns are largely similar over 22.5–35° channel slopes for debris flows of similar composition. Scour depth is generally largest slightly downstream of the fixed to erodible bed transition, except for clay‐rich debris flows, which cause a relatively uniform scour pattern. The spatial variability in the scour depth decreases with increasing water, gravel (= grain size) and clay fraction. Basal scour depth increases with channel slope, flow velocity, flow depth, discharge and shear stress in our experiments, whereas there is no correlation with grain collisional stress. The strongest correlation is between basal scour and shear stress and discharge. There are substantial differences in the scour caused by different types of debris flows. In general, mean and maximum scour depths become larger with increasing water fraction and grain size, and decrease with increasing clay content. However, the erodibility of coarse‐grained experimental debris flows (gravel fraction = 0.64) is similar on a wide range of channel slopes, flow depths, flow velocities, discharges and shear stresses. This probably relates to the relatively large influence of grain‐collisional stress to the total bed stress in these flows (30–50%). The relative effect of grain‐collisional stress is low in the other experimental debris flows (<5%), causing erosion to be largely controlled by basal shear stress. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatter-rich pyroclastic flow deposits on Santorini,Greece

Pyroclastic deposits exposed in the caldera walls of Santorini Volcano (Greece), contain several prominent horizons of coarse-grained andesitic spatter and cauliform volcanic bombs. These deposits can be traced around most of the caldera wall. They thicken in depressions and are intimately associated with ignimbrite and co-ignimbrite lithic lag breccias. They are interpreted as a proximal facies of pyroclastic flow deposits. Evidence for a flow origin includes the presence of a fine-grained pumiceous matrix, flow deformation of ductile spatter clasts, exceedingly coarse grain sizes several kilometres from any plausible vent, imbrication of flattened spatter clasts, intimate interbedding with normal pyroclastic flow deposits and the presence of inversely graded basal layers. The deposits contain hydrothermally altered, rounded lithic ejecta including gabbro nodules. The andesitic ejecta and the fine matrix are typically moderately to poorly vesicular indicating that magmatic gas had a subordinate role in the eruptive process. The andesitic clasts contain abundant angular lithic inclusions and some clasts are themselves formed of pre-existing agglutinate. We propose that these eruptions occurred when external water gained access to the vents, causing large-scale explosions which formed pyroclastic flows rich in coarse, semifluid but poorly vesicular ejecta. We postulate that large volumes of coarse pyroclastic ejecta and degassed lava accumulated in a deep crater prior to being disrupted by these large explosions to form pyroclastic flows.     

Geochemical contrast between the Sanbagawa psammitic schists (Oboke unit) and the Cretaceous Shimanto sandstones in Shikoku, Southwest Japan and its geologic significance

Abstract A systematic geochemical study of sandstones from the Cretaceous Shimanto Supergroup and psammitic schists from the Oboke unit in Shikoku has been carried out in order to clarify the depositional age of the protoliths of the Oboke psammitic schists. The geochemical data, together with chronological and geologic data, led to the following conclusions. (i) It is inferred that Oboke psammitic schists are metamorphically equivalent to sandstones in the Hiwasa Formation of the Shimanto accretionary complex, deposited in a trench area during the Campanian, in eastern Shikoku. (ii) The protolith attained to maximum metamorphic conditions within 20 million years after the deposition. (iii) The accumulation of a large amount of coarse-grained clastic sediments in the trench area induced offscraping and underplating of the sediments in the subduction zone, forming the Hiwasa Formation and Oboke unit, respectively.     

Terrestrial chondrules,glass spherules and accretionary lapilli from the suevite,Ries Crater,Germany

Until recently, no terrestrial analogues of meteoritic and lunar chondrules were known. Only rare glass spherules from the Lonar Crater, India, and black magnetic spherules from various localities have been recorded. The impact breccia suevite of the No¨rdlinger Ries Crater, Germany, contains both chondrules and glass spherules, and in addition, accretionary lapilli, all of which are found imbedded within the fine-grained matrix of the suevite. The chondrules display many of the textural features characteristic of meteoritic and lunar chondrules. Lithic chondrules and fluid drop chondrules are present, the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite. Fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition. On the whole, fluid drop chondrules, lithic chondrules and glass spherules are rare in the suevite, with fluid drop chondrules prevailing. Detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules. Accretionary lapilli also represent material formed as a result of impact.     

虹口八角庙-深溪沟炭质泥岩同震断层泥的X射线衍射分析结果

5.12汶川地震同震地表破裂带在虹口八角-深溪沟一带主要出露于三叠系须家河组的炭质泥岩中,同震断层泥在颜色、结构上与老断层泥和围岩类似。通过开挖探槽,系统采样,采用粉晶X射线衍射定量分析方法,研究了同震地表破裂带的围岩、断层角砾岩、老断层泥和新断层泥的矿物成分特征。同震断层泥的主要成分为石英和黏土矿物,含微量长石和白云石;断层泥的显著特征为高黏土矿物含量,从同震断层泥、老断层泥、角砾岩到围岩黏土矿物含量依次降低,黏土矿物以伊利石和伊蒙混层为主,含微量绿泥石和高岭石,矿物组成明显比地表破裂带北段同震断层泥简单。不同颜色的同震断层泥成分略有不同,黑色断层泥中伊利石含量明显高于白色断层泥;老断层泥中含有方解石和白云石,而同震断层泥不含方解石,只含微量白云石。同震断层泥中伊蒙混层高含量表明,在本次地震错动中有富含K的流体参与。     

Silicic phreatomagmatism in the Snake River Plain: the Deadeye Member

Non-welded rhyolitic pyroclastic units in the central Snake River Plain are interbedded with the much better exposed, large-volume ‘Snake-River type’ rheomorphic welded rhyolitic ignimbrites and rhyolite lavas. We document one such unit to investigate why it is so different from the interbedded welded ignimbrites. The newly recognised Deadeye Member of southern Idaho is a soil-bounded eruption-unit that comprises ashfall layers and a 4-m-thick ignimbrite that extends for >35 km. The ignimbrite is non-welded, lithic-clast poor and varies from massive to diffuse low-angle cross-bedded. It contains abundant angular clasts of non-vesicular black glass, and upper parts contain accretionary lapilli. The ashfall layers above it contain coated ash pellets and ash clumps, which record moist aggregation of fine ash. The magmas of the Deadeye eruption were closely similar in composition and temperature to those that generated the intensely welded rheomorphic ignimbrites of the central Snake River Plain. We infer that the marked contrast in physical appearance of the Deadeye ignimbrite compared to the other, more typical Snake-River-type welded ignimbrites was the result of emplacement at relatively low temperatures during an eruption in a lacustrine environment. Magmatic volatile-driven fragmentation of the rhyolitic magma was influenced by interaction with lake water that also led to cooling. The Deadeye Member is the first-recorded example of explosive silicic phreatomagmatism in the central Snake River Plain.     

Interrelations among pyroclastic surge,pyroclastic flow,and lahars in Smith Creek valley during first minutes of 18 May 1980 eruption of Mount St. Helens,USA

A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides.     

In situ formation of welded tuff-like textures in the carapace of a voluminous silicic lava flow,Owyhee County,SW Idaho

 The Badlands rhyolite, on the Owyhee Plateau of southwestern Idaho, can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes, its well-exposed near-vent features, and its response to pre-existing topography. However, samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures, including material which closely resembles the compressed, welded glass shards which are characteristic of ignimbrites. Formation of these tuff-like textures involved processes probably common to emplacement of most silicic lava flow units. Decompression upon extrusion causes inflation of pumice at the surface of the lava flow; some of this pumice is subsequently comminuted, producing loose bubble-wall shards, bits of pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose blocks and into open fractures deeper in the flow, where it can be reheated, compressed, and annealed to varying degrees. The end result is a dense vitrophyre layer (beneath the true upper, non-welded carapace breccia) which can be extremely texturally heterogeneous, with areas of flow-foliated lava occurring very near lava which in many aspects looks like welded ignimbrite, complete with flattened pumices. Identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow-foliated rock which otherwise appears to be lava. The textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites, at least at scales ranging from thin sections to outcrops. Voluminous silicic units with scattered fragmental textures, but with otherwise lava-like features, are probably true effusive lava flows. Received: January 30, 1995 / Accepted: January 22, 1996     

The Miocene Costa Giardini diatreme,Iblean Mountains,southern Italy: model for maar-diatreme formation on a submerged carbonate platform

In this paper we present a model for the growth of a maar-diatreme complex in a shallow marine environment. The Miocene-age Costa Giardini diatreme near Sortino, in the region of the Iblei Mountains of southern Sicily, has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions. Vesicular lava clasts, accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas-rich magma. Abundant xenoliths in the upper, late-deposited beds of the ring suggest rapid magma ascent, and deepening of the eruptive vent is shown by the change in slope of the country rock. The interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount. The occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle-lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity. Intrusions of nonvesicular magma, forming plugs and dikes, occur on the western side of the diatreme, and at the margins, close to the contact between breccia deposits and country rock; they indicate involvement of volatile-poor magma, possibly during late stages of activity. We propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment. This allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center. A subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water, at which time the hyaloclastite was deposited. Emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated, locally forming peperites. The sheltered, low-energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts. Ultimately, lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions.     

Comparing the transitional behaviour of kaolinite and bentonite suspension flows

Past research has demonstrated the dramatic effects that variations in suspended clay can have on the properties of flow by producing a range of transitional flows between turbulent and laminar states, depending on clay concentration and fluid shear. Past studies have been restricted to kaolinite flows, a clay mineral that has relatively weak cohesive properties. This paper extends these studies to suspension flows of bentonite, a clay mineral that attains higher viscosities at far lower volumetric concentrations within a flow. The results show that the types of transitional flow behaviour recognized in past studies can also be found in bentonite suspension flows, but at lower suspended sediment concentrations, thus demonstrating an even more dramatic effect on flow properties, and potentially on sediment transport and resulting bed morphology, than kaolinite flows. The paper proposes new stability diagrams for the phase space of bentonite flows and compares these to past work on kaolinite suspension flows. These new data suggest that the transitional‐flow Reynolds number can be used to delineate the types of transitional flow across different clay types and assess modern and ancient clay‐suspension flows. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia

Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due to the high organic carbon content of oil shale, many geochemical studies and petroleum exploration have been conducted. Although most of the oil shales are considered to be Early Cretaceous in age, a recent study reveals that some were deposited in the Middle Jurassic. The present study aims at establishing depositional ages and characteristics of the Jurassic and Cretaceous lacustrine deposits in Mongolia. The Lower Cretaceous Shinekhudag Formation is about 250 m thick and composed of alternating beds of shale and dolomite. The Middle Jurassic Eedemt Formation is about 150 m thick and composed of alternating beds of shale, dolomitic marl, and siltstone. The alternations of shale and dolomite in both formations were formed by lake level changes, reflecting precipitation changes. Shales were deposited in the center of a deep lake during highstand, while dolomites were formed by primary precipitation during lowstand. Based on the radiometric age dating, the Shinekhudag Formation was deposited between 123.8 ±2.0 Ma and 118.5 ±0.9 Ma of the early Aptian. The Eedemt Formation was deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation rate of the Shinekhudag Formation is between 4.7 ±2.6 cm/ky and 10.0 ±7.6 cm/ky. Shales in the Shinekhudag Formation show micrometer‐scale lamination, consisting of algal organic matter and detrital clay mineral couplets. Given the average thickness of micro‐laminae and calculated sedimentation rate, the micro‐lamination is most likely of varve origin. Both Middle–Upper Jurassic and Lower Cretaceous lacustrine oil shales were deposited in intracontinental basins in the paleo‐Asian continent. Tectonic processes and basin evolution basically controlled the deposition of these oil shales. In addition, enhanced precipitation under humid climate during the early Aptian and the Callovian–Oxfordian was another key factor inducing the widespread oil shale deposition in Mongolia.     

Late quaternary hydrothermal explosion breccias at Kawerau Geothermal Field,New Zealand

Two large (10⁶–10⁷ m³ erupted volume) hydrothermal explosions occurred from craters on the eastern margin of Kawerau Geothermal Field at c. 14,500 and 9,000 yrs B.P. Explosion products are interbedded within C14 dated pyroclastic fall deposits and contain clasts of hydrothermally altered ignimbrite, rhyolite and tuff, in a silty hydrothermal clay matrix. No magmatic ejecta are found. Some ejected blocks record earlier pre-eruption episodes of shallow hydraulic fracturing and silica cementation. Drillhole stratigraphy indicates that explosion extended to about 190 m below present ground level. The explosion is analysed as a rock/water interaction with eruptive energy provided by flashing of about half the available water. Although surface heat flow and shallow temperatures are now low at eastern Kawerau, the hydrothermal explosions demonstrate the previous existence of a high temperature shallow geothermal system, probably related to a major fault feeding water up through the basement.     

‘Column on column’ structures as indicators of lava/ice interaction,Ruapehu andesite volcano,New Zealand

Lava flows of the Mangawhero Formation (ca. 15–60 ka) on Ruapehu volcano erupted during the last glaciation. In a distal flow lobe at Tukino, on the east side of the mountain, small secondary columns (10–20 cm thick) have formed on the sides of large, rectangular, primary (0.5–3 m thick) cooling columns. Thick (10 m+) zones of such small columns form a lateral and basal outer rind of the lobe. As they do not mark glassy zones of quenching, these secondary columns are interpreted as being formed by a second cooling event at temperatures below the boundary between the low creep and elastic regimes (∼ 600 °C) by rapid influx of copious amounts of water. Temperature drops deduced from extensional strains of the two sets of columns were used to gauge the viability of such a two-stage process. Absence of reliable data on andesite contraction coefficients was overcome by using a sliding scale to assess a large range of values. The estimates indicate that two-stage chilling is feasible. After flowing across relatively ice-poor terrain, the lava flow must have interacted with a valley glacier that provided water for further chilling the already formed primary columns and formation of the outer rind small columns. Given this evidence for lava/ice interaction, it is likely that prominent, thick flows elsewhere in the Mangawhero Formation may have been constrained to their ridge-top locations by ice conditions similar to those described by Lescinsky and Sisson [Lescinsky, D.T., Sisson, T.W., 1998. Ridge-forming, ice-bounded lava flows at Mount Rainier, Washington. Geology, 26, 351–354].     

K-Ar geochronology of the late cenozoic volcanic rocks of the Cordillera Occidental, southernmost Peru

Twenty-four K-Ar radiometric ages are presented for late Cenozoic continental volcanic rocks of the Cordillera Occidental of southernmost Perú (lat. 16° 57′–17° 36′S). Rhyodacitic ignimbrite eruptions began in this transect during the Late Oligocene and continued episodically through the Miocene. The development of andesitic-dacitic strato volcanoes was initiated in the Pliocene and continues to the present.The earliest ignimbrite flows (25.3–22.7 Ma) are intercalated in the upper, coarsely-elastic member of the Moquegua Formation and demonstrate that this sedimentary unit accumulated in a trough, parallel to Andean tectonic trends, largely in the Oligocene. More voluminous ash-flow eruptions prevailed in the Early Miocene (22.8–17.6 Ma) and formed the extensively preserved Huaylillas Formation. This episode was coeval with a major phase of Andean uplift, and the pyroclastics overlie an erosional surface of regional extent incised into a Paleogene volcano-plutonic arc terrain. An age span of 14.2–8.9 Ma (mid-Late Miocene) is indicated for the younger Chuntacala Formation, which again comprises felsic ignimbrite flows, largely restricted to valleys incised into the pre-Huaylillas Formation lithologies, and, at lower altitudes, an extensive aggradational elastic facies. The youngest areally extensive ignimbrites, constituting the Sencca Formation, were extruded during the Late Miocene.In the earliest Pliocene, the ignimbrites were succeeded by more voluminous calcalkaline, intermediate flows which generated numerous large and small stratovolcanoes; these range in age from 5.3 to 1.6 Ma. Present-day, or Holocene, volcanism is restricted to several large stratovolcanoes which had begun their development during the Pleistocene (by 0.7 Ma).The late Oligocene/Early Miocene (ca. 22–23 Ma) reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the Cordilleras Occidental and Oriental of the Central Andes.     

Sedimentary environment and development controls of the hydrocarbon source beds:Middle and Upper Proterozoic in northern North China

Problem on development control of marine source bed hold in Chinese petroleum industry progression. The Hongshuizhuang Formation,Tieling Formation and Xiamaling Formation in the Middle and Upper Proterozoic are important hydrocarbon source beds in northern North China, and investigation of their sedimentary environments and the controls has great significance for petroleum exploration in North China. Based on sedimentology (sequence stratigraphy), palaeoecology, sedimentary geochemistry, and sedimentary palaeogeography, their development pattern is discussed. All these studies indicate that the development controls of the hydrocarbon source beds include a favorite palaeogeographic location, exceeding propagation of biomes in low and middle latitudes, anoxic environments, enrichment of phosphorus element and the adsorption of clay minerals during the preservation of organic matter in the marine carbonates.     

Cretaceous radiolarian fauna from the Kiselyovsky subterrane, the youngest accretionary complex of the Russian continental far east: Paleotectonic and paleogeographic implications

Abstract The Kiselyovsky subterrane is the northeastern section of the Kiselyovsko-Manominsky terrane, a distinguishable tectonic unit in the north of the Sikhote-Alin Range. The terrane has been treated as part of the accretionary wedge belonging to the Khingan-Okhotsk active continental margin, but its structure and stratigraphy have been poorly understood. This paper presents new data on the subterrane structure, lithology and radiolarian biostratigraphy. The following lithostratigraphic units are established in the terrane: a ribbon chert unit, a siliceous mudstone unit and a elastics unit. Abundant Valanginian to late Hauterivian-early Barremian radiolarian assemblages are obtained from the upper part of the chert unit in addition to the known Jurassic radiolarians. The radiolarian age of the lower part of the siliceous mudstone unit (red siliceous mudstone) is determined as early Hauterivian-early Aptian. The unit's upper part (greenish-gray siliceous mudstone and dark-gray silicified mudstone) and the clastics unit contain Albian-Cenomanian assemblages. The arrangement of the units is treated as a chert-elastics sequence, whose vertical lithologic variations indicate environmental changes from a remote ocean to a convergent margin, reflecting an oceanic plate motion towards a subduction zone. The subterrane structure is a stack of imbricated slabs composed of various lithostratigraphic units, and is complicated by folding. The structure's origin is related to subduction-accretion, which occurred in the Albian-Cenomanian. The data presented provide a unique basis for accretionary wedge terranes correlation in the circum-Japan Sea Region, and the Kiselyovsky subterrane is correlated in this study with the synchronous parts of the East Sakhalin, Hidaka and Shimanto terranes. The Albian-Cenomanian radiolarian assemblages were deposited in the Boreal realm, while Valanginian ones are Tethyan; this indicates a long oceanic plate travelling to the north. The former assemblages contain an admixture of older species, redeposited by bottom traction currents and turbidite flows in trench environments.     

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.