Shumilova, S. Tkachev, S.
Isaenko, S. Shevchuk, M. Diamond-like glass. The grain size of the material varies between silt and sharp-edged blocks up to the size of 1 m. In the majority, even the smaller fraction of limestone particles does not show any roundness. Frequently, limestone cobbles are covered with multiple sets of scratches and polish.
For the cross-bedded diamictite exposed at the edge of a flat chain of hills a glacial deposit, e. The multiple, small-scale cross-bedding units as well as the transport over short distance point to a close-by, short-term process of formation.
Talk:Río Cuarto craters
It is interpreted as the result of a big Lake Chiemsee tsunami that was triggered in the Holocene Chiemgau impact event. The deposit also raises issues relevant to a Lake Chiemsee glacier. Here, we in particular point out that the peculiar findings in the Nalbach area are revealing remarkable similarities to impact features in the Holocene large Chiemgau impact strewn field in southeast Germany, and meanwhile the possibility that the Nalbach impact is a companion to the Chiemgau impact is seriously being discussed.
Click on the image to open the full text! Under the scanning electron microscope SEM : The odd world of the iron silicides from the Chiemgau impact meteorite crater strewn field click to enlarge. Contribution to the mineralogy meeting of the Russian Academy of Sciences in Syktyvkar:. Shortly after the meeting between the 19th and 22nd May in Syktyvkar the Proceedings volume has been published:. We remind of the fact that in the beginning of the research on the Chiemgau impact the group of quite experienced local historians and amateur archeologists had detected the metallic iron silicides and, after having been aware of their relation to crateriform structures, had published the possible meteoritic origin of the matter.
Ultimately it is evident that these local historians who later got together with scientists from geosciences, astronomy, archeology and historical scholarship to establish the Chiemgau Impact Research Team CIRT , are proved right! As the case may be we suggest to save the file on the computer and to activate it with a pdf reading program. In addition to the report on the microtektites from the foothills of the Alps we present here the LPSC abstract paper click to open the full article :. On reentry in the atmosphere and cooling, the glass bodies exhibiting characteristic shapes fall to Earth where they form part of the impact ejecta.
By definition tektites that are smaller than 1 mm are called microtektites. Although the origin of tektites from impact events is generally accepted the exact mode of formation is not well understood. Comparison of their size and spacing has been presented by McLennan et al. Spherules in the Burns formation have relatively uniform shape and size Fig. At Eagle and Endurance craters, they have a rather uniform size distribution of about 4 mm average and are almost perfectly spherical McLennan et al.
Spherules at Victoria crater are smaller, with an average diameter of about 1 mm Edgar et al. At all outcrops observed to date, the spherules do not disrupt lamination and are highly dispersed relative to bedding, so they are not observed to concentrate along bedding planes or scour surfaces Fig. All factors considered, the Burns spherules are most likely to have formed as diagenetic concretions McLennan et al. However, see the following section for a discussion of alternatives. Lower, dark-toned unit is overlain by upper, light-toned unit, which represents a diagenetic alteration front see Figure 5c of Grotzinger et al.
Note that spherules are highly dispersed throughout both facies and are not concentrated along stratification boundaries. Inset shows plot of frequency versus nearest-neighbor-distance for spherules solid line from four locations compared to a numerical random distribution dashed line. This distribution shows quantitatively what can be observed qualitatively in the underlying image: that the spherules are highly dispersed, which generally supports of a model of formation by precipitation at point sources by iron-bearing pore fluids.
After McLennan et al. Well-stratified sandstone in Burns formation showing low-angle truncation surfaces and cross-stratification, typical of traction sedimentation processes in turbulent flows. Note dispersion of spherules throughout deposit and absence of concentration along bedding planes and scour surfaces. The image was obtained on sol Though direct observation of the events triggered by a large impact on a terrestrial planet has never been made, insight into the sediment dispersal dynamics can be gained from the literature on explosive volcanic eruptions and nuclear tests.
Surges generated by subaerial, pyroclastic eruptions are low-concentration, high-velocity currents, which are dominated by high degrees of turbulence Carey , Orton The stratification and sorting characteristics of volcanic base surge deposits are largely determined by the particle concentration and velocity profiles across the flow-boundary zone from which the lithofacies aggrades Branney and Kokelaar The particle concentration and velocity profiles follow clear trends based on proximity to the source and relative position within the flow, producing predictable patterns in stratification and sorting.
Deposits formed from these flows show distinct proximal to distal and bottom to top changes consistent with decreases in flow velocity and sediment accumulation rate Chough and Sohn In more vent-proximal areas and within valleys or channels, where sediment accumulation rates from suspension are high, thicker, massive, and coarser-grained deposits accumulate Crowe and Fisher , Chough and Sohn , Druitt , Orton As the cloud loses material and sediment concentration declines, deposition from the traction carpet dominates, and stratified deposits form Schmincke et al.
Thus, deposits progress from massive to well stratified, both outward from the vent—though local topography complicates this Schmincke et al. Grain size also generally decreases in these two directions, and sedimentary structures change from higher- to lower-velocity bed forms Crowe and Fisher , Chough and Sohn In the deposits formed by the Sudbury impact event, basal, disorganized, pebble to boulder breccias, composed of underlying lithologies, commonly with little to no lithic or accretionary lapilli, overlie a bedrock surface that is, in places, severely jointed. The near-surface rock layers were fractured and rotated by the strong earthquake produced by the impact Addison et al.
The presence of abundant boulders over 1 m in diameter that were entrained by the flow indicates that the leading edge of the base surge was traveling at very high velocity. Additionally, the shock wave see Wohletz et al. The breccias commonly are coarse-tail graded upward Addison et al. They are overlain by upper-flow-regime, parallel-laminated sandstones composed of angular, aphanitic grains of various sericite- and chlorite-dominated lithologies Cannon et al.
At site 1, where this sandstone is best exposed, all bed forms are upper flow regime with an upward progression in some sections from parallel lamination to undulatory layering. Undulatory layering associated with parallel lamination similar to that present in the Sudbury impact deposits has been described from base surge deposits in the Laacher See area Schmincke et al. The absence of accretionary lapilli in the lower sandstones may reflect processes similar to those proposed to operate in ground-hugging pyroclastic density currents developed during eruptions on Tenerife Brown et al.
Here, the absence of accretionary lapilli in the basal portion of the deposits is attributed to a two-stage process. Ash pellets formed in the uppermost lofted, moist, and cool parts of the plume above the base surge. They then dropped into the high-velocity, ground-hugging turbulent current, accreted concentric layers, and were sedimented Brown et al. The leading front of the ground-hugging flow advanced in front of the overlying, billowing ash cloud and, thus, did not receive ash pellets to transform into accretionary lapilli.
The thickness of the lower sandstones is probably controlled by fluctuations in velocity and concentration of sediment in the highly turbulent flow. At site 3A, where these sandstones are mostly limited to low-velocity zones in depressions and behind obstacles, overall flow conditions did not fall within the depositional realm until the main upper ash plume had advanced over the area and accretionary lapilli began accumulating. At site 2, the flow conditions accommodated sand and granule deposition from the leading edge of the surge, but arrival of the upper ash plume over this area was accompanied by a change in conditions promoting erosion of an unknown thickness of the sand.
Deposition of accretionary lapilli filled this broad scour surface and individual smaller scours superimposed upon it. Cross-stratification, consisting of alternating accretionary lapilli laminae and laminae of sand-sized material, developed during infilling of the scours and behind obstacles, such as boulders or small bedrock promontories. At site 1, no noticeable erosion occurred between deposition of the sandy succession and the overlying accretionary lapilli.
Site 3B has a gradual transition as well, but in a massive assemblage of sediment. The accretionary lapilli—rich units overlying the upper-flow-regime, parallel to undulatory beds of sand- to granule-sized material possess many of the attributes of the sandy succession. The common parallel-lamination and rarer development of cross-stratification dipping toward the impact site similar to type III and V dunes of Schmincke et al. As in the underlying sand dominated beds, this type of layering developed when sediment in suspension rapidly dropped into the traction carpet and was deposited.
Changes in velocity of the highly turbulent flow resulted in deposition of accretionary lapilli, with very coarse-grained sand, granules, and small pebbles forming the matrix, interbedded with coarse-grained sandstone. A decrease in size of the accretionary lapilli up through this unit denotes a drop in flow competency, culminating in a sand-dominated succession with accretionary lapilli concentrated as stringers at the base of coarser sand-granule—dominated beds and, more rarely, isolated in massive beds.
These massive beds with scattered accretionary lapilli probably formed as a result of an increase in the rainout of material from suspension, which will suppress the formation of parallel lamination Arnott and Hand Further dropping of flow competency upward through the succession resulted in the development of lower-flow-regime bed forms scour-fill festoon cross-stratification, dunes, and sand waves.
Only rare accretionary lapilli are found at the base of this sandstone unit, i. Because of a large difference in mass between the lapilli and sand, the accretionary lapilli were probably remobilized from exposed accretionary lapilli layers and rolled in traction rather than transported into the area in suspension with the other material in the waning flow. A general model for deposition of accretionary lapilli—bearing and related units can be developed using data collected from sections through the impact deposit Fig.
This model does not reflect proximal to distal relationships, but rather decreasing energy level during sediment accumulation. Sites 3A and 3B are within m of one another, yet their deposits are at opposite ends of the spectrum Fig. Work on volcanogenic base surges and ignimbrites has highlighted that local topography can exert an important control on the type and thickness of the sedimentary succession that accumulates Schmincke et al.
Microtektites from the Chiemgau impact fallout - Chiemgau ImpactChiemgau Impact
It is envisaged that localized topographic differences also played an important role in defining the type of accretionary lapilli—bearing successions that developed in the Sudbury impact deposits. Examination of outcrops where basement topography is exposed confirms that thicker deposits of the basal breccia accumulated in bedrock lows, whereas adjacent higher levels were swept clean of debris and accumulated lithic lapilli—sized sediment.
Although the basement topography at site 3 is unknown, a similar scenario is plausible there. Depositional model for units associated with accretionary lapilli deposited by the Sudbury impact event. Decreasing energy of the flow does not necessarily reflect proximal to distal positioning as topography exerted a major control on the energy level that existed during deposition. The higher-energy deposits have a basal breccia zone with locally derived debris erosively overlain by upper-flow-regime, parallel-laminated and undulatory laminated antidune accretionary lapilli and sandstone.
This fines upward, where the upper-flow-regime bed forms are replaced by lower-flow-regime trough cross-stratification. At this point, accretionary lapilli become rare. With decreasing energy levels, the basal breccia zone does not form, and accretionary lapilli become less numerous and confined to thinner beds. Upper-flow-regime parallel-lamination and some antidunes formed from hydraulic jumps are still present. At the lowest energy levels studied, the flow was probably not in contact with the bed, and a thinner, massive deposit developed.
Coarse-tail normal grading in the basal breccia Addison et al.
Erosive truncation of the top of this unit at some locations indicates that there was an increase in competency prior to the arrival of the lofted portion of the flow where accretionary lapilli could form. The lofted portion of the ash cloud provided a source for the accretionary lapilli, which were sedimented by traction processes in the highly turbulent, basal surging flow.
The coarser accretionary lapilli in beds deposited on the, in places, eroded top of the sandstones form the base of a fining-upward succession to small accretionary lapilli and finally sands, forming another powering down series of beds. This decrease in energy is also reflected in the upper-flow-regime bed forms in the accretionary lapilli—rich section, which are replaced upward by lower-flow-regime bed forms in the overlying sand-rich sediments. The ejecta-bearing sediments exposed in the drill hole at site 3B are massive, except for parallel lamination in the uppermost fine-grained sandstones.
Their inverse to normal grading is similar to some layers deposited from base surges in the volcanogenic Songaksan tuff ring Chough and Sohn There, the inverse grading is related to dispersive pressure within a suspension or a traction carpet with a dilute, turbulent overriding surge Chough and Sohn The normally graded portion was probably deposited from suspension. This depositional scenario may be applicable to site 3B, but the lack of parallel lamination or other sedimentary structures also indicates that movement from suspension was not into a traction carpet, but rather resulted in individual particles being sedimented.
This implies that sedimentation was not initiated at site 3B until the more diffuse, later stages of the flow were passing this area, and at this stage, the flow may have separated from the substrate so traction was not possible—as in the pyroclastic flow transforming into a buoyant plume in the Mount St.
Helens eruption Sparks et al. Another type of massive impact deposit has been described by Branney and Brown from the Stac Fada impact layer in Scotland. This impact layer is up to 10 m thick and composed of reverse graded and then disorganized bedded, matrix-supported breccia without accretionary lapilli. This section fines upward coarse tail grading to massive, matrix-supported pebble breccia with accretionary lapilli. Faint bedding developed immediately below a capping, thin layer of clast-supported rainout ash pellets.
Branney and Brown hypothesize that this deposit formed from a high-concentration, steady, granular fluid-based density current.
This is a higher concentration flow than that described for the Sudbury impact, possibly denoting a closer position to its source. As no layering is developed in the majority of the deposit, the accretionary lapilli are scattered throughout the upper portion of the layer. Aside from thin, massive impact deposits, probably formed by the waning suspension cloud, or representing tsunami reworking Pufahl et al.
They have well-developed stratification, and accretionary lapilli were sorted by traction processes according to their size and mass.
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Velocity variations in the surge, caused by turbulent eddies, separated the different medium-grained sand to pebble size fractions in transport into distinct beds. Only where sedimentation from suspension formed a massive layer, or where a limited number of accretionary lapilli were eroded from a clast-supported lapilli bed and rolled into a sand-dominated area, were isolated lapilli observed in thicker sandy units.
In the latter example, the lapilli are commonly found at or near the base of the sandstone bed as continued sand deposition buried the source of the lapilli. Two alternatives to the depositional model outlined previously have been suggested for the Burns formation. McCollom and Hynek argued that the sequence was a volcanic base surge deposit that experienced in situ isochemical acid-sulfate alteration in an acid-fog— like setting. They interpreted the spherules to represent diagenetic concretions formed during alteration.
These alternative models have not been widely accepted in part because they are inconsistent with the regional geological context, which provides no evidence for contemporaneous sources of volcanism or age-appropriate impacts of sufficient size see McLennan and Grotzinger In addition, there are other, more specific factors that are inconsistent with these models. Here, we specifically consider the possibility that the Burns formation spherules formed as impact-induced accretionary lapilli, or from condensation. In either case they would have been free particles interacting with the current that transported other sediments from the site of impact to the site of deposition, and thus the Sudbury impact deposit constitutes an important analog.
If the spherules are accretionary lapilli or condensation droplets, as might be predicted to exist in an impact deposit, they should show concentrations along bedding planes, as is the case for the Sudbury impact deposit. In contrast, in all outcrops observed to date by Opportunity , spherules are always highly dispersed relative to bedding McLennan et al.
Figure 12 shows three separate scenarios for the potential origin of spherules in the Burns formation. In the first case Fig. The shear strength of such a fluid is high enough that viscous forces dominate over gravitational forces, and the larger, heavier particles do not preferentially settle out to form discrete beds Nemec and Steel Note that ash plumes rising from the surface where a surge cloud loses contact with the ground, as sometimes happens from the initial ash plume or from retrogressive collapse of loose material Branney and Kokelaar can deposit ash and accretionary lapilli directly from suspension with no traction this likely occurred at site 3B.
A Baverian meteorite crater strewn field
However, deposits produced directly from suspension result in massive bedding in which deposits are poorly sorted and lack well-defined stratification;this contrasts with the Burns formation, in which millimeter-scale lamination is pervasively developed Grotzinger et al. Models for relationships between spherule distribution and sedimentologic texture. Left Spherules are dispersed within a massive matrix of poorly sorted, finer-grained sediments.
This texture is commonly observed in debris flows, or even hyper-concentrated flows, where flows may be nonturbulent and suspended particle concentrations are very high Jakob and Hungr Viscous fluid forces dominate the flow, and a combination of high fluid shear strength coupled with hindered settling of particles results in dispersion poor sorting of all grain sizes.
The presence of excellent stratification excludes this option for the Burns formation, even though the spherules are highly dispersed.
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- Distal Impact Ejecta Layers!
Center Flows with low sediment concentrations are fully turbulent, and particles will become hydraulically sorted according to grain size. Heavier particles, such as spherules, will be systematically concentrated as erosive lags along scour surfaces, where smaller less massive particles are removed by turbulent eddies in the scour pits of bed forms.
The Burns formation does not show evidence for concentration of the spherules as this scenario would predict. Right spherules here are highly dispersed relative to primary bedding features, including scour-related truncation surfaces and other stratigraphic disconformities. The absence of concentrated spherule beds points to a different origin, perhaps as diagenetic concretions. In this case, the mineralizing pore fluids move through the previously deposited sediments, and precipitation occurs at discrete points.
Concretions then form by growing radially in an outward direction from the point. In the second scenario Fig. This turbulence results in velocity fluctuations during deposition that will produce beds of coarser- and finer-grained sediment. Movement of sediment in traction under turbulent flow can also lead to the development of bed forms, the migration of which may create scour surfaces, and multiple flow events may cause stacking of beds separated by hiatal surfaces.
In this type of flow, gravitational forces may dominate, so that general behavior according to Stokes law is predicted. Hydraulic segregation of particles with different settling velocities is why sorting occurs, and why outsized particles for example, lapilli are commonly concentrated along bedding planes in successions of flow-emplaced strata see Figs.
This is a distinctive textural attribute of the well-stratified Sudbury deposits, as well as other well-stratified volcanic surge deposits. In contrast, the Burns formation lacks evidence for hydrodynamic sorting of spherules despite being well stratified and showing clear evidence for scouring by currents, and also reworking above erosional surfaces. Equation 1 shows that the settling velocity goes as the square of the grain radius, so the outsized spherules—if sediment particles—are predicted to drop out very rapidly.
In the Burns formation, they are on the order of times larger than the grains that make up the stratified matrix in which the spherules are embedded Grotzinger et al. Again, Eq. Finally, it is worth noting that the smaller g of Mars has no substantial effect on the relative settling velocities of the grains. In the third scenario, spherules are dispersed across all strata and show no concentration, even at obvious erosional surfaces, such as the lower—middle unit boundary Wellington contact at Burns cliff Grotzinger et al.
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Spherules are not concentrated along these or any other scour surfaces, despite clear evidence that the surfaces truncate spherule-bearing strata and represent erosion of spherule-bearing matrix sediments. This strongly indicates that the spherules must have developed in situ, after deposition of the sediments, favoring an origin as depicted in this third scenario.
In summary, the textural relationship between the outsized spherules in the Burns formation and the well-laminated sandstone matrix does not support their origin as sedimentary particles. This applies regardless of whether the spherules are interpreted to be accretionary lapilli or condensation droplets.
In each case, these spherules should react in the flow as heavy particles that would preferentially travel in traction and be sedimented together at velocities that would segregate the majority of the sand to the saltation and suspension populations Saxton et al. Concentrations of spherules should be particularly abundant along scour surfaces, where erosive lags would have developed.
In addition, shielding from the current in scour troughs would have led to preferential deposition of material in traction transport, i. Indeed, this is exactly what is observed in the spherule-bearing strata of the Sudbury impact deposits. These deposits highlight the improbability of the spherules in the strata observed at Meridiani Planum being related to an impact event.
The Sudbury impact deposits also provide clues as to what a spherule deposit on Mars, which is related to an impact, may look like. This study clearly outlines a means of distinguishing between impact-generated spherules and concretions. The differences produced by physical processes are of primary importance because the use of mineralogy or geochemistry can provide misleading results. The Sudbury impact accretionary lapilli present in the northern portion of their outcrop area have been massively replaced by ferroan dolomite. Thus, their mineralogy and geochemistry cause them to appear to be carbonate-rich concretions rather than lapilli generated by an impact on sialic crust.
If impact-generated lapilli on Mars prove to be just as susceptible to replacement, their distribution within the strata will be of paramount importance in formulating the correct interpretation of their origin. Differentiating between impact- and volcanic eruption—produced accretionary lapilli is difficult Branney and Brown Here, the associated lithofacies become important. In an impact event, the initial burst of energy is directed down into the substrate, producing intense shock waves.
In a volcanic eruption, the energy is released in an upward direction, with only minor resultant earthquake activity. Ground movement distal to the Sudbury impact event shattered the bedrock Addison et al. This debris was entrained by the leading edge of the base surge and swept into topographic lows.
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