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Clay Minerals (1998) 33, 51-64 Clay mineral alteration associated with meteorite impact in the marine environment (Barents Sea) H. DYPVIK AND R. E. F E R R E L L , a JR.* Department of Geology, University of Oslo, P.O. Box 1047, Blindern. N 0316 Oslo, Norway, and *Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, USA (Received 13 August 1996; revised 26 November 1996) A B S T R A C T : More than 50 samples from a Barents Sea borehole near the Mjolnir Structure (an extraterrestrial impact feature) were used to investigate changes in the clay assemblage associated with the submarine impact. Seismic evidence, the presence of shocked quartz and a prominent Ir anomaly restricted the potential impact affected zone to a 10 m interval, straddling the Jurassic/ Cretaceous boundary. Increased abundance (up to 30 wt%) of a smectite, a randomly interstratified smectite-illite with 85% smectite layers, forms the basis for a two-layer oceanic impact clay model that differs from published terrestrial cases. The smectite is assumed to represent seawater-altered impact glass from the ejecta blanket material that was mixed with resuspended shelf sediments by the collision generated waves. The smectite-rich interval is almost 5 m thick. It is overlain by a coarser unit (~2 m thick) containing abundant smectite, shocked quartz grains, and anomalous Ix contents at its base. The smectite-rich interval may have originated as a density/turbidity current, generated by the impact and the collapse and erosion of the crater rim. Seawater alteration of volcanic glass and changes in the tectonic regime of the provenance area, or changing oceanic current circulation patterns could produce similar variations in the clay mineral assemblage. The most compelling evidence for the possible impact derivation of this clay assemblage is the direct association with the Mjolnir Impact Structure and associated mineralogical and geochemical anomalies. The discovery of an Ir anomaly (Alvarez et al., 1980) and its interpretation as a global catastrophic event associated with the impact of a large bolide has produced a great deal of scientific interest and debate (McLaren & Goodfellow, 1990; Montanari et al., 1983; Sharpton & Grieve, 1990; Smit, 1982; Tappan, 1982). Much of the discussion has centred on the coincidence of an Ir anomaly and the mass extinctions at the Cretaceous/Tertiary Boundary (KTB). Many authors have linked the global changes produced by a large impact with the major faunal changes occurring at stage boundaries (McLaren & Goodfellow, 1990). Bolides larger than 1 km in diameter may strike the Earth at a frequency equal to 4.3 (-I-2.6) • 10 _6 yr -J, and nearly 82 craters of >30 km diameter should have been produced in the last 100 million years (Shoemaker et al., 1990). They may create high temperature and acidic conditions unfavourable for living organisms and promote large-scale die-offs. However, some authors have questioned the exact stratigraphic correlation of the impact structures with faunal boundaries and the global consequences of many impacts (Birkelund & HLkansson, 1982; Courtillot, 1990; Hansen et al., 1986; Hansen et al., 1987). The sedimentary record also contains a number of components that are interpreted as impact related (e.g. glass, shocked quartz, spinels, Ir anomalies, clay minerals). One of the best clay mineralogical descriptions is of material associated with the KTB in the Western Interior Basin of North America. It is the basis for the two-layer model of Pollastro & Bohor (1993). The lower layer is mainly kaolinitic 9 1998 The Mineralogical Society 52 H. Dypvik and R. E. Ferrell, Jr. and was formed during the post-depositional alteration of the silicic melt ejecta blanket layer. The upper laminated layer was derived from the fireball and is mostly smectite formed from the alteration of mafic glass. Their distinctive texture and impact components distinguish these clays from other clay beds and tonsteins in the vicinity of the KTB. The clay mineral differences are the product of the alteration of different starting materials in the acidic, organic-rich waters of ancient peat swamps. Secondary variations in the degree of ordering of the kaolinite or the illitization of the smectite due to weathering or burial diagenesis may occur locally. This two-layer sequence is best developed when the ejecta and fall-out materials accumulate in terrestrial environments. In the marine sedimentary sequences associated the KTB, there are differences of opinion regarding the significance of the increased quantities of smectite observed near the boundary. A single, pure Mg-smectite-rich deposit with a high Ircontent at Sterns Klint, Denmark, (Kastner et al., 1984) was interpreted to represent material formed from the alteration of impact glass. Subsequently, the discovery of multiple beds of Mg-rich smectite in the same section, led Elliott et al. (1989) to conclude that the clay minerals were alteration products of volcanic glass. Robert & Chamley (1990) suggested that the wide variation of clay minerals associated with the KTB indicates a global instability that can not be related to a unique extraterrestrial event. They interpreted the changes mainly to represent global changes in sea level and tectonic activity at that time. Additional examples from the Betic Cordillera and the BasqueCantabrian Basin were used by Ortega-Huertas et al. (1995) to argue that the clay mineral variations associated with the KTB are controlled by erosional processes in the source areas and the palaeogeography of the depositional basins. Most of the arguments regarding the nature of clay mineral changes associated with impact events are difficult to assess because the details of the stratigraphic associations are unclear. In the terrestrial sections which contain most of the data, the correlation with a particular impact crater is uncertain. Additional complications arise because most of the studied sections are in terrestrial settings with material derived from very distant, suspected terrestrial impacts. Only four suspected impact craters of marine origin are reported, although marine impacts should be several times more abundant than terrestrial ones because of the larger target area represented by the oceans. The Mjolnir Structure, a Late Jurassic submarine crater, provides an opportunity to assess directly the changes in clay mineralogy that are produced in the surrounding continental shelf deposits by the impact. STUDY AREA The Mjolnir crater was recently discovered by Gudlaugsson (1993) and geophysical and geological evidence for an extraterrestrial impact origin have been presented (Dypvik et al., 1996; Gudlaugsson, 1993). The structure, which is located in the Barents Sea (73~ 30~ Fig. 1), was formed by an impact in the Late Jurassic/Early Cretaceous. The 40 km diameter scar marks a dramatic event. The presence of shocked quartz records the extreme pressure (at least 15 GPa) and temperature (> 1000~ attained within a few seconds of impact. The approximately 1.5 km diameter bolide collided with Earth in a shallow shelf sea where the water depths were between 300 and 500 m. The sea floor consisted of soft silty clays underlain by several km of mostly marine claystone and sandstone units with interbedded thin silt layers. Samples were collected from a continuous core (IKU 7430/10-U-01) (Fig. 2) obtained about 30 km north-northeast of the Mjolnir Impact Structure in the Barents Sea (Fig. 1). The interval from 67.6-45 m below the sea bed represents clay-rich sediments of the Hekkingen Formation, that range in age from Late Kimmeridgian to Early Berriasian. The first seismic reflector above the chaotic zone of the impact structure can be projected to intersect the core at a depth of approximately 46 m (Dypvik et al., 1996). The location of the samples, a schematic lithologic description of the core, and the stratigraphic assignments are presented in Fig. 2. Dark grey, well-laminated, Kimmeridgian, fossiliferous, organic-rich claystone units, with some thin sandy laminae occur in the interval from the base of the core to 56.5 m. They are overlain by sandy to silty Volgian claystone, (56.5-50.0 m) containing some 2 - 5 cm thick, fining upward beds resembling distal turbidite deposits. The succeeding, possibly Late Volgian to Early Berriasian interval can be divided into an upper and a lower unit. The lower (50.0-47.6 m) is dominated by dark grey, weakly laminated claystone, while the upper (47.6-40.0 m) contains beds 53 Clay mineral alteration associated with meteorite impact o a o 30 ~ F:G. 1. The approximate location of the Mjolnir structure is marked by the large dot. The smaller dot marks the location of boring 7430/10-U-01, about 40 km to the North. The Jurassic palaeogeographic setting at the time of impact is indicated in the inset, based on a reconstruction by Lawver (pers.comm.). of sandier claystone that are glauconitic in part, moderately bioturbated, and relatively rich in pyrite and body fossils. The base of the upper unit is marked by a 19 cm thick, mudflake conglomerate composed of three minor fining-upward units. Shocked quartz (level 47.60-47.00 m, star in Fig. 2; illustrated in Fig. 3) representing -2% of the quartz grains observed in thin section and an Ir (1016 ppt) concentration peak at 46.8 m, more than 15 x higher than the average background value (Dypvik et al., 1996), have been found. Details of the mineralogical and geochemical analyses are presented in Dypvik et al. (1996). There is an approximately 80 cm thick zone of bioturbated silty claystone between the first layer containing shocked quartz and the Ir peak. These mineralogical and geochemical features, the seismic data, and the proximity of a crater, are the most direct indicators that the sampled interval has been influenced by an extraterrestrial impact. An erosional surface at 45.0 m marks the top of the interval sampled in detail (Fig. 2). METHODS X-ray diffraction (XRD) analyses were run on dried and crushed whole samples mounted in a sideloading holder. Samples for clay mineralogical analysis were washed with distilled water to remove salt, then suspended in a dilute Na phosphate solution. Following ultrasonic dispersion, the clay fractions (<2 gin) were extracted by gravity settling techniques. Oriented films for XRD were prepared by smearing the clay on glass plates. X-ray powder diffraction patterns of the airdried, ethylene glycol saturated, and heated (300~ and 550~ for 1 h) materials, as well as the bulk analyses, were obtained by step-scanning the interval from 2 ~ 20 to 50 ~ 20 in a Siemens D5000 diffractometer with a Cu tube operated at 40 kV tt. Dypvik and R. E. Ferrell, Jr. 54 CORE 7430 / 1 0 - U - 01 claystone / shale z siltstone sandstone crLLI .<~r" UJrr" .< s carbonates bioturbation parallel lamination , 17: t>l:> v.c.sand / granules [3E3 pyrite > glauconite ,,,z~ =<'1'" 13C .,~ ,,<,> Iridium z ,<co LIJCE LU 123 J z z uj ~ ~_j(,9 .J LUrr m 0 84 i z ,< C~ -J 0 l el si vf f m c vc~) m w p ~ b 0 I I I 1 I I 500 ppt I Iooo Ir FI~. 2. A generalized sedimentological log of core 74307/10 -U-01 is shown. Modified from Geir Elvebakk, IKU. The 47.6 to 47.00 m interval, from the conglomeratic zone and 40 cm above, is the only zone where shocked quartz has been identified (star). The highest Ir value is located 20 cm above. The core continues to 67.6 m, with lithologies similar to those below 55 m in the present log. Tick marks on the stratigraphic section indicate where samples were obtained for analysis. Clay mineral alteration associated with meteorite impact 55 Fro. 3. SEM/CL micrograph of a shock metamorphozed quartz grain from level 47.60-47.65 of core 7430/10-U01. The dark lamellae revealed by CL within this single grain were formed by recrystallization of planar fractures and planar deformation features. and 30 mA. Examples of composite XRD patterns for ethylene glycol saturated samples from selected intervals are presented in Fig. 6. A simple numerical representation of mineral abundance in the whole rock sample was derived from peak height percentage calculations which do not require mineral reference intensity correction factors. The minerals used in the calculation and the d-values of their principal peaks included: chlorite 14 ]k; smectite - 12.5 A; illite - 10 A; kaolinite 7 .~; quartz - 4.26 A; K-feldspar - 3.24 ~,; plagioclase - 3.18 A; calcite - 3.03/~; dolomite 2.89/k; siderite - 2.79 A; and pyrite - 2.71 A. The peak height percentage values were also recalculated after subtracting the carbonates and pyrite from the total in order to assess potential variations in the original allochthonous mineral components in the sedimentary environment. Quantitative estimates of clay mineral abundance were obtained by comparing NEWMOD simulated patterns with those produced by the diffractometer. The parameters used to generate the simulated patterns are presented in Table 1. An iterative least- squares minimization procedure reported by Huang et al. (1993) was used to calculate the fractional contribution of each simulated pattern to the observed ones. This procedure systematically changes the fractional multiplier for each simulated pattern until R 2, the best-fit estimator comparing the simulated XRD composite with the actual pattern, reaches a minimum value. An example of the calculation procedure is illustrated in Fig. 4. The pattern for a simulated medium-crystallite thickness, moderate Fe content illite ( D M I C A l l ) is presented with a composite pattern representative of the clay minerals in the interval from 49.2.0-49.9 m. The simulated peak intensities must be reduced by a fractional multiplier of 0.5 (equivalent to an approximation of the weight fraction of illite in the sample) in order to match the observed illite peaks. The procedure was repeated to calculate a fractional multiplier for each of the patterns in the databank. Some of the reference patterns were rejected during the iterations as they did not match the observed peaks. The sum of the fractional multipliers for all recognized 56 H. Dypvik and R. E. Ferrell, Jr. TABLE 1. Parameters used to simulate clay mineral XRD patterns with N E W M O D , Symbol Description K A O 11 Fine-particle kaolinite KAO7 Medium-particle kaolinite Layer types (fraction) kaolinite (0.5) kaolinite (0.5) kaolinite (0.5) kaolinite (0.5) D M I C A 11 Med.-part., moderate-Fe illite dimica (0.5) dimica (0.5) DMICA015 Fine-part., moderate-Fe illite dmica (0.5) dimica (0.5) CHLOR1 Med.-part., Fe-poor chlorite tritriehior (0.5) tritrichlor (0.5) TTCH511 Med.-part., Fe-rich chlorite DSM523 Fine-particle smectite CORRE1 Fine-particle corrensite S 185R0 Fine-part. smectite-rich smectite-illite tritrichlor (0.5) tritrichlor (0.5) dismec2gly (0.5) dismec2gly (0.5) tritrichlor (0.5) dismec2gly (0.5) dismec2gly (0.85) dimica (0A5) Ordering Layer 1 composition Layer 2 composition Crystallite thickness R = 1 fixed fixed low N = 3 high N = 7 R = 1 fixed fixed low N = 7 high N = 14 R = 1 1.0 Fe 1.0 K 1.0 Fe 1.0 K low N = 7 high N = 14 R = 1 0.8 Fe 0.7 K 0.8 Fe 0.7 K low N = 5 high N = 9 R = 1 R = 1 R = 0 Sil Fe 0.1 Hyd Fe 0.1 Hydx 0.9 Sil Fe 0.1 Hyd Fe 0.1 Hydx 0.9 Sil Fe 1.2 Hyd Fe 2.1 Hydx 0.9 Sil Fe 1.2 Hyd Fe 2.1 Hydx 0.9 0.5 Fe 0.5 Fe low N = 7 high N = 14 low N = 7 high N = 14 low N = 2 high N = 3 R = 1 R = 0 Sil Fe 0.1 Hyd Fe 0.1 Hydx 1.0 0.1 Fe 0.1 Fe 0.1 Fe 0.9 K low N = 3 high N = 7 low N = 3 high N = 7 Clay mineral alteration associated with meteorite impact 57 TABLE 1. (contd.) Symbol Description IS5R1 Fine-particle rectorite IS6R1 Fine-part. illiterich illite-smectite Layer types (fraction) dimica (0.5) dismec2gly (0.5) dimica (0.6) dismec2gly Ordering R = 1 Layer 1 composition Layer 2 composition 0.5 Fe 0.9 K 0.5 Fe Crystallite thickness low N = 5 high N = 11 R = 1 0.5 Fe 0.9 K 0.5 Fe lowN = 5 0.5 Fe 0.9 K 0.5 Fe low N = 5 high N = 11 (0.4) IS7R1 Fine-part. illiterich illite-smectite dimica (0.7) dismec2gly (0.3) IS8R1 Fine-part. illiterich illite-smectite dimica (0.8) dismec2gly (0.2) IS9R1 Fine-part. illiterich illite-smectite dimica (0.9) dismec2gly (0.1) R = 1 high N = 11 R = 1 0.5 Fe 0.9 K 0.5 Fe low N = 5 high N = 11 R = 1 minerals was then normalized to 1.0. The results are expressed as the weight fraction of a simulated clay mineral in the sample. The normalization was required because the simulations produced sums that were between 0.85 and 1.1, indicating that the simulations were not exact and/or non-crystalline materials were present. Analyses of replicate samples indicate a precision within _+0.03 wt% of reported values >10 wt% . Below I0 wt%, the relative error may be >100%. The calculated patterns used in the analysis were selected because they represent the best matches with observed peak positions, peak widths, and intensities produced by the Mjolnir samples. The quartz pattern was obtained by analysis of a polished plate o f Arkansas novaculite. Two kaolinites, two micas, two chlorites, one smectite, one corrensite, a randomly interstratified smectiteillite with 85% smectite layers, and five representa- 0.5 Fe 0.9 K 0.5 Fe low N = 5 high N = 11 tives of nearest-neighbour ordered illite-smectite (the percentage of illite layers varying from 5 0 - 9 0 % ) comprised the databank of simulated patterns (Table 1). These patterns attempt to simulate changes due to the thickness of the crystallites, changes in octahedral sheet composition, and interlayering of different ideal clay mineral layers. Crystallite thickness parameters are assumed to be directly correlated with minimum particle size. Hence, the reference materials with low crystallite thicknesses, between 2 - 5 and/or below 11, represent fine-particle sized clays. Medium particle sized clays are those with crystallite sizes ranging from 7 to 14 ideal layers. The values used for clay mineral abundance provide an improved quantitative representation (QR of Hughes et al., 1994) of clay mineral variations, compared to other peak height intensity methods. The method utilizes the entire XRD 58 H. Dypvik and R. E. Ferrell, Jr. COMPOSITE 49.2-49.9 m I-OOZ I~! DMICA11 ISIMULATED I--Z - _ || ~', ' 0 ' ' ' I ' 5 9 9 ' I ' ' ' 10 ' | 15 ' ' ' ' I ' ' ' 20 ' I 25 "* ' ' ' I 30 DEGREES TWO THETA (Cu) FIG. 4. Comparison of a simulated XRD pattern for a medium-crystallite size, moderate Fe content illite with the composite pattern for samples from the smectite-rich interval of the core. pattern and incorporates the possibility that several minerals with different compositions and crystallite sizes may contribute to a single peak. The possibility of human induced calculation and measurement errors are also minimized by the application of the least-squares fitting routine. Nevertheless, the values still fail to meet the rigorous requirements of a quantitative phase analysis, because they have not been confirmed by independent means and standards for calibration are not available. Differences due to variations in peak intensities of one of the phases caused by different mass absorption coefficients for other minerals in the sample matrix have also been neglected. RESULTS The whole-sample results (Fig. 5) display large variations due to the presence of variable quantities of diagenetic and biogenic minerals (pyrite, calcite, dolomite, siderite). Many of the samples have considerable quantities of pyrite that formed after deposition of the detrital silicates. With those secondary variations subtracted, only minor variations were found in the siliciclastic mineral composition of the whole-rock samples. However, an increase in the amounts of K-feldspar and decrease in plagioclase are seen between 56.8 to 56.3 m, as well as increasing amounts of illitesmectite above 52.5 m. The composite patterns of Fig. 6, produced by adding the intensities from six individual samples within each indicated depth interval, illustrate the general variation in the clay mineral composition of the Barents Sea materials. The larger smectite peak in the samples from the 49.2-49.9 m interval is readily apparent. The general similarity of the patterns from intervals below (55.0-56.7 m) and above (45.2-45.9 m) the smectite enriched zone, can also be observed. The changes in clay mineral abundance with regard to stratigraphic position are presented in more detail in Fig. 7. Variations in the illite components of the samples and the smectite-rich smectite-illite mixed-layered clay minerals are greatest at a depth of -49 m, near the Volgian/ Berriasian boundary. Samples from depths below Clay mineral alteration associated with meteorite impact 4~. oo li::FK",."~ fi~,\~ 59 ~x``~A~!!~!~i;iiiii~ii#i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~#i~`~ ................................. r I ~x~/////~ilili!iiiiiii!iii!i!i!ili#iiiiiiiiiiiii!ili#i!i!iii!i!#i!iii!i!!~ ~?~///////.e~ [] PYRITE T:.~&q sl. oo T i i i ~ \ \ ~ t9 ........................... ~//////.4iiiiiiiiiii~i~i~i~i~i~i~i ~//////~#i~i~i ::: [] SIDERITE 52.45 [] DOLOMITE 53.50 [] CALCITE 54.00 [] PLAGIOCLASE [] 55.00 ~o K - F E L D S P A R [] QUARTZ 0 s6 7s s~\\\'q =~\\\\'~ E ~TA ~.~////////A!i::i:,ii~ k~/////////////~l ~10A ~% .\\',.~ ~ / / / / / / / / / / / . d iiii~~~ :::::::: 0 ~0 ~ ~//////////////A::::::::::::::::::::::i::::::i::iii::iiii!iiiii::::i::::i::i::iiiii~/f~~~ ' []12A ~i4A 6v.ao !llliiiil.~K-~\\\'~ 0 % , , ~ 20 % 40 % 60 % ~ , 80 % 100 % % XRD FIG. 5. XRD peak height percentages o f minerals in the whole sample vs. depth. (14 A = chlorite; 12 ,~ = smectite; 10 ~, = illite; 7 ,~ = kaolinite). t,O <>- \\ \ ~ ~ ?A K/C ~ A S ~,- 5A t~ K/C z -~.9 i J m m m ' 0 ' ' ' I 5 ' ' ' ' l 10 ' ' ' ' I 15 ' ' ' ' I ' ' 20 ' ' I 25 ' ' ' ' I 30 DEGREES TWO THETA (Cu) FIG. 6. XRD patterns o f ethylene glycol saturated samples from selected intervals of the core. The intensities from six individual samples within each indicated depth interval have been added to produce the composites shown. 60 11. Dypvik and R. E. Ferrell, Jr. DEPTH (m) ,(ro, 4,i ., i ~"1 .o , ; I- ' t _ q p9 ,i Pi, ,,. o9 7C 0 0 DSmS23 9 SI85RO ( f 0.1 0.2 0 DMICAll IiJ 9 9 DMICA01SI] t o 9 :o y- A 9 0 P B D~ o R1 9 KAO111 CHLOR1 9 IS9RI 9 - 9 QUARhII 9 KAO7 TrCH511 J 0,30 0,1 0 . 2 0 . 3 0 , 4 0 . S 0 . 6 0 0.1 0.7 0.3 0 0.1 0.2 0.30 0.1 0.2 0.30 0.1 0.2 0.3 WEIGHT FRACTION Fro. 7. Variations in the calculated weight fractions of clay minerals as a function of depth. For explanation of the symbols, refer to Table 1. 52 m and near 45 m are less smectitic than those in the intervening section of the core. Changes in the weight fraction of the other clay minerals and quartz are not as marked with regard to depth. Illite is the most abundant clay mineral (Fig. 7) in the samples from the Barents Sea core. Two varieties of moderate-Fe content illite with different particle size distributions best represent the integral series of 10 A XRD peaks produced by the samples. The weight fraction of the finer crystallite size illite varies from ~0.4 to 0.5 in the lower parts of the core and decreases to a low of ~0.2 in the interval between 50-47.6 m (DMICA015, Fig. 7). The weight fraction of the coarser crystallite size illite ( D M I C A l l , Fig. 7) is generally <0.04 below 50 m, but increases to a maximum of -0.15 in the 50-47.6 m interval. The weight fraction of smectite-rich smectiteillite (SI85R0, Fig. 7) varies from <0.05 below 50 m to a high of -0.3 in the 50-47.6 m interval. The changes in smectite abundance are inversely correlated with those observed for the finer crystallite illite. The 9.2 to 9.8 ,~ broad peaks with minimal ethylene glycol expandability occurring in the XRD patterns were matched by an R = 1 ordered illitesmectite with differing percentages of illite layers. The most abundant illite-rich ordered illite-smectites are those with 70% and 90% illite layers (IS7R1 and IS9R1, Fig. 7). The 90% illite materials are more abundant than the 70% illite ones below 50 m, are less abundant in the 50-47.6 m interval, and about equal above 47.6 m. Their combined weight fraction approaches 0.3 in the interval above 47.6 m. Changes in the abundance of the other clay phases are not as notable as those for the illite and smectite containing materials. The medium crystallite size chlorite (TTCH511) exhibits no recognizable stratigraphic pattern and the medium-crystallite size Fe-poor chlorite (CHLOR1) is confined to the interval above 47.6 m. Fine-crystallite size kaolinite (KAO7) is slightly more abundant in the sections of the core below 50 m. The coarser kaolinite ( K A O l l ) is only present above a weight fraction of 0.02 in the interval above 47.6 m. Clay-sized Clay mineral alteration associated with meteorite impact quartz is ubiquitous and its abundance (Fig. 7) is not directly related to any of the recognized stratigraphic changes. The weight fraction of quartz is usually <0.1 in the clay-sized materials. DISCUSSION There are four major observations that must be considered in trying to formulate a model for the origin of the clay mineral alterations observed in the Volgian and Berriasian sediments near the Mjolnir crater. (1) Smectite abundance in the clay fraction begins to increase at a depth of ~52 m to a maximum o f - 3 0 wt% in the interval from 50 to 47 m, and then decreases. Smectite abundance below 52 m and above 46 m is low and usually <5 wt% of the clay fraction. (2) The mudflake conglomerate located between 47.6 and 47.4 m represents a major change in the energy of the depositional environment. (3) Shocked quartz is present in the conglomerate and extends upward to the 46.8 m level in the core. (4) The highest Ir value was produced by the sample just above the shocked quartz zone at 46.7 m. Other samples in the conglomerate and shocked quartz zone have Ir values greater than twice the average for samples below 49 m and above 46 m. There are other geochemical data that must be considered. Dypvik and Attrep (in prep.) have found increases in the abundance of Ni, Co, V, Cr, Zn, and total organic carbon (TOC) that parallel the increase in the abundance of smectite in the interval from a depth of 52 m to the base of the mudflake conglomerate. A dramatic decrease in the abundance of the siderophile elements and TOC occurs at the level of the conglomerate and values remain low to the top of the interval studied. The changes summarized above can best be explained by reference to the events associated with the bolide impact that created the Mjolnir Crater. The impact forced the water from the area and vapourized, fractured and melted the sea floor sediments to form glass that was primarily deposited as ejecta in, and close to, the crater. This is similar to the situation for the Montagnis Crater in Canada (Jansa et al., 1989; Jansa, 1993) where the ejecta blanket is confined to the area within two crater diameters of the impact site. The currents generated by the returning wave surge resuspended the ejecta materials and mixed them with resuspended bottom sediments. They floccu- 61 lated, settled to the bottom, and developed gel strength rapidly. They are now represented by the smectite enriched materials in the interval from 52 m to the base of the mudflake conglomerate. The smectite formed by the near-surface alteration of the impact glass after the restoration of normal marine conditions. Glass was more abundant near the top of the interval because of the slower hydraulic settling velocities of the less dense silicarich glass. Organic matter and the siderophile elements associated with decaying organic debris could also be concentrated near the top of this interval for similar reasons, augmented by material from organisms killed by the impact (Dypvik & Attrep, in prep.). This is a stage equivalent to the ejecta blanket of Pollastro & Bohor (1993), but in this marine environment larger quantities of essentially unaltered local sediments are mixed with the ejecta materials and the blanket is less extensive. Subsequently, other marine processes triggered instabilities in the crater rim or eroded the central area of the crater, and coarser materials were distributed by radiating density flows and shelf currents. These coarser deposits are represented by the mudflake conglomerate and the other overlying smectite-rich sediments containing the shocked quartz and higher concentrations of Ir. They represent material derived from areas nearer the crater where the sediments were more directly influenced by the composition of the bolide. The basal section of the conglomerate contained about the same quantity of glass (now smectite) as the underlying clay-rich interval, but the amount decreased near the top as erosion progressed deeper into the metamorphosed section of the crater. These deposits have some of the same characteristics attributable to the bolide and ash-fall layer of Pollastro & Bohor (1993), but have been transported and deposited from turbid flows. The affected interval is represented by the sediments occurring between 47.6 and 46 m. The changing character of sediments derived from an impact zone is also described from other shallow marine impact sites (Jansa et al., 1989; Newsom et al., 1990; Puura & Suuroja, 1992; Poag & Aubry, 1995). The impact-metamorphosed basement is generally covered by a lower autochthonous breccia with shocked quartz. This breccia is often overlain by an allochthonous breccia with glass, spherules and mixed lithologies, followed by a glass-suevite dominated unit. They may be 62 H. Dypvik and R. E. Ferrell, Jr. reworked by turbidity currents, debris flows and other erosional processes (Roddy, 1977; McKinnon, 1982; Melosh, 1982; Jansa, 1993; Poag & Aubry, 1995). The best evidence supporting the bolide hypothesis for the origin of the smectite enrichment in this sequence is its location. The core is close to a recognized crater structure and the seismic evidence (Dypvik et al., 1996) restricts the potentially affected zone to the interval with high smectite content. Auxiliary support for the extraterrestrial origin is provided by the coincidence of the Ir anomaly and shocked quartz with the smectite zone. The clay mineralogical changes described above are clearly not necessarily unique products of a bolide impact. Thus far, no vestiges of the parent glass to compare with glasses of known volcanic or impact derivation have been found. However, we should not expect to find much original glass in these Late Jurassic sediments, as the alteration of glass to smectite proceeds rapidly (Grieve et al., 1977; Pohl et al., 1977; Newsom et al., 1990; Sigurdsson et al., 1991; Puura & Suuroja, 1992). The smectite produced is dioctahedral, and the associated trace elements are within the ranges expected for clays originating in an oxygen-poor, marine environment (Dypvik & Attrep, in prep.). The smectite is comparable to the 95% smectite-5% illite mixedlayered clay mineral found as an alteration product of glass in the Ries Crater (Newsom et al., 1990). Other potential sources for the increased smectite in the proposed impact affected interval are difficult to evaluate. Burial diagenesis usually destroys smectite, and in this case the sediments have not been buried to significant depths. Local tectonic influences on the types of sediment supplied to the continental shelf are not very likely as the area is distant from the palaeoshoreline and there are no reports of other major Late Jurassic tectonic disturbances in the this area of the Barents Sea. Lower Cretaceous dolerites occur to the North of Mjolnir, on the islands of Svalbard and Franz Josef Land. Recent, unpublished age determinations show the dolerites of Svalbard to be post-Berriasian (S. Dahlgren, pers.comm., 1996), while those of Franz Josef Land are post-Barremian (A. Andresen, pers. comm.), in both cases younger than the Mjolnir impact. The available literature contains no references to the clay mineral composition of comparable sediments in this section of the ocean or the adjacent parts of the continents that would permit correlation of this smectite-rich zone with processes affecting the normal distribution of sediments. Thus the smectite probably does not have a detrital or volcanic origin. Nevertheless, some enigmas remain. Why did a wave with the energy to resuspend so much sediment not leave any evidence of basal scour? Why do the siderophile elements and TOC not have a distribution that mimics that of smectite? If it is assumed that the mudflake conglomerate represents the base of the wave-surge induced scour following the impact, then all of the impact derived material is confined to the interval between 47.7 and 46 m and the differences in the minor element composition of the materials above and below the contact could be explained by the development of anoxic sea bottom conditions prior to the impact. This would, however, make it impossible to account for the secondary smectite below the conglomerate unless the extreme temperatures associated with the impact were able to facilitate an in situ alteration of clay minerals at depths at least 4.5 m below the sediment/water interface. This would seem unlikely considering the poor thermal conductivity of sediments and the distance from the impact site. The sequence near the Mjolnir crater may be correlated with the Jurassic/Cretaceous unconformity (t~-hus, 1991) reported in other cores from the Barents Sea by Arhus et al. (1990). It is often expressed by Valanginian coquina beds directly succeeding black Volgian shales or occurs within lithologically homogeneous looking VolgianValanginian black shale successions. This unconformity could be the result of severe wave/current erosion of the sea bed, processes related to the Mjolnir impact. CONCLUSIONS Clay minerals occurring in a core near the Mjolnir Crater provide a distinct record that can be attributed to the impact of a bolide in a marine environment. Two distinct zones can be recognized by changes in the abundance of a smectite-rich smectite-illite formed from the alteration of impactgenerated glass. The clay mineral zones are not as distinct as those of purported impact origin occurring on the continents. The lower, partly anoxic, zone contains materials that are essentially equivalent to the ejecta blanket of the Pollastro & Bohor (1993) two-layer model, but smectite, rather than kaolinite, is the major alteration product in the Clay mineral alteration associated with meteorite impact marine environment. The original glass is diluted by the admixture of resuspended sea floor sediments. In the upper zone, shocked quartz and Ir occur with varying amounts of smectite. The sedimentary structures suggest that these deposits originated partly as density flows/currents originating from the collapse/erosion of the crater rim or core of the crater and are not directly equivalent to the ash-fall deposits of the terrestrial model. 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