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A Late Triassic Impact Ejecta Layer in Southwestern Britain

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Science  13 Dec 2002:
Vol. 298, Issue 5601, pp. 2185-2188
DOI: 10.1126/science.1076249

Abstract

Despite the 160 or so known terrestrial impact craters of Phanerozoic age, equivalent ejecta deposits within distal sedimentary successions are rare. We report a Triassic deposit in southwestern Britain that contains spherules and shocked quartz, characteristic of an impact ejecta layer. Inter- and intragranular potassium feldspar from the deposit yields an argon-argon age of 214 ± 2.5 million years old. This is within the age range of several known Triassic impact craters, the two closest of which, both in age and location, are Manicouagan in northeastern Canada and Rochechouart in central France. The ejecta deposit provides an important sedimentary record of an extraterrestrial impact in the Mesozoic that will help to decipher the number and effect of impact events, the source and dynamics of the event that left this distinctive sedimentary marker, and the relation of this ejecta layer to the timing of extinctions in the fossil record.

Major bolide impacts produce craters that may survive long after associated ejecta deposits have been lost through erosion or diagenetic alteration. There are five known late Triassic impact craters (1), including the 100-km-diameter Manicouagan crater (Fig. 1), which is one of the largest known Phanerozoic impacts (2). Earth experienced a series of worldwide extinctions in the late Triassic, constituting one of the top five Phanerozoic faunal crises (3,4), but the exact causes are disputed. The discovery of an ejecta deposit corresponding to one or more of these impact events permits a direct tie-in of the impact to the stratigraphic record and an assessment of its timing and related environmental and biotic consequences.

Figure 1

Late Triassic impact sites in relation to the southwest Britain impact deposit placed on a North American/Eurasian plate reconstruction of ∼214 Ma [after Sprayet al. (1)]. Impact symbols are scaled according to relative (not absolute) size of impact crater. Full extent of some error values is not shown. Large Igneous Provinces data are from (16). Carnian Extinction is derived from Benton (3).

Our deposit contains spherules and shocked quartz, which are diagnostic evidence of an impact ejecta layer. The ejecta deposit is located within a late Triassic continental red mudstone/sandstone succession in a temporary exposure near Bristol in southwestern Britain. The stratigraphic succession is a marginal facies of the Mercia Mudstone Group (Norian) positioned below the horizon of the widely recognized late Triassic Rhaetian marine transgression (Late Norian, Penarth Group) (Fig. 1 and fig. S1). The position of this layer in red continental sediments below the base of the Penarth Group places it within the upper part of the local Triassic succession but well below the Triassic/Jurassic boundary.

The ejecta layer was found 1 m from the base of a 9-m-thick deposit of Mercia Mudstone that unconformably overlies the regional Lower Carboniferous (Dinantian) limestone basement. The ejecta layer ranges in thickness from 0 to 150 mm (average of 25 mm from eight points that were sampled within a 200-m-wide region) and shows evidence of turbulent reworking in water such as grading, convolution, rippling, and mixing with local mud and lithoclasts. The layer contains up to 50% (by rock volume) emerald green spherules preserved within a clear calcite and/or pink potassium feldspar matrix. Spherules are mostly in the submillimeter-size range. They are usually round, but other shapes are found, including teardrops, dumbbells, fractured spheres, and shards (Fig. 2). Spherules are composed of disordered illitic clay (Fig. 3), and many are hollow, or were so before the arrival of diagenetic minerals. Spherule exteriors are generally smooth, whereas interiors have a botryoidal texture (Fig. 3). Spherules commonly enclose one or more smaller spheres (Fig. 2), in which the smooth surface is the inside and the botryoidal one the outside, reversing the order in the host spherule. Concentric zonation within spherule walls is commonly marked by clusters of micrometer-sized titanium oxide crystals.

Figure 2

Cathodoluminescence (left) and transmitted light (right) images of Triassic spherules and shards in a calcite matrix. Spherical, oblate, teardrop, and sphere-within-sphere forms, as well as shards, are apparent. Clay and empty space are nonluminescent; calcite is bright. (Polished standard optical thin section, width of field ∼2 mm.)

Figure 3

A Triassic clay spherule set in a calcite matrix. The outer wall of a second spherule occupies top left of the image. (Scanning electron microscope secondary electron image; the spherule is 0.5 mm.)

Spherules of this type are generally considered to be pseudomorphs after glass beads (microtektites) that solidified from molten and vaporized material generated during shock melting and explosive vaporization of terrestrial or extraterrestrial materials (5–7). Spherule shapes result from surface tension and aerodynamic flight of molten glass, followed by quenching in either air or water (8–10). Our spherules are similar in shape and texture to those found in the end-Cretaceous (Cretaceous-Tertiary event, commonly called K/T) impact deposit, particularly Type 1 spherules in the K/T (10), which have similar simple, distorted, conjoined, compound and fractured morphologies (8–10). K/T spherules at some sites are also typically hollow, concentrically zoned, and radially textured, having been altered from glass to clay (8,9, 11). Alteration starts with hydration of the glass, followed by conversion into stable clay minerals (9). Botryoidal inner-wall textures (Fig. 3) mark the limit of crystallization advancement through hydrated glass, and the hollow interiors of spherules result from the loss of the remaining glass during later diagenesis (9). Compound sphere-within-sphere forms, with their associated textural reversal, represent vesicular glass (9).

Quartz grains are abundant in the late Triassic ejecta deposit, but concentration varies throughout. Some of this quartz was probably entrained from local sediment during reworking, but we have etched representative quartz residues in hydrofluoric acid to look for shocked quartz, which is diagnostic of high-energy impact events (12). Separated grains were mostly angular, in the 50- to 300-μm range, and although about 50% showed no response, the remainder were strongly etched, displaying abundant planar layers arranged in parallel sets with multiple orientations. These may be impact-generated planar deformation features (PDFs) (13), each marked by a layer of quartz glass that has been preferentially removed during etching (Fig. 4). U-stage measurements of these planar features, conducted by established optical procedures (14), provided rational Miller indices and confirmed their identity as shock effects. Orientation data included high counts of {101̄2} (∼26%) and {101̄3} ( ∼42%) (Fig. 5), confirming impact-generated shock pressures (13).

Figure 4

A quartz grain from the Triassic impact deposit displaying two differently oriented parallel sets (approximately north-south and northeast-southwest) of shock lamellae (PDF). There are up to three additional subsidiary sets in other orientations. Acid etching has removed quartz glass, and some lamellae reveal quartz annealment (syntaxial regrowth) features. (Scanning electron microscope secondary electron image.)

Figure 5

Histogram of frequency percentage of angles betweenc axis and poles to the planes of PDF angles for 33 grains in part of one thin section. Data were plotted using ± 5° error circles with rotation to best fit (14). Five grains show 1 PDF direction, 17 show 2 PDF directions, 8 show 3 PDF directions, 2 show 4 PDF directions, and 1 shows 5 PDF directions. In some cases additional PDF directions were detected but could not be measured. The grains reveal a mixed population of impact stresses, but 10 showed only {101̄2} or {101̄2} + {101̄3}, which is the highest shock state before complete phase transformation.

Ar-Ar dates were obtained from inter- and intragranular authigenic K-feldspar, which was separated from the deposit and washed in a dilute acid to remove carbonate. Laser step heating has yielded old ages in the first few steps of 39Ar release and slightly noisy plateaus, although most of the Ar release occurs at relatively low temperatures (table S1; fig. S2). We obtained a low-precision plateau age of 214 ± 2.5 million years ago (Ma) (2σ errors). Because the K-feldspar (5 to 10 μm) is authigenic, it postdates the age of spherule deposition, but dissolution of the glass of spherule cores may have provided the ions necessary to cause early K-feldspar diagenesis, a common feature of continental red-bed deposits (15).

Our Ar-Ar age is consistent with a Norian age of the layer on the basis of the local stratigraphy, indicating that the impact event occurred before the end of the Triassic, which has recently been placed at about 201 Ma (16). This Ar-Ar age is within the dating errors of the two best-constrained of the late Triassic craters (Fig. 1), which are the Manicouagan crater in northeastern Canada (214 ± 1 Ma, U-Pb) (17) and the Rochechouart crater in central France (214 ± 8 Ma, Ar-Ar) (18). We estimated the possible ejecta layer thickness at various distances from these potential source craters to determine which crater may have produced the deposit. Several variables influence the thickness and distribution of an ejecta blanket, including impact dynamics (size, velocity, impact angle, and Earth's rotation) and impact site characteristics (crystalline or sedimentary target), but basic crater size and distance models (19–21) suggest a difference in the ejecta layer thickness in southwestern Britain from the two sources. A deposit from Manicouagan (100-km-diameter crater and about 2000 km away from the layer on the basis of a late Triassic plate reconstruction) (Fig. 1) would be about 6.6 to 15.5 mm thick, whereas one from Rochechouart (25-km-diameter crater and 650 km away) would be 1.1 to 1.6 mm thick (table S2). Local thickness variation and mixing of the actual deposit prevent accurate averaging, but our figure of ∼25 mm includes at least 10 mm of impact-derived spherules and dust. From first approximation thickness modeling, therefore, Manicouagan may be a source for the layer, but we cannot rule out the possibility that the ejecta may be from the Rochechouart crater or from both craters as part of a multiple-impact event (1). The deposit could be a sedimentary record of an impact event of global significance at this time and, if so, there is no corresponding biotic response in the fossil record (Fig. 1).

Given the Ar-Ar date of ∼214 Ma, we can predict the position of the deposit in British and North American successions. On the basis of magnetostratigraphic work linked to cyclostratigraphy (22, 23), the position of the layer can be correlated with paleomagnetic chron E14 (fig. S1). This lies within the lowest part of the Passaic Formation in the Newark Basin and within the Twyning Mudstone Formation in North Somerset, England. No ejecta deposits have hitherto been reported from either of these successions. The only other ejecta deposits associated with an impact event in the late Triassic are the discovery of shocked quartz at the Triassic/Jurassic boundary in Italy (24) and a small iridium anomaly at the Triassic/Jurassic boundary in the Newark Basin succession (4). Our ejecta layer is located in a stratigraphic succession below the boundary and will provide a valuable time horizon in sedimentary successions, but only if it is widespread. In the generally arid continental environments of the Triassic, the preservation potential of microtektites may have been reduced by the lack of standing water, considered essential for the hydration and palagonitization process (9). Microtektites may have remained as glass, which can be removed during diagenesis (9). More work is needed to confirm the age of our deposit and to look for related deposits elsewhere that might be part of a global event.

Supporting Online Material

www.sciencemag.org/cgi/content/full/1076249/DC1

Figs. S1 and S2

Tables S1 and S2

References

  • * To whom correspondence should be addressed. E-mail: spherules{at}abdn.ac.uk

REFERENCES AND NOTES

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