Clastbearing Dikes Formed by Injection

The most widespread group of injected dikes forms at the excavation stage during the transient cavity growth and radial tension of its floor. In the true crater floor two varieties of such dikes may be distinguished: a) DI -impactite dikes (impact melt matrix), and, 2) DM - mylolisthenite (from Greek myle - mill, and olistainein - to slide, Rondot 1994) and polymict lithic breccia dikes (matrix composed of clastic material, which is fluidal in the case of mylolisthenite), but some transitional types may exist. Some dikes formed involving multiple injections. Spatial distribution of these varieties in the crater floors is often radial and circumferential, but also depends on the initial irregularities within the target rocks - contacts, faults

Fig. 2. Zhamanshin impact structure, Kazakhstan. Core sample of the polymict lithic breccia dike from the drill hole in the central uplift from 177 m depth. The breccia contains some particles of amber (amb) and organic debris derived from Paleocene deposits. Scale in mm.

Lithic Hole Driller

Fig. 2. Zhamanshin impact structure, Kazakhstan. Core sample of the polymict lithic breccia dike from the drill hole in the central uplift from 177 m depth. The breccia contains some particles of amber (amb) and organic debris derived from Paleocene deposits. Scale in mm.

etc. The thickness of these dikes is usually from cm to several meters, but in large impact craters they can reach up tens of meters. The larger fragments are usually concentrated in the central part of such dikes. The rock fragments are often shock metamorphosed, and sometimes show thermal and hydrothermal alterations. These dikes usually are partially disturbed at the early modification stage during the collapse of the transient cavity, and are displaced back upward together with the host rocks.

Injected dikes of tagamite (or another impact melt rock), mylolisthenite and polymict lithic breccia comprising wandering clasts are widespread in many impact structures. They are found in large structures whose diameters are from hundreds of km to many tens of km: Vredefort (French et al. 1989, Therriault et al. 1997, Henkel and Reimold 1998, Gibson and Reimold 2001), Sudbury (Dressler 1984, Müller-Mohr 1992), Popigai (Masaitis et al. 1998), Manicouagan (Currie 1972, Grieve and Floran 1978, Dressler 1990), Puchezh-Katunki (Masaitis and Pevzner 1999), Charlevoix (Robertson 1968, 1973,), Siljan (Hjelmquist 1966, Rondot 1976) etc., as well as in the middle-sized craters with the diameters from some tens to ten km: Sierra Madera (Wilshire et al. 1971, 1972), Slate Island (Halls and Grieve 1976, Sharpton et al. 1996, Dressler and Sharpton, 1997), Rochechouart (Lambert 1981, Bischoff and Oskierski 1987, Rondot 1995), Carswell (Pagel et al. 1985), Ries (Hüttner 1977, Stöffler 1977, Chao et al. 1978, Stöffler et al. 1987), Zhamanshin (Masaitis et al. 1993), Wells Creek (Roddy 1977), etc., and small ones, with diameters of only several km, e.g. Flynn Creek, Steinheim (Roddy 1977), and others.

The lithology of wandering clasts allows to estimate their stratigraphic displacements or depth of penetration in the brecciated crater basement, including its central uplift.

In the Sierra Madera impact structure the wandering clasts, found in polymict lithic breccia dikes and lenses, were displaced by about 1200 m (Wilshire et al. 1972). Numerous clast-bearing dikes were mapped in the Rochechouart impact structure (Lambert 1981, Bischoff and Oskierski 1987). Dikes in the floor of this crater are represented mostly by mylolisthenites, composed of crushed crystalline rocks (Rondot 1995). Polymict lithic breccia and other dikes occur abundantly in the central part of the Slate Island impact structure. They include specific rock fragments, showing that the amplitude of stratigraphic displacement may be as much as 5 km (Halls and Grieve 1976, Dressler and Sharpton 1997). In the Charlevoix impact structure mylolisthenite dikes rarely include sedimentary fragments (Robertson 1968, 1973, Rondot 1976). The estimated penetration of clasts into the basement is about of 1 km or more. The most frequent dikes (their arrangement is radial and concentric) occur close to the center of the impact structure and in the ring trough, but they are absent directly in the center (Rondot 1976).

In the largest and oldest impact structure known in the world, the Vredefort structure (e.g. Henkel and Reimold 1998, Gibson and Reimold 2001), wandering clasts (as such carbonates, quartzites) from the sedimentary cover of the target region were trapped by impact melt (granophyre), and transported downwards at the distance of some tens of km from their initial position. These relatively thick granophyre dikes resulted from melt injection into the central part of crater bottom, and have radial and circumferential arrangement (French and Nielsen 1990, Therriault et al. 1997, Gibson and Reimold 2001). Specific lithology of wandering clasts have been determined for the relatively small and fresh Zhamanchin crater (Boiko et al. 1991). Clasts of amber and other Paleocene siliciclastic rocks were found in an injected polymict lithic breccia dike, that cuts the brecciated rocks of the central uplift (Fig. 2). The particles of amber are now located about 100 m below the original position of this organic mineral in the undisturbed Paleocene beds, which intitially overlain folded Paleozoic rocks of the target (Masaitis et al. 1993).

Table 1. Dikes bearing wandering lithic clasts (Compiled after Wilshire et al. 1972, Halls and Grieve 1976, Stöffler 1977, Mashchak and Ezersky 1982, Bischoff and Oskierski 1987, Lambert 1987, Stöffler et al. 1988, Müller-Mohr 1992, Rondot 1994, 1995, Hunton and Shoemaker 1995, Sturkel and Ormö 1997, Warme and Kuehner 1998, Masaitis and Pevzner 1999)._

Table 1. Dikes bearing wandering lithic clasts (Compiled after Wilshire et al. 1972, Halls and Grieve 1976, Stöffler 1977, Mashchak and Ezersky 1982, Bischoff and Oskierski 1987, Lambert 1987, Stöffler et al. 1988, Müller-Mohr 1992, Rondot 1994, 1995, Hunton and Shoemaker 1995, Sturkel and Ormö 1997, Warme and Kuehner 1998, Masaitis and Pevzner 1999)._

Matrix lithology

Location

Source of clasts

Direction of clasts displacements

Stage of cratering

Mode of origin

DI

Impactite (impact melt rock)

Beneath the true crater floor, in the ejected blocks

Uppermost target layers

Downwards (+ upwards)

Excavation

Injection into fissures

DM

Mylolisthenite, polymict lithic breccia

DE

Polymict lithic breccia

In the crater wall

Uppermost and Lowermost target layers

Downwards and upwards

DS

Monomict lithic breccia

In the water saturated target rock

Local target layers

Upwards

Compression, excavation, early modification

Squeezing out

DF

Sandstone, sand, clay

Beneath the apparent crater floor (in breccias and impactites)

Debris from crater lake bottom and dike walls

Downwards

Late modification

Infilling of open fissures

The deepest registered penetration of melt injection into crater basement rocks occurs in the Puchezh-Katunki crater (Fig. 3). Brecciated crystalline rocks of the central uplift are intersected by numerous mylolisthenite (Fig. 3) and tagamite (impact melt rock) dikes. The latter were traced on cores from deep drill hole (5374 m) to a depth of about 4.3 km (Fig.4). Sedimentary clasts in the breccia dike were found at the depth of about 0.5 km from the surface of central uplift (Masaitis and Pevzner 1999). According to the analysis of this core, the depth of impact melt penetration exceeds several times the depth of penetration of polymict breccia injections comprising wandering clasts.

Fig. 3. Puchezh-Katunki impact structure, Russia. Schematic section of the lower part of the deep drill hole in its central uplift. Core lithologies: A = gneisses, amphibolites; B = gneisses, amphibolites, schists, quartzites, calciphyres. The amount and thickness of tagamite dikes decrease downwards, as well as the intensity of brecciation. In contrast, rock density increases downwards. Sedimentary clasts are found in mylolisthenite dikes to a depth of 1050 m. The upper part of the section (0-600m), which consists of crater lake deposits, suevites and allogenic breccia, is not shown (from Masaitis and Pevzner 1999, modified).

Fig. 3. Puchezh-Katunki impact structure, Russia. Schematic section of the lower part of the deep drill hole in its central uplift. Core lithologies: A = gneisses, amphibolites; B = gneisses, amphibolites, schists, quartzites, calciphyres. The amount and thickness of tagamite dikes decrease downwards, as well as the intensity of brecciation. In contrast, rock density increases downwards. Sedimentary clasts are found in mylolisthenite dikes to a depth of 1050 m. The upper part of the section (0-600m), which consists of crater lake deposits, suevites and allogenic breccia, is not shown (from Masaitis and Pevzner 1999, modified).

Fig. 4. Puchezh-Katunki impact structure, Russia. Core sample of a mylo-listhenite dike in the gneiss of the central uplift. The drill hole is located 2.3 km south of the center of the impact structure, depth 356 m. Scale in mm.

The DI and DM injection dikes are formed during the short interval when downward motion of the transient cavity floor reverses, and fractures are caused by tensile stresses (Broberg 1988, Stoffler et al. 1987, Melosh 1989). The dike formation results from downward injection of gas- or melt-saturated,). crushed and compressed material following the Z-flow model (Melosh 1989. The low viscosity of this material is evident from its propagation over long distances, its ability to inject into very thin cracks and from the zoned inner structure of such dikes caused by hydrodynamic forces.

Blocks of target rock injected with these clast-bearing dikes and derived from the transient cavity floor are sometimes ejected, and can be incorporated within the allochthonous breccia lens. Such blocks cut by DM and DI dikes are found in the Ries and Popigai craters (Stoffler et al. 1987, Masaitis et al. 1998). In the Popigai crater these dikes cut brecciated

Fig. 5. Popigai impact structure, Russia. A core sample of a fluidal mylolisthenite dike composing of crushed leucocratic crystalline rock and including clasts of Upper Proterozoic quartzite (qt). The dike cuts brecciated pyroxene gneiss (dark gray on the photograph) of the annular uplift in the northwestern sector of the structure. Drill hole 0928, depth 313 m. Scale in mm.

blocks and contain wandering clasts from all principal lithologies known in the sedimentary target rocks: from uppermost Cretaceous to lowermost

Upper Proterozoic layers (Fig. 5, 6). The clast material derived from overlying beds (that initially occurred in normal stratigraphic sequence) is injected in all subjacent rocks, which compose now the ejected blocks. Some specific structural features of DM type dikes allow to reconstruct their mode of origin. For example, the clast dimensions sometimes exceed the thickness of a host dike, showing that open fractures collapsed (or closed) after infilling (Fig. 7). Very thin branching veins of microbreccia in the gneiss and limestone blocks (Fig. 6) probably formed by absorption of debris into opening fissures at the time of underpressurization just behind the propagating compression wave. These microbreccia veins are mostly composed of loose sandy material derived from Cretaceous beds, originally located in the upper portion of the target section. At the moment of injection this material must have been in a state of suspension under low viscosity.

The DI and DM groups of dikes mark the extent of fracturing within the crater floor, and also the extent of downward displacement of clasts from higher target levels. In the case of strongly eroded impact structure, these dikes may serve as the last indicators of the former crater.

Fig. 6. Popigai impact structure, Russia. A Cambrian limestone block of allogenic breccia that is cut by numerous branching microbreccia dikes, composed of sandy material, that contains organic debris derived from Cretaceous beds of the target. Western sector of the structure, Variegated Rocks cliff.

Fig. 6. Popigai impact structure, Russia. A Cambrian limestone block of allogenic breccia that is cut by numerous branching microbreccia dikes, composed of sandy material, that contains organic debris derived from Cretaceous beds of the target. Western sector of the structure, Variegated Rocks cliff.

Vredefort Granophyre

Fig. 7. Popigai impact structure, Russia. A polymict microbreccia dike cutting the cataclased gneiss of a large block in allogenic breccia. The dike consists of sandy matrix saturated with organic debris derived from Cretaceous beds. Embedded are slightly rounded fragments of Cretaceous (?) carbonificated wood (w), that shows evidence of desiccation, Permian black slate (s), Cambrian limestone (l), angular gneiss clasts and small clasts of impact glass are also present. Western sector of the structure, Variegated Rocks cliff. Scale in mm.

Fig. 7. Popigai impact structure, Russia. A polymict microbreccia dike cutting the cataclased gneiss of a large block in allogenic breccia. The dike consists of sandy matrix saturated with organic debris derived from Cretaceous beds. Embedded are slightly rounded fragments of Cretaceous (?) carbonificated wood (w), that shows evidence of desiccation, Permian black slate (s), Cambrian limestone (l), angular gneiss clasts and small clasts of impact glass are also present. Western sector of the structure, Variegated Rocks cliff. Scale in mm.

The third variety of injected dikes, e.g. DE type dikes, formed by injection of ejected polymict lithic breccia, and can be observed in crater walls or in fractured target rock outside of craters. Probably these dikes originated from ground surge transportation of ejected clastic material, represented by a mixture of various rock fragments. These polymict breccias are similar in lithology to allogenic lithic breccias forming lensoid bodies inside craters and ejecta blankets outside of them. Downward injection of these dikes causes downward displacement of some clasts. In some cases dikes may contain clastic material ejected from lowermost levels of the target as well. The dikes composing of such breccia were found in the vicinity of Lockne impact structure too (Sturkel and Ormo 1997), probably such clast-bearing dikes also occur in some other impact craters.

Fig. 8. Kara impact structure, Russia. Radial and circumferential orientations of dikes filled in by sandy material are shown by arrows; all dikes are located close to the crater edge. Numbers of dikes are indicated in circles. CU= central uplift of the structure (from Mashchak 1990).

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