Petrographic Studies of Rocks from The Chesapeake Bay Impact Structure (USA): Implication for Moderate Shock Pressures in Sedimentary Breccias

Shock petrographic investigations were carried out on samples collected from drill cores from the Chesapeake Bay impact structure (USA). The late Eocene Chesapeake impact structure is, at 85 km diameter, currently the largest impact structure known in the United States, buried at shallow to moderate depths beneath continental margin sediments underneath southeastern Virginia. To better define the variety of the samples collected from the shallow drill cores and the shock degrees experienced by the target rocks and breccias in the Chesapeake impact crater, thin section analyses were conducted on more than 50 samples from the various zones of the impact structure. The study involves measurements of the orientations of planar deformation features (PDFs) using a universal stage attached to a petrographic microscope. The aim of this study is to determine the shock pressures of various clasts in the shallow breccia fill of the crater. As a result, we note that the overwhelming numbers of shocked grains, which are now present in the sedimentary breccia, are derived from the basement granitoids. Our studies involved samples from four shallow drill cores (Exmore, Windmill Point, Kiptopeke, and Newsport News).The breccia fill is termed the Exmore breccia, which is dominated by particulates of silt, shocked and unshocked granitic fragments, shale, clay, and free shocked quartz grains. The Kiptopke and Windmill Point samples contained rare fragments showing a variety of different shock effects, whereas the Newporte News samples, show several fragments and impact melt with the evidence shock metamorphism was noted. The most abundantly observed shock indicators are shock fracturing, indicative of shock pressures of less than about 10 GPa, as well as 1-2 sets of PDFs in quartz grains, which is indicative of moderate shock pressures of up to about 20 or 25 GPa.


INTRODUCTION
Impact cratering is a rapid surface-modification process, which happens when a large meteoroid (asteroid or comet) hits a planet or a satellite (e.g., Koeberl, 1998). Besides, it is a unique geological process in which vast amounts of energy are released in a small area in a very short time (e.g., Grieve, 1990). Impact craters on Earth are produced by the hypervelocity impact of asteroids and comets at a velocity between 11 and 72 km/s and the magnitude of the energy constructed based on 10 multi-channel seismic reflection profiles transecting the bay and the 3 single-channel profiles on the inner continental shelf, as well as 56 boreholes drilled inside and outside the crater rim (Poag et al., 1994). The seismic profiles define the outer rim structure of the crater (e.g., Koeberl et al., 1996;Poag et al., 2004).
Much of the shallow part of the crater is filled with a chaotic sedimentary deposit known as the Exmore breccia. The Exmore breccia contains angular clasts of older sedimentary material, and granite to metamorphic basement rocks in sandy matrix. A first petrographic and geochemical study of samples from the Exmore breccia (imaged on seismic profiles in the environs of the central uplift with a maximum thickness of 1.2 km) showed that the breccia is composed of a range of clastic components (the various pre-impact sediments and crystalline granitoid basement) set in to fine-grained clastic-matrix of the same components. The first evidence for an impact crater came from the morphology and the occurrence of breccia (Poag et al., 1994). Final confirmation came from the identification in cores of partially melted basement rocks and multiple sets of planar deformation features in quartz and feldspar basement clasts (Koeberl et al., 1996).
The relatively recent discovery of the crater (Powars et al., 1993;Poag et al., 2004) has contributed to a better understanding of the geological framework of the middle and outer Virginia Coastal Plain. Moreover, the existence and location of the crater helps to explain the structure, stratigraphy and ground-water quality in the area.
The Chesapeake Bay impact structure is also the source crater for the North American tektitesglassy distal ejecta that are found in a geographically extended strewn field along the eastern and central part of the North American continent (e.g., Koeberl et al., 1996;Deutsch and Koeberl, 2006).
Recently, the Chesapeake Bay impact structure was the subject of a large international and interdisciplinary deep drilling project; the goal was to obtain a deep, continuously cored hole into The results from the deep drillcore supplement the information that has been obtained from the sampling of the shallow drillcores (including those described in the present manuscript).

GENERAL GEOLOGY
The geological cross section of the Chesapeake Bay impact crater and its circular features was derived from seismic surveys and detailed examination of sedimentary cores (Fig. 2). The Chesapeake Bay impact structure lies beneath the shallow waters of the Chesapeake Bay and a thin veneer of coastal plain sediment (Powers and Bruce, 1999;Poag et al., 1994;Poag et al., 2004). The structure includes an inner basin surrounded by a ring of raised basement rock, encircled by a flat-floored terrace zone and bounded by the outer rim by a zone of concentric faulting (Powars and Bruce, 1999;Koeberl et al., 1996). The crater is overlain by up to 650 m of Early Cretaceous to late Eocene sedimentary material and underlain by granodioritic basement rocks The pre-impact coastal plain rocks consisted of a seaward-thickening wedge of mainly lower Cretaceous to upper Eocene age, poorly lithified, and mainly silicic-clastic sedimentary rocks (Fig. 2).
Information from borehole samples indicates that the structure of the Chesapeake Bay crater is partially filled with the so-called Exmore breccia, which is mainly composed of autochthonous sedimentary material and granitic to metamorphic, with minor basement rock clasts in a sandy matrix (e.g., Poag et al., 1994;Koeberl et al., 1996). After the formation of the crater, younger marine and non-marine sediments deposited on the coastal plain completely buried the structure.
The crystalline basement (interior structure) of the Chesapeake Bay crater is expressed in the structure and thickness of the overlying breccia and of the post-impact sedimentary section. In particular, both the breccia and the post-impact section are notably thinner and structurally raised where they cross the peak ring and central peak (Fig. 2). During the impact process, the impactor penetrated through the water column, the full thickness of the existing Coastal Plain sediments, slammed in to the basement rock, and vaporized, creating a catastrophic explosion and ejected material into the atmosphere (e.g., Poag et al., 2004;Powars and Bruce, 1999). The basement rocks lining the crater cavity were melted, and the basement rocks in the region beneath and around the crater were faulted and fractured. The impact produced an inverted sombrero-shaped 85-km-wide complex crater that was immediately filled with sediments and rim collapse material and eventually buried by younger sedimentary deposits (Powars and Bruce, 1999). From geophysical and geological data, the Chesapeake Bay crater has been reported as one of the bestpreserved complex peak-ring structures documented on Earth (Poag et al., 1994(Poag et al., , 2004.

METHODOLOGY
The samples for this study of shock metamorphism of the clastic sediments were collected by Poag (2004) (US Geological Survey) from cores drilled earlier into the Chesapeake Bay crater.
For the present study, 50 samples that are representative of the different lithological types
Figure 2. Schematic radial cross section showing half of "inverted sombrero" shape of the Chesapeake Bay impact crater, constructed from drill core and seismic data (from Gohn et.al 2006). The location of the cross-section is indicated in Fig. 1. The measurements of the planar deformation features (PDFs) were conducted using universal stage (e.g., Emmons, 1943). More precisely, this method allows the determinations of the angle between the c-axis of a quartz grain and the poles of the planes of the planar deformation features. The data obtained from the universal stage measurements are then plotted with the aid of a stereographic equal-area net projection. After measuring the orientations of the PDFs in quartz grains, the data are arranged in table form to provide information about the axis of orientation and plane of orientation of each quartz grains in thin section. From the data, a histogram is plotted, with the X-axis representing the polar angle and the frequency on the Yaxis. The overwhelming number of shocked grains is derived from basement granitoids. Only rarely it was possible to observe weak to moderate shock deformation in sediment-derived particles.
Granitoids are widely present, even though the clastic sedimentary components are important throughout the drilled breccia sequences, carbonates are relatively rare. On the other hand, mafic components are also extremely rare (Poag et al., 2004). Most thin sections contained 10 to 15 mineral fragments, but fine grained material (<1 mm grain size) may have contained several grains. Exmore samples do frequently contain very small proportions of shock particles, but here too, very weak and weak shock degrees are dominant. only or impact melt breccia. In Newport News samples, several fragments and impact melt with evidence of shock metamorphism were noted.

Planar deformation feature (PDF) measurements
Confirmation of an impact origin requires conclusive evidence that the rocks and minerals have undergone shock metamorphism, which is defined by high pressures and temperatures (up to 100 GPa and 1000 0 c), and strain rates associated with impact cratering (from 5 GPa to > 50 GPa).
The type of metamorphism depends on the shock pressure experienced. Planar deformation features (PDFs) is the designation currently used for distinctive and shock produced microstructures that were formerly given a variety of names (e.g. "planar features", "shock lamella"). In contrast to planar features, with which they may occur, PDFs are not open cracks.
Instead they are sets of closed, extremely narrow , parallel planar regions (Fig. 3e, f).
Most of the information from impact structures comes from dense, coherent quartz bearing crystalline rocks (French, 1998). There is a relatively little information about the effects of shock deformation in other kinds of quartz-bearing rocks, e.g. porous sandstone or fine grained shale.
Several studies have demonstrated that shocked sandstones and shale's also develop PDF in quartz, and even diapeletic quartz and feldspar glasses, similar to those observed in other craters in shocked crystalline rocks (Fig.3). Despite These similarities a growing amount of data now indicate that sedimentary rocks respond differently to shock pressure than do crystalline rocks (Greive et. al., 1996). The more important difference between the sedimentary porous rocks and crystalline rocks is that a shock wave passing through sediments will generate more heat than in passing through crystalline rocks. extremely narrow , parallel planar regions (Fig. 3e, f).
Evidence of shock metamorphism is abundant in rocks and mineral clasts from the Exmore breccia in the Chesapeake Bay impact structure was described earlier by Koeberl et al. (1996) and Poag et al. (2004). A first petrographic study from the Exmore breccia showed that the breccia is composed of a range of clastic components, such as various pre-impact sediments and crystalline granitoid basement set in to fine-grained clastic matrix of the same components. More than 50 specimens from four boreholes (Exmore, Windmill Point, Kiptopeke, and Newporte News) into the shallow outer annulus of the Chesapeake Bay crater were examined for the presence of distinctive mineral deformation features. Most of the samples analyzed did not show any significant deformation features at all. A specimen from Windmill Point and Newport News Shock metamorphism in these samples is manifested by a number of quartz grains with single occasionally with multiple sets of planar deformation features (Table 4). The overall percentage of PDF-bearing quartz grains in the investigated core samples is far less than 1 vol%. The results include data for 2 grains with 1 sets of PDFs, 17 grains with two sets each and 3 grains with three sets (Table 3) each, and also the data showings the frequency of the angles between the quartz C-axes and the poles to the planes of PDFs in degrees. Figure 4 shows a histogram with orientations of the poles to the planes of PDFs relative to the C-axis of the quartz grains.
The orientation of 45 sets of PDFs was determined in 22 quartz grains from the Exmore breccia.
Most of the remaining sets are oriented parallel to r {101 1} and {213 1 }, and basal the plane is absent. Seventeen grains with 2 sets could be reliably indexed at shock diagnostic orientations.
Most abundantly observed PDFs in the Exmore breccia in the quartz grains are one to two sets of planes, which are indicative of moderate degrees of shock. Mainly {101 3} and {101 2} are the dominant orientation, which provide conclusive evidence that the rocks and minerals have undergone shock metamorphism; that is, subjected pressures of 10-25) (Fig 4). Table 4. Number of quartz grains with a number of sets of PDF planes and orientations from the Chesapeake Bay impact structure, USA.