Hydrodynamic role of groundwater in bolide impact: Evidence from the Kentland structure, Indiana, USA*
Published:December 10, 2018
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Raymond C. Gutschick†, 2018. "Hydrodynamic role of groundwater in bolide impact: Evidence from the Kentland structure, Indiana, USA", Ancient Oceans, Orogenic Uplifts, and Glacial Ice: Geologic Crossroads in America’s Heartland, Lee J. Florea
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†Deceased. Correspondence: Barry Voight, email@example.com.
The extremely important role of groundwater has been largely overlooked in studies of meteorite and comet impact processes. Beyond the radius of plasma generation, impacts can produce massive shattering in saturated porous rocks. Fluid pressure rise reduces rock strength and facilitates hydrofracture, to produce intraformational monomict breccias, faulting, and generation of mobile polymict breccia slurries. Decompression of a deep “transient” crater accounts for complex central uplift and gravitational collapse of tremendous slide blocks that in turn cause injection and ejection of fluidized breccia. As pore fluid pressures equilibrate, frictional strength increases, and the structural form is locked into stability. Evidence is reported here for Kentland, Indiana, where quarry rocks display relatively low pressure-temperature (elastic to ductile transition, 100 kb–100 °C) impact phases of the model of D. Stöffler. Breccias include monomict, polymict, mixed polymict-fault, and conventional fault types. The monomict breccias are associated with aquifer beds and formed by pervasive shockwave transmission on impact. Polymict breccias are derived from all rock types and formed from late stage injection-ejection pseudoviscous slurries. These processes can apply to similar impacts like Wells Creek, Flynn Creek, Decaturville, Sierra Madre, and many others.
The process of bolide impact is divided into stages for convenient reference, as first considered by Gault et al. (1968; cf. Short, 1966; Roddy, 1968, 1976). The first-stage of contact and compression begins with first contact of the striking bolide at tens of km per second. The projectile compresses and squeezes target material out of the path, and simultaneously decelerates because of target resistance. Strong shock waves are created at the interface between compressed and uncompressed material, creating shock pressures as much as 102 GPa. The duration of its entire stage is only about one second or less. Most of the initial kinetic energy of the projectile is transferred to compression, heat, and acceleration of target rocks, initiating the phase of excavation (Melosh, 1989; Stöffler, 1971). Subsonic excavation flow opens a “transient” crater many times larger than the projectile that produced it. The shock wave expands and weakens, degrades to a stress wave and ultimately decays into an elastic wave.
The modification stage follows, during which the walls of large craters collapse, while central uplifts occur, e.g., Sierra Madera, Ries, and Manicouagan (Grieve, 1991; Melosh, 1989). Alterations to the complex form are achieved mainly by collapse of an initially deep transient crater, with structural and stratigraphic uplift dominant near the center, accompanied by downward and inward slumping of rim materials. Gravity is claimed to be the principal force driving these modifications (Melosh, 1977, 1982), but an alternative mechanism is the so-called hydrodynamic theory of central peak formation, in which a central jet of target fluid rises behind the projectile and only partly collapses (Baldwin, 1963, 1981). Obviously, preservation of the uplift implies that the fluid is complex and has, or gains strength. The process is considered as a kind of dynamic decompression or rebound, in materials possessing both mobility, e.g., complex viscosity, and strength.
What is the effect of pore fluids on impact development? The idea has also been expressed that materials surrounding an impact crater may be mobilized by pore fluids (Kieffer and Simonds, 1980). The concept may apply for impacts on Earth and Mars but not for dry lunar or Mercurian craters. Explosion craters on earth are known to be influenced by subsurface water, notably the Prairie Flat Crater produced by a TNT sphere embedded in saturated silt and clay, and nuclear crater KOA in Eniwetok Atoll; possibly the exaggerated development of central peaks in Martian craters have a similar origin (Melosh, 1989; cf. Greeley et al., 1980; Fink et al., 1981). The question remains whether significant differences generally exist between cratering phenomena in wet and dry materials, and whether the presence or absence of pore water can account for such differences. The question is debatable; for example, Melosh (1989) argues that brecciation should create new porosity and decrease pore fluid pressure, and thereby prevent fluidization by water.
Some alternative explanations for mobile behavior of crater materials have been proposed, notably acoustic fluidization (Melosh, 1979); even this theory, invented to account for mobility in “dry” environments, remains speculative and would require modification to account for interstitial fluid interactions in a “wet” environment. Finally, much emphasis to date has concerned the behavior of loose rock debris in crater formation, rather than intensely fragmented but stratigraphically coherent materials.
This paper examines the role of ground water in bolide impact, and evidence is assembled from the Kentland uplift structure that bears particularly on crater excavation and modification stage processes, and the role of brecciation and pressurized pore fluid in promoting rheodynamic behavior.
THE KENTLAND ANOMALOUS STRUCTURAL COMPLEX
The Kentland anomalous structural complex is defined by an uplifted, erosionally truncated dome with central core, as revealed in six quarry pits as much as 120 m deep, supplemented by exploratory core drill holes over 100 m deep. Geological mapping of the pit floor and walls has kept pace with quarry operations from 1937 to the present, providing three-dimensional documentation of the structure (Shrock, 1937; Boyer, 1953; Gutschick, 1961, 1976, 1982, 1983, 1987, and unpublished company reports to the present; Laney and Van Schmus, 1978). Regional gravity and seismic refraction and reflection studies have outlined the boundary of the anomaly with surrounding flat-lying Paleozoic strata (Tudor, 1971) (see Figs. 1 and 2).
At the heart of the centrally uplifted dome is the complexly deformed Ordovician core, ~1.8 km in diameter. Erosional truncation of the dome, commensurate with uplift, has exposed the roots of impact. The Oneota Dolomite, the oldest exposed bedrock in the core, is stratigraphically uplifted ~700 m. The youngest rocks in the truncated central uplift are Lower Silurian Salamonie Dolomite. Upper Silurian (Niagaran and Cayugan) reef dolomite, Middle Devonian carbonates, and lower Mississippian rocks have been removed by erosion and are absent in the quarries; however, they are present along the flanks of the central uplift (Fig. 3).
By analogy with Stöffler’s (1971) model for the Ries basin impact, features observed in the Kentland quarries such as shattercones, breccias, microstructural elements in quartz, pseudotachylites (Gutschick, 1987; Dietz, 1947, 1972; S. de Silva, 1994, personal commun.; Q.S. Huss, 1994, personal commun.) suggest the impact deformation fit stages 0–1, indicating low to moderate impact pressures and temperatures. Kentland quarries thus represent an ideal field laboratory for that range of structures and dynamic metamorphic features. It is likely that eroded Upper Silurian to Lower Mississippian rocks were affected by higher dynamic pressures and temperatures than rocks now exposed.
Kopf (1982) recognized a role for groundwater in hydrotectonic elements related to some cryptoexplosion structures. He used such groundwater-related evidence to support an endogenic origin for these structures, thus challenging bolide impact interpretations. An alternative view is considered below.
It is reasonable to assume that at the time of impact the Kentland aquifers maintained similar qualities as do the present ones. Reconstruction of the stratigraphic column from quarry exposures and core drilling in the central uplift (Fig. 3) reveals three major aquifer zones: Silurian-Devonian carbonates, Trenton (Galena) dolomite, and Knox dolomite–St. Peter Sandstone (Rupp, 1989, 1991; State of Indiana Department of Natural Resources, Division of Water, 1990). These units total 450 m thickness, including the Cambrian Potosi Dolomite, not exposed or cored, to complete the Knox Group. All aquifers are considered heterogeneous.
Probably, impact dynamics also affected the Cambrian section below the Knox Group. This adds two other important aquifers, Galesville-Ironton (Davis Formation) and Mount Simon–Eau Claire sandstones. The aggregate thickness of aquifers is 1340 m, compared to 1585 m for the total sedimentary column. All other formations, ~245 m thick, are either aquitards or low quality aquifers which lack significant free flow permeability; still, some may have been saturated with significant porosity.
Tudor (1971) concluded from his gravity survey that Precambrian rocks were uplifted 600–900 m, and advocated an endogenic origin for the Kentland structure. However, the positive gravity anomaly can be accounted for by density variations in the sedimentary column and do not require basement involvement (Laney and Van Schmus, 1978). The Cambrian section has yet to be drilled to confirm its relationship to the aquifers and assess its breccia content.
Rocks at Kentland have sustained considerable deformation, as is manifest in three types of breccia (fault, monomict, and polymict), and shattercones (Gutschick, 1983, fig. 13 therein; 1987). Focus here is on the origin and roles of monomict and polymict breccias, as these are common to many impact structures (Sharpton and Grieve, 1990). Monomict and polymict breccias are distinct in appearance, habitat, timing, and genesis. Monomict breccias contain angular clasts and pulverized matrix of a single lithology that are formed by dilatant crushing of a host bed. These breccias are found throughout the Kentland structure associated with aquifer-rock-units—typically competent dolomites and sandstones; in contrast, rocks of low permeability, such as fine-grained limestones and dolomites, and shales, lack monomict breccias. Monomict breccias account for considerable dilation of the stratigraphic section.
Polymict breccias are characterized by angular clasts derived from diverse rock types in the central uplift, set in a fine-grained, mortar-like groundmass. Variations in these breccias occur, depending on the sources of the clasts. The clasts-in-matrix texture resembles concrete, and indeed the emplacement mechanism inferred for these breccias—commonly found in small clastic dikes—is analogous to pressurized cement grout. Polymict breccias are closely associated with faults and related fractures, and are absent within large rock blocks unaffected by faulting. These breccias are faintly streaked with flow lineations and swirl traces, probably reflecting preferred grain orientations; tabular clasts are aligned parallel to fault contact walls, and locally the breccias are interbedded with the strata along fault walls.
DYNAMICS OF THE KENTLAND BOLIDE IMPACT
Kentland quarries reveal much concerning the dynamics of impact, as they contain the exhumed roots of central uplift rocks subjected to lower stages of shock metamorphism. Rocks or impact products associated with high stages of shock metamorphism are missing at Kentland, including the crater and enclosed polymict breccia lens. Coesite has been reported by Cohen et al. (1961), but remains unconfirmed. The reconstruction of events presented here is based on geologic maps and sections by the author, as well as analysis of structural details, and consideration of the role of formation fluids. The physical model for the impact process at Kentland is a gigantic, rapidly striking and penetrating piston-and-cylinder assemblage, producing the sequence of contact and compression, followed by shock wave expansion and complex interference; explosive decompression, with inward and upward expansion producing the central uplift; and compressional readjustment due to partial collapse of the central uplift. How do the breccias fit into this pattern?
Shockwave Expansion and Rock Fragmentation
Expansion of the shock wave and near-field excavation flow are the two major processes of the so-called excavation stage of bolide-induced deformation. The “detached” shock wave (Melosh, 1989; Bjork et al., 1967) expands away from the impact site, characterized by pressure and particle velocity as established by Hugoniot relations, with initially high values declining roughly as 1/r2. The shock front is abrupt, with shock wave thickness only several meters or less in rocks, depending on rate-dependent pore closure and other processes. Pressure contours are approximately hemispherical and grade outward from vapor to melt, to the limit of crushing at the Hugoniot elastic limit. Below a surface spall zone defined by the tensile rarefaction wave, tensile and shear stresses break the rock into Grady-Kipp fragments to great depths below the impact site (Grady, 1980; Melosh, 1989).
In this process, competent aquifer rocks were affected by complex coupled elastic and hydrodynamic shock waves that caused intense hydrofracture and brecciation; non-aquifer-bearing rock units were less affected and unbrecciated. Brittle shattering was aided by high dynamic fluid pressures, in part reflecting thermal expansion or vaporization under pressure which reduced the effective confining pressure on the medium, and permitted the occurrence of transient effective tensile stresses. Rupture occurred when these stresses exceeded critical values.
With critical temperature increase, shock waves may convert water into steam. This seems less likely for aquifers in stage 0 shock metamorphism of Stöffler (1971), for which temperatures <100 °C (cf. conodont Color Alteration Index, CAI = 1.5 [50°–90 °C]) in Platteville-Galena rocks (Votaw, 1980; Jackson and Van Der Voo, 1986; Nasser and Howe, 1993).
Monomict brecciation at Kentland is thus interpreted to have formed during the shock wave expansion. In addition to shattering, transient fluid pressures generated tensile hydrofractures, a process that may have become explosive when groundwater was flashed to steam. The brecciation-hydrofracture processes were accompanied by dilation, and the associated increased aquifer reservoir permeability enhanced subsequent fluid transfer into the central uplift.
Rheodynamic Hypotheses for Formation of Central Uplift
Modification from a deep, bowl-shaped transient crater to a shallow complex form may be achieved by gravitationally driven collapse, associated with uplift of rocks underlying the crater (Melosh, 1989). The process may also involve components of “rebound” energy, and central uplifts and peak rings have indeed been compared to the central jet raised in a lowviscosity fluid following the impact of a projectile (see Melosh, 1989, fig. 8.19).
The term hydrodynamics has been applied to such a mechanism (Melosh, 1989, p. 147), although it is obvious that the term must be used only in a qualitative sense, as the ultimate fate of a disturbance in a perfect Newtonian fluid is a plane surface lacking surface relief. Clearly the medium must have strength in order to retain a dynamically produced structural form. Thus the term rheodynamic is introduced to imply dynamic deformation of a medium with qualities of both mobility (e.g., viscosity) and strength. The Bingham fluid is the simplest example of such materials (Melosh, 1982), but another approach, probably more realistic, is to consider that mechanical properties may change over the duration of the process. Thus, a rheodynamically generated structure may be “frozen” into position, a view which recalls the “frozen tsunami” theory of Baldwin (1972, 1981) to explain the morphology of complex impact craters. Here the view is explored that shattered rock may be fluidized by an influx of pressurized groundwater. The concept is inapplicable to lunar or Mercurian craters produced in a dry environment, but may apply elsewhere.
At Kentland, the field evidence is consistent with rheodynamic rebound of the transient crater, augmented by upward tumescent release of depressurized, dilatant water and steam initially associated with monomict brecciation. Boundary decompression led to rapid inflow from surrounding aquifers, at a rate constrained by hydraulic diffusivity. Collectively, these processes—in which groundwater played a vital role—resulted in significant uplift of the central region. The rate of uplift may also have been constrained by hydraulic diffusivity, and thus may not have been “nearly instantaneous.”
Faulting with associated polymict brecciation formed throughout the uplift, with rock types from all formations juxtaposed by faults mixed into a slurry. With rheologic mobility enhanced by supercritical pore fluid gradients, pseudoviscous polymict grout generated along faults was emplaced in an array of related fault splays, fault-associated fractures, and hydrofractures. These clastic intrusives indicate high pore fluid pressures at the time of faulting and clastic dike injection (Voight, 1973). Although it cannot be demonstrated at Kentland, due to subsequent surficial erosion, polymict breccias can potentially be ejected into the crater by polymict feeder dikes (Sharpton and Grieve, 1990, their fig. 4).
Rheodynamic mobility may be qualitatively expressed by the factors which produce and influence it:
where RM is rheodynamic mobility of liquefied slurry; DB is the characteristics of dry breccia (polymict); GW represents the amount, distribution, and properties of groundwater; EA is entrained air; Pp is pore fluid pressure; S is (three-dimensional) state of stress; P-T G are pressure-temperature conditions and gradients; and TFS is transient free space (development of openings and paths for transmission of slurry).
In the case of the Kentland impact, detailed structural mapping also indicates collapse by massive coherent blocks of rock and slurries sliding off the rising central uplift. Breakaway separation occurred along the Kentland Quarry Fault (KQF) of Gutschick (1976, 1987), which apparently circumscribes the central uplifted core (Gutschick, 1987, fig. 5 therein). Sliding was facilitated along the bedding shear between weakly cemented, cataclastically deformed St. Peter Sandstone and overlying hard, brittle, intricately fractured Joachim Dolomite, probably aided by high fault pressures. The KQF is characterized by planar, polished slickensided surfaces, mullion, quartz lamellae, breccias and ductile gouge (Gutschick, 1983, figs. 12 and 13 therein).
New perspectives into meteorite and comet impacts are gained from consideration of the important role of groundwater in impact processes. Consideration of these processes leads to the following conclusions.
(1) On earth and some other planets, groundwater is stored in large quantities in aquifers and in lesser quantities in aquitards. In both instances the available pore space is water saturated, and thus water must be considered in treatments of bolide impact dynamics. Impacts preserving lower stages of metamorphism like the Kentland structure furnish this evidence.
(2) Groundwater transmits high-energy shock waves, and aquifers are compressed as a consequence of shock-wave expansion and thermal strains established in poro-elastic or poro-plastic media.
(3) Pore fluid pressures reduce rock and rock-fragment interface strength, and encourage brittle behavior via the Terzaghian principle of effective stress, expressed in dynamic terms.
(4) Heat produced by dynamic impact may locally cause explosive steam generation; the process is complex and is coupled to dynamic fluid pressures, in association with thermodynamic pressure-boiling relations.
(5) Tensile and shear shattering and hydrofracture processes are prevalent due to (3) above and account for monomict brecciation associated with aquifers.
(6) Fracture initiation occurs with shock-wave expansion in saturated, fluid-pressurized media. Dilation occurs as a result of fracturing, thus momentarily reducing fluid pressures—but only after the fractures had formed with passage of the shock wave.
(7) As a consequence of (3), resistance to faulting is reduced in the core of the central uplift; gravitational collapse of the uplift, including breakaway sliding of coherent rock mass blocks, is likewise facilitated (Hubbert and Rubey, 1959).
(8) Fault-related generation of polymict breccia slurries due to (3) and (4) above, and pseudoviscous injection of slurries into adjacent fault blocks, are aided by hydrofractures. The clastic breccia injection features observed at Kentland and elsewhere are evidence that strongly suggests high fluid pressures at the time of deformation.
(9) Some central peak (and peak scale rings) may be explained by rheodynamic rebound in a collapsing transient impact cavity. The gross fluid-like aspect of uplift may be accounted for by pressurized groundwater and pseudoviscous slurries engendered by supercritical (relative to liquefaction) interstitial-fluid pressure gradients in pervasively shattered media.
(10) Ultimately, fluid pressure gradient dissipation associated with uplift and massive dilation causes a recovery in frictional strength via (3), providing a mechanical basis for “freezing” the morphology of structures developed by rheodynamically generated uplift.
(11) Nearer to the projectile contact region, groundwater may serve as a flux to reduce the melting temperature of the rock matrix (Kieffer and Simonds, 1980).
(12) Buildup of the hydraulic head of rheodynamic polymict slurries may cause effusive ejection into the crater to account for the polymict breccia lens resting on in situ monomict brecciated strata formed earlier. It can also account for a buoyancy factor in uplift.
The author values helpful discussions on impact dynamics with Gordon Bennett, David Stearns, and Barry Voight.
The author values helpful discussions on impact dynamics with Gordon Bennett, David Stearns, and Barry Voight.
The author values helpful discussions on impact dynamics with Gordon Bennett, David Stearns, and Barry Voight.
Ancient Oceans, Orogenic Uplifts, and Glacial Ice: Geologic Crossroads in America’s Heartland
This volume, prepared for the 130th Annual Meeting of the Geological Society of America in Indianapolis, includes compelling science and field trips in Indiana, Illinois, Kentucky, Michigan, and Ohio. A wealth of geologic and human history collides in the Midwest, a confluence that led to the growth of America's industry over the past two centuries. Guides in this volume depict this development from the establishment of New Harmony, the birthplace of American geology, through the construction of Indianapolis's modern skyline. Underpinning this growth were the widespread natural resources-limestone, coal, and water-that built, powered, and connected a growing nation. Take a journey through the Heartland to sand dunes, outcrops, quarries, rivers, caves, and springs that connect Paleozoic stratigraphy with the assembly of Gondwana, continental glaciation with Quaternary geomorphology and hydrology, and landscape with the human environment.
- brittle deformation
- fluid pressure
- ground water
- hydraulic fracturing
- hydraulic head
- impact breccia
- impact craters
- impact features
- Kentland impact structure
- metamorphic rocks
- Newton County Indiana
- shock waves
- transient phenomena
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