2.1 OVERVIEW
The area of interest lies along the 38th parallel, in the Ozark Dome, a broad region of upwarped strata in central and southern Missouri and northern Arkansas, whose structural origin dates to the Late Ordovician. Sediments of Late Cambrian to Early Ordovician age were deposited in two cycles of marine transgression such that the Ordovician strata lie disconformably on the underlying Cambrian layers (Bunker et al., 1988) . Pennsylvanian sediments, remnants of which still remain in Ordovician paleosinkholes, have since been almost entirely removed by erosion (Bunker et al., 1988).

This sedimentary column rests upon the eroded surface of a 1.44 Ga granitic rhyolite province whose origin has been linked to the opening of the Reelfoot Rift, an abortive Eocambrian rift system related to the opening of the Iapetus Ocean during the breakup of Laurentia (Stark, 1997). Rocks of this volcanic terrane and their associated intrusives are best exposed in the St. Francois Mountains, which lie approximately 60 kilometers to the east of the study area, a region noted for its extensive hydrothermal ore deposits (Brown, 1989). Less extensive igneous activity involving highly basic magmas continued into Late Cambrian time, and Ervin & McGinnis (1975) suggested that the end of magmatic activity in the St. Francois Mountains also signaled an end to the rifting event.

During the Cretaceous, the Reelfoot Rift was again reactivated, this time as the Mississippi Embayment, the failed third arm of a triple junction which opened the Gulf of Mexico (Ervin & McGinnis, 1975; Sykes, 1978; Keller et al., 1983; Braile et al., 1986; Horrall et al., 1993; Marshak & Paulson, 1996, Kolata & Nelson, 1997; Stark, 1997). The abortive nature of both rifting events acted to preserve the region as part of the interior stable craton, therefore the record in the rocks has not been lost to extensive deformation. However, faulting has been extensive, especially adjacent to the rift area (Bunker et al., 1988). This rift region is again active today as the New Madrid Seismic Zone.

In addition to the general structural features mentioned above, the 38th parallel is home to a series of events known as cryptoexplosive structures. As implied by the name, these features are controversial in origin, with current debate divided between a volcanic or impact cause (Snyder & Gerdeman, 1965; Heyl, 1972; Kisvarsanyi & Kisvarsanyi, 1976; Lidiak & Zietz, 1976; Sykes, 1978; Grieve, 1987; Nicholaysen & Ferguson, 1990; Unklesbay & Vineyard, 1992; Rampino, 1997; Luczaj, 1998).

Five of the causes of circular features listed in the introduction are geologic in origin. The following sections emphasize the characteristics of those causes as they occur within the geologic framework of southeastern Missouri.

2.2 STRATIGRAPHY
The stratigraphic column [Fig. 2.1] consists of a series of depositional cycles resulting from the downwarping of the crust toward the southeast, away from the Transcontinental Arch (Bunker et al., 1988). Both the present Ozark Dome and adjacent Illinois Basin to the east constitute areas of maximum subsidence.

In southeastern Missouri, two of the causes, structural dome and erosion/collapse basin, may be related to the stratigraphy. In this area, local structural �domes� result from preferential deposition of sediments around basement topographic highs rather than crustal warping. Erosion or collapse basins form in areas of extensive carbonate deposition where regional uplift has exposed the rocks, resulting in a karst topography.

2.2.1 Apparent Structural Dome. The Dresbachian Lamotte Sandstone, the basal member of the stratigraphic sequence in southeastern Missouri, reaches a maximum thickness of about 160 meters (Unkelsbay & Vineyard, 1992). Vertical relief of the underlying Precambrian surface at the time of initial deposition was about 450 meters, so that topographically high areas remained unsubmerged (Bunker et al., 1988). Further transgression of the seas resulted in deposition of the Bonneterre Formation, which consists of shaly beds of limestone and dolomite. Some of the higher portions of the St. Francois Mountains still remained above sea level during this period, and surrounding these relict islands are sandier areas in the carbonates as well as algal reefs. These bioherms became the locus for many of the region�s lead deposits (Unklesbay & Vineyard, 1992). In this area of near-horizontal bedding, apparent structural �domes� occur over these basement topographic highs.

2.2.2 Erosion or Collapse Basin. Two paleokarst erosional surfaces are present in the stratigraphic column of southeastern Missouri: The Eminence Formation, which is the uppermost of the Cambrian rock units in the Ozarks, and the Jefferson City Formation, the uppermost Ordovician unit in the study area.

The Eminence Formation consists of a sandy dolomite with abundant chert nodules. Much of the chert is found in large masses, suggesting that it originated as algal colonies (Unklesbay & Vineyard, 1992). The Jefferson City Formation is a cherty dolomite which, like the Eminence Formation, underwent karstification sometime after uplift (Unklesbay & Vineyard, 1992). Pennsylvanian sediments, which once covered this paleokarst, have since been completely removed by erosion, except for a few remnants remaining within some paleosinkholes (Bunker et al., 1988; Unklesbay & Vineyard, 1992) Presently, the Ozark Dome is a dissected plateau of extensive modern karst formation (Unklesbay & Vineyard, 1992).

2.3 ST. FRANCOIS MOUNTAINS
The St. Francois Mountains are the most extensive outcrop of Precambrian igneous rocks in the region (Bickford et al., 1986). Today the terrain is one of exhumed paleo-topography, the eroded remains of an extensive Precambrian volcanic system. Associated with the volcanic units are several intrusive plutons which are circular to arcuate in plan: Kisvarsanyi (1981) counted several granitic ring complexes, six ring intrusions, twelve central plutons, and one major cauldron-subsidence feature in the area, while Brown (1989) discusses four more cauldron collapse structures on the southern margins of the region. A brief description of the characteristics of these plutons follows:

The most voluminous granitic plutonic rocks occur as subvolcanic batholiths and stocks, the central plutons of the ring intrusions, and as ring dikes. Rocks of intermediate composition are the syenites of the ring plutons and trachytic hypabyssal intrusions (Kisvarsanyi, 1981), although Brown (1989) has cautioned that the potassium content of some rocks identified as trachyte could be secondarily introduced. Mafic intrusions are more scarce and consist of small layered gabbroic intrusions and diabase dikes (Amos & Desborough, 1970; Kisvarsanyi, 1981).

Youngest (~500 Ma) may occur    together    Oldest (~1.44 Ga) Late diabase dikes Small layered gabbro intrusions Tin granites assoc. with resurgent domes Trachytic rocks Syenite and/or amphibole granite ring intrusions Biotite granite batholiths Rhyolitic flows and ash falls Fig. 2.2 - Age relations of crystalline rocks (Kisvarsanyi, 1981; Amos & Desborough, 1970.

The following information on the felsic and intermediate rocks of the St. Francois Mountains and surrounding Precambrian are summarized from Kisvarsanyi, (1981): The oldest rocks of the St. Francois Mountains are rhyolites [Fig. 2.2]. Exposed contacts show an alkali-feldspar biotite granite both intruding and underlying the rhyolite. Chemical and mineralogical evidence suggests the rocks are co-magmatic, indicating that the plutons are roofed by their own volcanics. Typically, the granite grades with depth from a relatively fine-grained granophyre into a coarse-grained, hypidiomorphic rapakivi granite known from exposures farther north in the terrane as the Butler Hill Granite. These granites are the most widespread and abundant rock types known from drill cores, and extend well to the south near Ellington, Missouri, where granophyre again crops out locally at the surface. Following caldera collapse and cauldron subsidence after the upper crustal magma chambers were emptied, less silicic amphibole-biotite granites and syenites were emplaced along ring faults and fractures. Variations in texture and composition occur within individual ring intrusions, possibly due partly to magmatic differentiation.

The emplacement timing of trachytic rocks is not well understood, since they have not been seen in contact with the ring plutons described above. However, based on observed cross-cutting relationships, they are known to post-date the rhyolite and predate the tin granites, discussed below. They are concentrated along fracture zones, most noticeably in the northern portion of the terrane. This group of rocks includes trachytes, syenites, trachyandesites, and trachybasalts.

The last stage of granite emplacement is associated with resurgent doming. These granites occur as small circular to oval plutons of medium to coarse-grained pegmatite within some cauldron-subsidence structures. Because these rocks are relatively enriched in some of the large ion lithophile elements including tin, they are sometimes referred to as �tin� granites. They are expressed on aeromagnetic maps as circular magnetic lows surrounded by a ring of relative magnetic highs (the ring plutons). The tin granites have been observed cutting all other rock types except for the olivine diabase dikes (discussed below).

Mafic rocks are the youngest unit in the exposed Precambrian of the St. Francois Mountains (Amos & Desborough, 1970, Kisvarsanyi, 1981). They consist mainly of tholeiitic olivine diabase dikes and layered intrusions of gabbro which are small compared with the extensive granites (Kisvarsanyi, 1981). However, Amos & Desborough (1970) note that sheets up to 900 feet (275 meters) thick and dikes up to 3000 feet (915 meters) wide are common. The following information on these mafic rocks is taken from Amos & Desborough (1970): The layered gabbroic intrusions are the older of the two rock types. Chilled margins of this type are of olivine diabase. The gabbro filling the center shows two textural variations: laminated and ophitic. Igneous lamination is the most common type of layering, however rhythmic and cryptic layering are also present. The lamination may be present in the darker bands of rhythmically layered rocks. When it is, it is typically parallel to the layers, which in most cases dip less than 15�. When igneous lamination occurs alone, it is nearly always horizontal. Cryptic layering is indicated by changes in plagioclase and olivine composition. In the lower portions of the intrusive, the olivine is magnesian and the plagioclase is calcic labradorite. Alkalinity and iron content increase upwards. Essential minerals of the two rock types are the same and differ only in proportion, except that olivine may or may not be present in the gabbros. Especially noticeable is the gabbros� marked increase in opaque mineral content. Major constituents (>10% by volume) are plagioclase (labradorite), augite, and olivine. Secondary minerals include chlorite, sphene, pyrite, and marcasite.

Kisvarsanyi (1981) likened the St. Francois terrane to that of the Sara-Fier granite complex in northern Nigeria, noting the similarity of the tectonic setting, geophysical signatures, mode of occurrence, petrology, mineralogy, and geochemistry. She noted that the only major differences between the two terranes were their ages and levels of erosion: Most of the volcanic pile has been removed from the Nigerian province, revealing the circular to ellipsoidal ring intrusions. This exposure shows their alignment along structural lineaments, the overlapping nature of the rings themselves, and suggests that the ring centers migrate with time (Kisvarsanyi, 1981).

2.4 STRUCTURAL FEATURES
Fracture patterns and impact structures are the primary causes of circular features associated with structural geology. However, each of the geologic causes may be structurally controlled: Sinkholes tend to align along existing fractures or lineaments (Price, 1984). Intrusive plutons, which in this area may be closely related to structural highs, are also likely to be controlled by the structural grain of the region (Kisvarsanyi, 1981).

2.4.1 The Reelfoot Rift. The Reelfoot Rift is a northeast trending zone of crustal extension of Eocambrian age, which branches in �Y� fashion and has been likened to the triple junction of the Benue trough in Africa (Ervin & McGinnis, 1975; Keller et al., 1983). The east-trending branch is known as the Rough Creek Graben (Stark, 1997; Kolata & Nelson, 1997). Fault systems and troughs extending from this branch continue to the east, ultimately disappearing beneath the Grenville Front (Heyl, 1972; Stark, 1997). The west-trending branch, known as the St. Louis Arm [Fig. 2.3], consists of the Cottage Grove Fault System in Illinois and the St. Genevieve Fault System in Missouri (Stark, 1997; Kolata & Nelson, 1997).

Associated with the Reelfoot Rift are a series of northwest-trending faults and lineaments which lie perpendicular to the strike of the Rift (Kisvarsanyi & Kisvarsanyi, 1976; Clendenin et al., 1989; Horrall et al., 1993). The Simms Mountain Fault, the Black Fault, and the Ellington Fault in Missouri, and the Bolivar-Mansfield Fault in Arkansas, are examples of these faults (Clendenin et al., 1989; Horrall et al., 1993). In addition, Clendenin et al. (1989) mapped a fault in the subsurface they called the Shannon Fault. Other northwest-trending features include the Bloomfield Lineament Zone, an area of northwest-striking lineaments first noticed by Horrall et al. (1993) on Landsat-2 imagery, and the Missouri Gravity Low, a 700 km long negative gravity anomaly with a maximum amplitude of -40 mGal (Arvidson et al., 1984; Horrall et al., 1993) [Fig 2.3].

Northwest-striking faults occur in extent from single fractures to large braided systems (Clendenin et al., 1989). Both Clendenin et al. (1989) and Horrall et al. (1993) consider the northwest-trending faults to be transform faults related to the original rifting episode. The dominant strike of lineaments, both from ground mapping of joints and inspection of Landsat-1 images, also follows this northwest trending pattern (Gay, 1976; Kisvasanyi & Kisvarsanyi, 1976).The Missouri Gravity Low extends northwest from the Reelfoot Rift, where it coincides with the horst block known as the Pascola Arch (Arvidson et al., 1982), to the Midcontinent Rift, where it forms the accommodation zone separating the Nebraska and Kansas segments of that rift (Berendson, 1997). Arvidson et al. (1982, 1984) consider that it is best modeled as a 4 to 6 kilometer crustal excess at the Moho. The intersection of the Missouri Gravity Low with the Reelfoot Rift has clearly controlled the emplacement of two mafic plutons, which were identified by their positive gravity signature: the Bloomington to the north and the Covington to the south (Arvidson et al., 1982). A portion of the Missouri Gravity Low underlies the study area [Fig. 2.3].

Ervin & McGinnis (1975) argue that the Reelfoot Rift was part of the widespread rifting activity which took place in central North America during the late Proterozoic. As Bickford et al. (1986) point out, this predominantly granite-rhyolite terrane, now exposed in the St. Francois Mountains of Missouri, is consistent with derivation from anatexis of the continental crust during a period of extension. They suggest that the absence of a large volume of mafic rocks is an indicator that rifting did not proceed long enough to tap the basaltic heat source.

2.4.2 The 38th Parallel Lineament. Several basement-cutting fault systems also extend westward from the St. Louis Arm as well: The Big River Fault System, Palmer Fault System, and Newburg Fault zone (McCracken, 1971; Heyl, 1972). Both branches of the Reelfoot Rift and their extensions are known as the 38th parallel lineament after their location, which approximately follows the 38th parallel of latitude (McCracken, 1971; Heyl, 1972, Kisvarsanyi & Kisvarsanyi, 1976) [Fig. 2.3].

In addition to the above-mentioned faults, eight Phanerozoic structures, known as cryptoexplosive features also lie along the western branch of the 38th parallel lineament (Snyder & Gerdeman, 1965; McCracken, 1971; Heyl, 1972; Kisvarsanyi & Kisvarsanyi, 1976; Lidiak & Zietz, 1976; Sykes, 1978; Grieve, 1987; Nicholaysen & Ferguson, 1990; Unklesbay & Vineyard, 1992; Rampino, 1997; Luczaj, 1998) [Fig. 2.3].

As some of these cryptoexplosive structures display shock metamorphic features, some writers have characterized them as resulting from a string of bolide impacts (Grieve, 1987; Rampino, 1997). However, others have determined, on the basis of disturbed strata and cross-cutting relationships, that the features are of various geological ages ranging from Cambrian to Cretaceous, and therefore could not have formed during a single event (Snyder & Gerdeman, 1965; Luczaj, 1998). Snyder & Gerdeman noted that nearly all of the cryptoexplosive structures were located at the intersections of the 38th parallel lineament with other known regional faults or folds, and that most of the structures have an association with basic igneous activity. Nicholaysen & Ferguson (1990) have shown that it is possible for shock metamorphic features such as shatter cones to form from explosive venting of fluids which are associated with alkaline ultramafic magmas: Since these alkaline magmas are first-formed partial melts with CO2 pressures higher than 26 kbar, the associated fluids must enter the vapor phase during ascent. If this phase transition is sufficiently fast, it may drive a shock wave. Rapid exsolution of the volatile component could cause cataclysmic disruption of the rock. They contend that this phenomenon could be responsible for all of the 38th parallel cryptoexplosion structures, and that the differences in morphology are caused by: 1) the depth where the volatiles originated, 2) their subsequent evolutionary paths, and 3) the nature of the near-surface wall rocks which they intruded. Whatever their origin may be, the features themselves are diverse in their morphologies, and this is reflected in the length of the ensuing description. Nicholaysen & Ferguson (1990) grouped their different kinds of rock-failure mechanisms into six �types�, four of which are present in the 38th parallel lineament structures, and the individual descriptions below are listed by these �types.�

Type 2 (type 1 is not represented in the lineament) - �Fluid with magma globules is explosively erupted in diatremes; a swarm of pipes is usually present.� The Avon Diatremes in Missouri are of this type (Nicholaysen & Ferguson, 1990). These are located over an area of 100 square miles, near the intersection of the St. Genevieve and Cottage Grove fault systems (Heyl, 1972), at the south end of the northwest-trending Farmington anticline (Snyder & Gerdeman, 1965; McCracken, 1971; Heyl, 1972; Ervin & McGinnis, 1975), and at the northwest terminus of Horrall et al.�s (1993) Bloomfield lineament zone. The dike material consists of three types: kimberlite to alkalic peridotite, breccias composed entirely of the country rock, and breccias composed of a combination of country rock and basalt lapilli, which appears to be volcanic ejecta (Snyder & Gerdeman, 1965; Heyl, 1972). The intrusions contain anomalously high amounts of thorium, niobium and rare-earth elements, and one diatreme to the southwest also contains barite and flourite (Heyl, 1972). Major constituent minerals of the igneous rocks are olivine, augite, and phlogopite (Snyder & Gerdeman, 1965; Nicholaysen & Ferguson, 1990). Nicholaysen & Ferguson (1990) also reported small, concentrically arranged melilite crystals. The age of the diatremes has been calculated at Devonian to pre-Mississippian (Snyder & Gerdeman, 1965; Luczaj, 1998).

Type 3 - �Fault-bounded rock mass is created by fluid pressure and magmatic pressure in an underlying unit.� Nicholayen & Ferguson�s type example is the Rose Dome in southeastern Kansas [Fig 2.3]. This feature, together with the nearby Silver City and Neosho Domes, forms a northeast-trending anticline (Snyder & Gerdeman, 1965; Heyl, 1972; Luczaj, 1998). The Rose and Silver City Domes are both characterized by concentric and ring faults and a ring graben (Luczaj, 1998), and both were formed by laccoliths of alkalic peridotite into the Pennsylvanian strata (Heyl, 1972; Luczaj, 1998). In addition, the Rose Dome contains large blocks of Precambrian granite at the surface, which has been displaced at least 1700 feet (518 meters) from the basement (Snyder & Gerdeman, 1965; Heyl, 1972; Luczaj, 1998). Major constituent minerals in the peridotite laccoliths are phlogopite, olivine, augite, and amphibole (Snyder & Gerdeman, 1965). Radiogenic dating of the peridotites places the age of both domes in the mid-Cretaceous (Heyl, 1972; Luczaj, 1998).

Type 4 - �Localized updoming occurs in cover strata; gas escapes explosively at the crest without causing shock deformation.� Hicks Dome [Fig 2.3] is the example of this type of explosive rock-failure mechanism (Nicholaysen & Ferguson, 1990). This structure is located in Illinois at the intersection of the main Reelfoot Rift with its St. Louis Arm (Heyl, 1972; Ervin & McGinnis, 1975; Luczaj, 1998). It has a ring fault system as well as several surrounding northwest and southeast striking dike-like breccias and numerous small peridotite, kimberlite, and lamprophyre intrusions, all nearly identical in composition to those of the Avon Diatremes (Snyder & Gerdeman, 1965; Heyl, 1972; Nicholaysen & Ferguson, 1990). Luczaj (1998) dates the structure as Pennsylvanian or younger.

Type 5 - �An entire rock unit is rapidly brought to the fluid-loaded condition; while this fluid-loaded unit moves centripetally during venting and cratering a front with increased pressure in the crystalline matrix advances outward.� The Crooked Creek, Decaturville, and Weaubleau structures [Fig 2.3] are all examples of this type (Nicholaysen & Ferguson, 1990). The Crooked Creek structure is located at the intersection of the Palmer fault system with the northwest-trending Cuba fault (McCracken, 1971; Heyl, 1972; Kisvarsanyi & Kisvarsanyi, 1976; Lidiak & Zeitz, 1976; Luczaj, 1998), and the Decaturville structure is located at the intersection of the lineament with the north-northwest trending Proctor anticline and the northwest-striking Red Arrow fault (Snyder & Gerdeman, 1965; Heyl, 1972; Lidiak & Zeitz, 1976; Luczaj, 1998). The Weaubleau disturbance consists of an intensely faulted and brecciated zone covering an area of about 30 square miles at the intersection of the 38th parallel lineament with the northwest-striking Humansville anticline (Snyder & Gerdeman, 1965; Lidiak & Zeitz, 1976; Luczaj, 1998). Most of the faults associated with the Weaubleau feature consist of low-angle thrust faults which lie to the east and northeast of the brecciated area. Compression forces for the thrust faulting were from the southwest (Snyder & Gerdeman, 1965). Breccias consist of coarse blocks of Ordovician and Kinderhookian age overlain by a very well-sorted water-laid conglomerate (Snyder & Gerdeman, 1965). Undeformed Pennsylvanian rocks overlie the conglomerate, effectively dating the events as post-Kinderhookian, pre-Pennsylvanian (Snyder & Gerdeman, 1965; Luczaj, 1998). Although no ring structure or explosive activity is apparent at this site, both Snyder & Gerdeman (1965) and Nicholaysen & Ferguson (1990) believe it to be different from the Decaturville and Crooked Creek structures only in that it was protected from erosion by a relatively rapid burial, calling attention to its similarity to the Jurassic Ries Basin structure in Germany.

Both the Crooked Creek and Decaturville structures are the only 38th parallel lineament cryptoexplosive features which exhibit the shock metamorphic effects (shatter cones) that some writers maintain can only be caused by meteorite impact (Grieve, 1987; Rampino, 1997). Neither of the two features is associated with an igneous intrusion, however, an irregular swarm of diatremes intruding Cambrian sediments occurs just south of the Decaturville structure. The dike breccia is similar to the sedimentary breccia type at Avon (Snyder & Gerdeman, 1965). Exposed at the surface in the center of the Decaturville site is an allocthonous Precambrian granite pegmatite block surrounded by a zone of sericite. One and a half miles east of the structure coarse flakes of mica lie on the surface, and Snyder & Gerdeman (1965), suggest this may indicate the location of another center of activity. Luczaj (1998), dates the Decaturville structure as post-Pennsylvanian and the Crooked Creek structure as early Ordovician to pre-Pennsylvanian. Thus, if they formed from a meteorite impact, it was not during the same event.

The Hazelgreen and Furnace Creek structures [Fig 2.3] are not listed among Nicholaysen & Ferguson�s (1990) rock-failure mechanisms, although they do include Furnace Creek among the 38th parallel lineament sites of �violent escape of gas and magma�. �The Furnace Creek volcanics� crop out at the intersection of the Big River and Palmer fault systems (Snyder & Gerdeman, 1965; Heyl, 1972; Luczaj, 1998). A short distance to the southwest is the similar feature known as the Dent Branch structure (McCracken, 1971; Heyl, 1972), which some include as a ninth 38th parallel lineament cryptoexplosion feature (McCracken, 1971; Heyl, 1972; Kisvarsanyi & Kisvarsanyi, 1976).

Snyder & Gerdeman (1965) describe the Furnace Creek crater as a funnel-shaped structure filled with layered ejecta of basic igneous material mixed with clasts of wall rock. The lower portion of the ejecta is a tuffite consisting of highly altered dark green to black lapilli of about 1/4 to � cm in diameter and fragments of granite and rhyolite porphyry about 7 cm in diameter in a matrix of volcanic material, sand, and white carbonate. The central portion is similar in composition, but poorly bedded, and has been interpreted as water-laid (Snyder & Gerdeman, 1965). The upper layer is an evenly bedded, fine-grained mixture of volcanic ash and comminuted country rock, which shows graded bedding. This layer is thickest in the central portion of the crater. Major constituent minerals of the lapilli are chlorite and carbonate alteration products, although some incompletely altered fragments reveal feldspar, quartz, and a brown mica. There are also abundant small crystals of magnetite. Rare minor constituents are barite and iron sulfides. Iron sulfides also occur in the formations adjacent to the Furnace Creek structure. Near the crater silicification of sediments is extensive, with the Lamotte sandstone altered to a quartzite in places (Snyder & Gerdeman, 1965). The age of Furnace Creek has been constrained to Upper Cambrian since the rocks in the uppermost disturbed layer and the lowermost undisturbed layer are Bonneterre (Snyder & Gerdeman, 1965; Luczaj, 1998). Drill core data show the structure extending into the Precambrian basement (Snyder & Gerdeman, 1965).

The Hazelgreen feature is similar to the Furnace Creek structure in content and vertical distribution of material, however the morphology of a possible crater is unknown since the evidence is based on a section in a single drill core. Snyder & Gerdeman (1965) estimated the source distance at 2 to 3 miles (3.2 to 4.8 km) from the core location. Northwest-trending faults intersect the 38th parallel lineament nearby (Luczaj, 1998). Basement rock from the core consisted of gneissic granite containing numerous veinlets of serpentine, which is considered unusual for the area (Snyder & Gerdeman, 1965). The age of this structure is also Upper Cambrian, although earlier than that of Furnace Creek, since here the ejecta is found in the middle of the Lamotte sequence (Snyder & Gerdeman, 1965; Luczaj, 1998).

2.5 SUMMARY
All five geologic causes of circular features given by Everett et al. (1986), structural dome, erosion/collapse basin, intrusive pluton, fracture pattern, and impact (cryptoexplosive) structure, may be present in the study area. Each may be related back to the Precambrian rifting event known as the Reelfoot Rift, which has effectively defined the structural fabric of southeastern Missouri. This chapter has covered the modes of occurrence, morphology, and where appropriate, the petrology and relevant geophysical characteristics of these features so that they may be readily compared to the circular lineaments seen in the area of interest.