Northeast Limb Research Articles

Observation of Reconnection Inflow of a Solar Flare

We find an important piece of evidence for magnetic reconnection inflow in a flare on March 18, 1999. The flare occurred on the north-east limb, displaying a nice cusp-shaped soft X-ray loop and a plasmoid ejection typical for the long-duration-events. As the plasmoid is ejected, magnetic reconnection occurs at the disconnecting point. A clear ingoing pattern toward the magnetic X-point is seen. The velocity of this apparent motion is about 5 km sec−1, which is an upper limit on reconnection inflow speed. Based on this observation, we derive the reconnection rate as MA = 0.001 − 0.03, where MA is a Alfvén Mach number of the inflow.

Clear Evidence of Reconnection Inflow of a Solar Flare

We found an important piece of evidence for magnetic reconnection inflow in a flare on 1999 March 18. The flare occurred on the northeast limb, displaying a nice cusp-shaped soft X-ray loop and a plasmoid ejection typical for the long-duration events. The EUV observation of the same flare shows us a bubble-like void ejection. The core of this EUV void corresponds to the soft X-ray plasmoid. Moreover, as this void is ejected, magnetic reconnection occurs at the disconnecting point. A clear ongoing pattern toward the magnetic X-point is seen. The velocity of this apparent motion is about 5 km s-1, which is an upper limit on reconnection inflow speed. Based on this observation, we derive the reconnection rate as MA = 0.001-0.03, where MA is an Alfven Mach number of the inflow.

Comparison of the 1998 April 29 M6.8 and 1998 November 5 M8.4 Flares

We combined, and analyzed in detail, the Hα and magnetograph data from Big Bear Solar Observatory (BBSO), full-disk magnetograms from the Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO), coronagraph data from the Large Angle Spectrometric Coronagraph (LASCO) of SOHO, Fe XII 195 A data from the Extreme ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray telescope (SXT) data of the M6.8 flare of 1998 April 29 in National Oceanic and Atmospheric Administration (NOAA) region 8375 and the M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares have remarkable similarities: 1. Partial halo coronal mass ejections (CMEs) were observed for both events. For the 1998 April 29 event, even though the flare occurred in the southeast of the disk center, the ejected material moved predominantly across the equator, and the central part of the CME occurred in the northeast limb. The direction in which the cusp points in the postflare SXT images determines the dominant direction of the CMEs. 2. Coronal dimming was clearly observed in EIT Fe XII 195 A for both but was not observed in Yohkoh SXT for either event. Dimming started 2 hr before the onset of the flares, indicating large-scale coronal restructuring before both flares. 3. No global or local photospheric magnetic field change was detected from either event; in particular, no magnetic field change was found in the dimming areas. 4. Both events lasted several hours and, thus, could be classified as long duration events (LDEs). However, they are different in the following important aspects. For the 1998 April 29 event, the flare and the CME are associated with an erupting filament in which the two initial ribbons were well connected and then gradually separated. SXT preflare images show the classical S-shape sheared configuration (sigmoid structure). For the 1998 November 5 event, two initial ribbons were well separated, and the SXT preflare image shows the interaction of at least two loops. In addition, no filament eruption was observed. We conclude that even though these two events resulted in similar coronal consequences, they are due to two distinct physical processes: eruption of sheared loops and interaction of two loops.

Search for velocity variations in Fe XIV 5304 Å coronagraph observations near activity minimum

The LASCO C1 coronagraph on the SOHO satellite observes the solar corona from 1.1 to 3.0 R ⊙, and contains a Fabry-Perot interferometer which can image the corona in the 1.8 million K Fe XIV green line. We designed an observing program with reduced spatial coverage and reduced profile coverage at only three wavelengths to study coronal heating in off-limb structures at high temporal resolution. We illustrate the observations from 31 March 1997 of a bright loop system above an active region off the northeast limb.

The Radial Structure of the Cygnus Loop Supernova Remnant: Possible Evidence of a Cavity Explosion

We observed the northeast limb toward the center region of the Cygnus Loop with the ASCA Observatory. In our previous paper (Miyata et al.), we analyzed the data obtained with the X-ray CCD cameras (SISs). We found a radial variation of electron temperature (kTe) and ionization timescale [log(τ)], whereas no variation could be found for the abundances of heavy elements. In this paper, we reanalyzed the same data set and new observations with the latest calibration files. Then we constructed the precise spatial variations of kTe, log(τ), and abundances of O, Ne, Mg, Si, and Fe over the field of view (FOV). We found a spatial variation not only in kTe and in log(τ) but also in most of heavy elements. As described in Miyata et al., values of kTe increase and those of log(τ) decrease toward the inner region. We found that the abundance of heavy elements increases toward the inner region. The radial profiles of O, Ne, and Fe show clear jump structures at a radius of 0.9Rs, where Rs is the shock radius. Outside of 0.9Rs, abundances of all elements are constant. On the contrary, inside of 0.9Rs, abundances of these elements are 20%-30% larger than those obtained outside of 0.9Rs. The radial profile of kTe also shows the jump structure at 0.9Rs. This means that the hot and metal-rich plasma fills the volume inside of 0.9Rs. We concluded that this jump structure was the possible evidence for the pre-existing cavity produced by the precursor. If the ejecta fills inside of 0.9Rs, the total mass of the ejecta was roughly 4.1 M☉. We then estimated the main-sequence mass to be roughly 15 M☉, which supports the massive star in origin of the Cygnus Loop supernova remnant and the existence of a pre-existing cavity.

Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau

Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau

Metal-Rich Plasma at the Center Portion of the Cygnus Loop

We observed the center portion of the Cygnus Loop supernova remnant with the ASCA observatory. The X-ray spectrum of the center portion was significantly different from that obtained at the North-East (NE) limb. The emission lines from Si and S were quite strong, while those of O and the continuum emission were similar to those obtained at the NE limb. Based on a spectral analysis, the Si and S emission lines originated from a high-kTe and low-ionization plasma, whereas O and most of the continuum emission arose from a low-kTe and high-ionization plasma. We suppose that Si and S emitting gases are present at the interior of the Loop, while the O lines and continuum emission mainly arise from the shell region. Therefore, we subtracted the spectrum of the NE limb from that of the center. The obtained abundances of Si, S, and Fe were 4 ± 1, 6 ± 2, and $1.3^{+0.6}_{-0.3}$times higher than those of the cosmic abundances, respectively, and ∼ 40 times richer than those obtained at the NE limb. These facts strongly support the idea that some of the crude ejecta must be left at the center portion of the Cygnus Loop. The low abundance of Fe relative to Si and S suggests a type II SN with a massive progenitor star as the origin of the Cygnus Loop.

X‐Ray and Optical Imagery of the SN 1006 Supernova Remnant

We present a high-resolution X-ray image of SN 1006: a mosaic of four ROSAT HRI images that together cover the entire supernova remnant. A well-defined shock front is observed surrounding the remnant shell. Comparison of the X-ray and radio images shows a close correspondence in general morphology, especially for the brightest features. Along the northeast limb of the shell, where the remnant is brightest in both bands, the X-ray/radio correspondence is striking in its detail. The data are consistent with a model in which the X-ray emission in this region results primarily from synchrotron radiation by the same population of shock-accelerated electrons that produce the radio emission. New optical images show Hα emission over a much greater portion of the remnant than has previously been reported. Newly discovered faint filaments in the south, like the much brighter ones along the northwest limb, delineate the primary shock as it propagates into the interstellar medium. In the northwest, a narrow X-ray peak traces just inside the crisp optical filaments and provides a laboratory for observing the effects of a fast, nonradiative shock propagating nearly perpendicular to our line of sight. The calculated profile of the thermal X-ray emission from a rapidly ionizing postshock plasma closely matches the observed profile, supporting a model in which nonequilibrium thermal processes dominate the X-ray emission in the northwest. The measured displacement of 8'' between the optical filaments and the X-ray peak, combined with previously measured proper motions for the filaments, indicates a timescale of only ~120 yr for X-ray heating and ionization behind the shock. In addition to the narrow optical filaments around the rim of SN 1006, we find faint, diffuse Hα emission within the remnant shell to the southwest. This emission probably represents nonradiative sheets viewed obliquely.

ITAL]Yokhoh[/ITAL] Soft X-Ray Telescope Images of the Diffuse Solar Corona

During the interval 1992 May 3-15, an extended region (out to 1.5 solar radii) of diffuse, stable corona crossed the northeast limb of the Sun. This region underlaid a coronal streamer as revealed by the Mauna Loa Coronagraph of the High Altitude Observatory. During this passage, the soft X-ray telescope on Yohkoh obtained a number of high-quality pairs of images, closely spaced in time, through the two thinnest analysis filters. Analysis of these data indicates that (1) the temperature increases steadily with height and (2) the variation of temperature with radius is consistent with a conserved inward heat flux. These results imply that the magnetic field configuration was substantially open out to 1.5 solar radii and that there was no significant coronal heating below that height in that region. It appears that this region was being heated by nonthermal energy deposited beyond 1.5 solar radii.

The Balmer-dominated northeast limb of the Cygnus loop supernova remnant

We present a comprehensive investigation of the Balmar-dominated northeast limb of the Cygnus Loop supernova remnant. Data presented include H alpha (O III), and X-ray images, UV and visible spectrophotometry, and high-resolution spectroscophy. The two relatively bright Balmer-dominated filaments visible on the POSS prints are seen to be part of a very smooth and regular complex of filaments. These filaments mark the current location of the blast wave and are seen to bound the sharply limb-brightened X-ray emission, including the previously reported X-ray, 'halo.' The (O III)/h beta ratio throughout the region is approximately 0.1, except for regions in which the shock is undergoing a transition from nonradiative to incomplete radiative to incomplete radiative conditions. At these locations (O III) emission from the cooling region is quite strong, while collisionally excited Balmer-line emission can be weak because of photoionization of the preshock medium by UV from the nascent cooling region. As a result (O III)/H beta is greater than 100 in some locations. The nonradiative/radiative transition is best studied along the length of the northwestern of the two brightest filaments, where the shock velocity and swept-up column go from approximately 180 km/s and 10(exp 17)/sq cm at one end to approximately 140 km/s and 8 x 10(exp 17)/cm at the other. There are also a number of locations of such incomplete radiative emission where the shock has recently encountered denser regions with characteristic sizes of approximately 10(exp 18) cm. There is a considerable amount of evidence that the shock has decelerated from approximately 400 km/s to less than 200 km/s in the last 1000 yr. We interpret this as the result of the blast wave hitting the wall of a cavity which surround the supernova precursor and succeed in matching a wide range of data with a reflected shock model in which the density ofthe cavity wall is approximately 1.2/cu cm and the density in the interior of the cavity is about 0.09/cu cm. This analysis supports previous suggestions that the Cygnus Loop was a cavity explosion. We opredict that the X-ray shell will continue to brighten for the next 1000 years or so, during which time the shock front will become radiative throughout the northeast region. The similarity of the time scales for post-shock cooling, shock deceleration, and the 1ressure evolution of the region violates assumptions of steady flow, and greatly complicated interpretation of shock specptra in the region. This may be a common and important effect in many supernova remnants. High-resolutoin spectroscopy of the H alpha line profile shows that the broad component is surprisingly narrow (approximately 130 km/s). Consistency between this line width and the shock velocity inferred from spectrophotometry requires either rapid postshock equilibration of T(sub i) and T(sub e), or that the shock front has declerated from 180 km/s to 150 km/s in the last approximately 200 yr. While we cannot unambiguously distinquish between these models, the required deceleration is consistent with extrapolation of the shock decleration rate inferred over the last 1000 years. The width of the narrow component of the h alpha line profile is approximately 3 km/s, which indicates that the preshock medium has been heated to approximately 25,000 K by a shock precursor. The most promising types of precursor are heating either by fast neutrals which overtake the shock or by cosmic rays. In the latter case the observed properties of the precursor constrain the cosmic-ray diffusion coefficient to be between about 10(exp 23) sq cm/s. The characteristics of the precursor seem to be about right to explain the presence of weak (S II) and (N II) emission previously reported by Fesen & Itoh (1985).

Bent chert tubes in the Phosphoria Formation of Wyoming: Pressure solution strain markers

Tubular chert in the Phosphoria Formation in the Gros Ventre Mountains of Wyoming appears to have been deformed by pressure solution during the flexural-slip folding of the Gros Ventre uplift. We suggest that these chert tubes can be used as strain markers. The tubes are 0.4 to 1.2 m in length and 5 to 12 cm in diameter, and are bent where they occur in two concentrated zones which parallel bedding. The orientation of the bent chert tubes is regionally consistent. The tubes plunge to the northeast in the direction of bedding-plane dip at an angle greater than dip. This is the orientation that would be produced by shear parallel to bedding on the northeast limb of the northwest-trending Gros Ventre anticline. Pressure solution is indicated by the truncation of concentric bands by the walls of the tubes observed in cross sections. The tubes are not bend by soft-sediment deformation or the action of paleocurrents. A value of maximum shear strain of {gamma} {approximately} 1.5 and a maximum volume loss of 46% is obtained by treating the zones of bent chert tubes as shear zones.

The Muller anticline, Papua New Guinea; basement-cored, inverted extensional fault structures with opposite vergence

Throughout the NW-SE trending Papuan Foldbelt, two dominant structural styles are present. In the northeast the 1 km thick, thrust-imbricate slices of Miocene limestone attest to the classical thin-skinned nature of the deformation. In the southwest, adjacent to the undeformed foreland, fold structures are 1 or 2 orders of magnitude greater in size, are strongly asymmetrical and commonly have relatively thicker stratigraphie sections on the crests. Such structures are the result of Mio-Pliocene compression that caused thrusting along faults that are hypothesized to have been extensional in the Mesozoic and Palaeogene. Geological traverses from the undeformed foreland heading northeast across the 150 km by 50 km Muller Range in western Papua New Guinea, provided detailed surface structural data. When combined with analysis of existing geological maps, exploration wells, regional gravity surveys and extensive seismic in the foreland, moderately constrained balanced and restorable cross-sections through the Muller anticline could be constructed. Interpretations of gravity data are consistent with the Muller anticline, being cored by basement and seismic data extrapolated beneath the mountains, show the outcrop of basement in the centre of the Muller Range to be elevated 8 km above regional. The eastern Muller anticline (EMA) has a shallow northeast limb and steeper southwest limb, suggesting a NE-dipping fault below, such that basement was thrust to the southwest. The Cecilia, Wai Asi, Juha and Lavani anticlines to the southwest of the EMA are interpreted to be formed by shortening of the whole sedimentary section to balance the shortening in basement. There are no such structures to the southwest of the western Muller anticline (WMA) where the southwest limb is gently dipping and the northeast limb is steep. Hence, the WMA is interpreted as having formed above a SW dipping fault along which basement was thrust to the northeast. The marked difference in shortening of the southwest limbs requires a tear fault between the EMA and WMA. The tear fault is here interpreted to coincide with the northwestern end of the Cecilia and Lavani anticlines and the marked change in strike of the Muller anticline. By analogy with other very large, asymmetrical, basement-involved structures in the Papuan Foldbelt it is suggested that the faults underlying the Muller anticline were active as extensional faults in the Mesozoic, soling at a mid-crustal detachment. The extensional faults beneath EMA and WMA had opposite vergence and were separated by a transfer zone. Subsequent compressional reactivation of the extensional faults in the Pliocene gave rise to the present Muller anticline.

Growth Fault-Bend Folding at Southeast Lost Hills, San Joaquin Valley, California

Synsedimentary deformation has created complex structural and stratigraphic relationships along the west side of the San Joaquin Valley, California. This case study of Lost Hills anticline documents how a simple structural process--fault-bend folding--results in complex growth stratigraphy. Lost Hills anticline is a fault-bend fold caused by a step in the Lost Hills thrust. The thrust steps up to the northeast at 27° from a detachment at the base of the Tertiary to an upper detachment in the Reef Ridge formation. The lower and upper detachments are at 20,000 ft (6 km) and 13,000 ft (4 km). Displacement on the lower detachment is approximately 4 mi (6.5 km); folding consumes 1.2 mi (2 km) and the remaining 2.8 mi (4.5 km) continues northeast beneath the San Joaquin Va ley. Forward- and back-limb dips of 42° and 27°, respectively, are consistent with fault-bend fold theory. Pliocene-Pleistocene sediments onlap a time-transgressive unconformity on the northeast (front) limb, but have uniform dip and thickness on the southwest (back) limb. This characteristic growth fault-bend fold geometry indicates basinal sediments rolled through an axial surface onto the back limb, whereas contemporaneous sediments deposited above the front limb are undeformed. Basinal sediments onlapping the fold crest indicates fault slip is greater than ramp length. Onlapping growth sediments indicate folding began during deposition of the Etchegoin Formation (~8 Ma) and continued to the late Pleistocene or Holocene, corresponding to an average displacement rate on the Lost Hills thrust of 2,700 ft/m.y. (820 m/m.y.). Continuous burial of the fold during growth caused each material ( .e., physical) point in the fold to be at its maximum depth of burial. Contours of equal density and seismic velocity are relatively flat, so that the anticline causes little or no gravity anomaly and less-than-expected pullup in seismic time sections. The geometry of growth sediments imaged on seismic lines indicates Lost Hills and adjacent anticlines grew by uniform displacement to the northeast with no significant rotation. Oblique convergence between the Pacific and North American plates is manifested as synchronous, but spatially separate, transcurrent and compressive displacements. Thus, Lost Hills anticline and perhaps other folds in the area are more appropriately described as compressive folds in a transpressive region than as transpressive folds. Duplicated lower Tertiary strata beneath the Lost Hills-Kettleman Hills trend are potential targets for petroleum exploration.

A deep H-alpha image of faint Balmer-line filaments in the northeast Cygnus Loop supernova remnant

A deep H-alpha image of a field outside the optically bright northeast limb of the Cygnus Loop supernova remnant is presented. Two filaments which are just visible on the red POSS print form part of two long and unbroken lines of filaments which generally bound a region of diffuse emission and fainter filaments. However, the diffuse emission extends out to a newly detected line of extremely faint filaments 3 arcmin to the northeast. The brightest filaments are resolved into multiple components with widths less than 2 arcsec. Comparison with the POSS print leads to a rough estimate of the proper motion of 6 arcsec per century, although temporal variations in the relative brightnesses of the components of a filament could confuse attempts to more accurately measure the true proper motion. The morphology of the emission strongly suggests that it arises in a single large sheet. No direct evidence is found favoring the existence of the dense cloud cores which have been hypothesized as a way of evaporatively enhancing the X-ray emissivity and producing forbidden line emission in the filaments.

Eruptive prominences of 1980 april 27 observed during STIP interval — X

Observations and analyses of two similar eruptive prominences on the north-east limb observed on 1980 April 27 at 0231 and 0517 UT, which are associated with the Boulder active region No. 2416 are presented. Both the eruptive prominences gave rise to white-light coronal transients as observed by C/P experiment of High Altitude Observatory on the Solar Maximum Mission. Type II and moving type IV radio bursts are reported in association with the first Hα eruptive prominence at 0231 UT.

Rapid formation and degradation of barrier spits in areas with low rates of littoral drift

A small barrier beach exposed to low-energy waves and a small tidal range (0.7 m) along Nantucket Sound, Mass., has experienced a remarkable growth phase followed by rapid attrition during the past century. In a region of low longshore-transport rates, the barrier spit elongated approximately 1.5 km from 1844 to 1954, developing beyond the baymouth, parallel to the adjacent Nantucket Sound coast. Degradation of the barrier spit was initiated by a succession of hurricanes in 1954 (Carol, Edna and Hazel). A breach opened and stabilized near the bay end of the one kilometer long inlet channel, providing direct access for exchange of baywater with Nantucket Sound, and separating the barrier beach into two nearly equal limbs. The disconnected northeast limb migrated shorewards, beginning near the 1954 inlet and progressing northeastward, filling the relict inlet channel behind it. At present, about ten percent of the northeast limb is subaerial: the rest of the limb has completely filled the former channel and disappeared. The southwest limb of the barrier beach has migrated shoreward, but otherwise has not changed significantly since the breach. A new mechanism is proposed for spit elongation when the inlet thalweg parallels the beach axis, in which material scoured from the lengthening inlet is the dominant source for spit accretion (perhaps initially deposited as a linear channel-margin bar which later becomes subaerial). The lengthening spit causes the parallel inlet to elongate, which in turn further lengthens the spit, in a self-generating fashion. This mechanism provides both a source of sediment for elongating the barrier spit, and a sink for material scoured from the lengthening inlet. The proposed mechanism for spit growth may be applicable to other locations with low wave energy, small tidal prisms and low longshore sand transport rates, suggesting that estimates of directions and rates of longshore sand transport based on spit geomorphology and development be scrutinized on a case-by-case basis.

Diagenesis and Migration of Hydrocarbons in Monterey Formation, Pismo Syncline, California: ABSTRACT

End_Page 999------------------------------Kerogen in the Miocene Monterey Formation in the Pismo syncline consists of amorphous material of algal origin. Most of the rocks presently contain 1 to 5 wt. % total organic carbon. Thermal alteration index determinations and pyrolysis (thermal extract) data suggest that the organic material at the center of the fold is mature, whereas the organic material on the limbs of the fold is immature. The Monterey Formation in this region entered the liquid hydrocarbon window at a present day depth of approximately 1,700 m. The liquid window appears to be coincident with the opal-CT to quartz reaction (80 ± 10°C). Hydrocarbons were expelled from the Monterey rocks deep in the center of the fold and began migrating as a result of microfracturing. The hydrocarbons migrated up into the southwest limb of the fold through macrofractures. This limb of the fold is characterized by brittle dolomitic and opal-CT rich rocks that were intensely fractured prior to hydrocarbon migration. The potential reservoirs on this limb of the fold are in fractured Monterey. In contrast, on the northeast limb of the fold, Monterey rocks consist of silty and sandy siliceous rocks that tend to be more resistant to fracturing; hydrocarbons migrate up into this limb of the fold through relatively low angle faults and spread into structural traps along the fault in the adjacent Pliocene Pismo Formation through conjugate she rs. The potential reservoirs on this limb are in structural traps in the Pismo Formation. A third potential hydrocarbon target is the basal sand in the Pismo Formation; almost everywhere in the fold this unit is highly bituminous. End_of_Article - Last_Page 1000------------

Late Cretaceous Eugeosynclinal Sedimentation, Gravity Tectonics, and Ophiolite Emplacement in Oman Mountains, Southeast Arabia

The Oman Mountains border the Gulf of Oman from the Arabian Sea to the mouth of the Persian Gulf. The Jebel Akhdar anticline forms the central core of the mountains and exposes a section of massive limestones which range in age from Permian (transgressive on a Precambrian nucleus) to late Tertiary; the latter fringe the uplift. Intercalated in this sequence, between well-dated latest Cretaceous sediments, is a thick contorted assemblage of rocks composed, from base to top, of turbidite limestone and radiolarian chert (Hawasina Group), massive exotic blocks of Permo-Triassic limestone, and a thick sheet of serpentinic igneous rock (Semail igneous series). This association of rocks, which is comparable to that known from many complex Laramide orogenic belts, can be studied with advantage in Oman where the sequence has been little disturbed since its emplacement during the latest Cre aceous (Campanian-Maestrichtian). The absence of recognizable latest Cretaceous fossils in the Hawasina, the common occurrence of well-preserved Permian to middle Cretaceous species, and the contorted features of the strata have led some geologists to postulate that these sediments were deposited outside of their present location during a prolonged pre-Late Cretaceous interval and then tectonically emplaced during the latest Cretaceous. However, it is concluded from field data presented herein that the Hawasina was deposited in its present location during latest Cretaceous time. Regional stratigraphic correlations show that northeast Oman was situated far out on the Arabian platform where quiet carbonate sedimentation persisted from Permian to middle Cretaceous time during prolonged regional tectonic quiescence. Sedimentary and tectonic quiescence ended during latest Cretaceous time when the thick Aruma pelagic shale, originating from a northerly source, was deposited across northeastern Oman concomitant with major normal faulting. Hawasina facies, north of the Aruma province, give evidence in the form of basal boulder beds, turbidites, and massive terminal gravity slides of sharp contemporaneous tectonism. Lower Hawasina deep-water turbidite sediments have yielded an inverted faunal sequence of shallow-water fossils, whereas the overlying cherty facies contains only pelagic Radiolaria. Thus the whole sequence cannot be older than the reworked youngest middle Cretaceous fossils present in the basal beds. Distribution, grading, and constitution of the carbonate clastic material in the Hawasina, and the alignment of exotic limestone blocks indicate that the sediment source area was a northwest-southeast-trending uplift of Permian to middle Cretaceous carbonate rocks north of the Oman Mountains. The absence of terrigenous clastic material and terminal submarine volcanism suggest that the source area was a submerged seamount. Erosion from this high is believed to have been by means of turbidity currents activated by repetitive and at times strong block-fault movement. Hawasina sediments compare with present deep-water sediments in the Puerto Rico Trench where faunally barren siliceous oozes of abyssal facies are interbedded with calcareous turbidites rich in reworked older and contemporaneous shallow-water fauna. The absence of contemporaneous (latest Cretaceous) shelf fauna in the Hawasina is attributed to the seamount source area being deeper than neritic, whereas the absence of contemporaneous pelagic calcareous fauna was the result of dissolution during descent to abyssal depths. The latest Cretaceous Aruma Shale is compared with Holocene pelagic globigerinal sediment occurring above 4,500-m depth. The deep trough which received Hawasina sediments appears to have been bounded by a steep block-faulted northeastern limb whereas the southwestern limb became shallower gradually through the neritic Aruma belt to the Arabian carbonate platform. At the close of the time of Hawasina deposition, volcanism and catastrophic tension-relief faulting dislodged the remnants of Permo-Triassic limestone sedimentary rocks from the seamount, and these descended into the trough as huge gravity slides (e.g., Jebel Kawr, 250 sq mi), leaving the seamount as a denuded basement uplift. Regional tension relief was accomplished finally by crustal separation and flood eruption of Semail ultrabasic magma which blanketed the abyssal landscape and cooled slowly, under great hydrostatic pressure, along the axi of the trough. According to this interpretation, the sedimentation of Hawasina beds is considered to have been contemporaneous with the deposition of the latest Cretaceous Aruma Shale, and major block-fault tectonism in the orogenic belt is correlated with known latest Cretaceous tension faulting on the southwest basin slope. Compressional features in the Hawasina are attributed to contemporaneous gravity slumping of the beds on the steep northeast limb of the trough, combined with the subsequent End_Page 626------------------------------ bulldozing effect which massive slides of exotic blocks and the thick Semail eruptives had on the incompetent sediments. Oil prospects in the Hawasina belt are difficult to explore and assess for several reasons, not the least of which is the masking effect which contorted chert has on seismic and gravity exploration for more simple structures in the competent sub-Hawasina limestone succession, which is hydrocarbon-bearing below Aruma Shale at Fahud, Natih, and Yibal oil fields.

Structural Geology of the Sierra de la Gavia, Coahuila, Mexico

Sierra de la Gavia, in east-central Coahuila, Mexico, is one of a number of northwesterly aligned, mountain-forming anticlinal structures which constitute part of the Coahuila Marginal Folded province of northeastern Mexico. This province, an uplifted segment of the southwestern limb of the Rio Grande Embayment, lies north and east of the folds of the Sierra Madre Oriental. It includes several geologic elements: the Parras and Sabinas Basins, the paleogeographic Coahuila and Tamaulipas Peninsulas, and fold belts east of the Coahuila Platform and north of the Tamaulipas Arch. The fold, roughly 60 miles in length, rises abruptly out of bolson plains which border its northeast and southwest flanks. Lower-Upper Cretaceous-Quaternary rocks exposed within the Sierra include several thousand feet of detrital and chemico-detrital sedimentary rocks. Lower Cretaceous formations (Barril Viejo, Patula, La Mula, Taraises-Cupido, La Pena, Aurora-Cuesta del Cura), which are more than 7000 feet thick, form the major part of the mountain mass. Upper Cretaceous rocks (Indidura) and alluvial debris, together more than 500 feet thick, constitute minor mappable units. Younger Cretaceous units (Parras, Difunta), largely obscured by alluvium, form flanking outcrops, and generally occupy synclinal areas between Gavia and adjacent structural highs. In gross aspect, the axis of the structure is doubly inclined, striking generally northwest-southeast and plunging in both these directions. Areal mapping, however, discloses the presence of five separate areas of closure. Inclinations of the axial plane vary markedly along strike. Large-scale drag folding modifies both northeast and southwest limbs of the structure in certain areas. Minor en echelon normal (?) faults, thought to be shear fractures, occur at the structural culmination of the fold. It is suggested that Sierra de la Gavia and similarly aligned structures in the Coahuila Marginal Folded province owe their origin initially to movement of recurrent nature occurring along a suggested wrench fault of regional magnitude—a fault expressed at the surface in the abrupt change in strike and character of the frontal folds of the Sierra Madre Oriental between Monterrey, Nuevo Leon, and Torreon, Coahuila. Local northeast overturning on the southwest flank of Gavia, near its northwest end, is thought to be a surface reflection of the buttressing mass of the northeast edge of the Coahuila Peninsula.

Intertongued Upper Cretaceous Volcanic and Nonvolcanic Rocks, Central-Western Montana

A thick sequence of intertongued volcanic and nonvolcanic rocks, exposed on the northeast limb of a syncline immediately west of Garrison, central-western Montana, is a western facies of the Elkhorn Mountains Volcanics of Late Cretaceous age. This sequence, here named the Golden Spike Formation, ranges from 4000 to 8000 feet in thickness. In the northwestern sixth of the 9-mile-long outcrop belt, nonvolcanic rocks predominate. Nonvolcanic detritus which forms this relatively thin fluviatile sequence was derived from tectonic uplifts a short distance to the west or northwest of Garrison. In the southeastern area, a relatively thick sequence comprises mostly lavas and volcaniclastic rocks. Most volcaniclastic rocks were probably deposited by epigene processes ( e.g. , weathering, erosion, and redeposition by mudflows, landslides, or running water) and were derived from centers of Elkhorn volcanism to the east. This depositional pattern resulted in intimate interfingering of volcanic and nonvolcanic beds; the authors observed only limited mixing of these two components in individual beds. Throughout the formation many volcanic tongues wedge out within a short distance to the northwest. The virtual absence of volcanic debris in the fluviatile rocks deposited to the northwest indicates either that explosive volcanic activity was minimal in the Elkhorn volcanic field during deposition or that prevailing westerly winds kept pyroclastic material out of the Golden Spike area.