摘要:

Abstract—Although mapped initially as a piercement dome, subsequent discovery of shock metamorphism in clasts of an impact breccia, shatter cones in outcrops of uplifted target rocks and morphological and geophysical characteristics consistent with a complex crater, confirmed a meteorite impact origin for the Haughton structure, Devon Island.Results of three field investigations carried out prior to 1984 defined a complex crater, 20 km in diameter, formed in a lower Paleozoic sedimentary sequence overlying gneisses of the Precambrian basement. The distribution of allochthonous breccia overlying the disturbed target rocks and of the sediments deposited in the crater‐filling lake were mapped. A Miocene or possibly Holocene age for the crater was based on paleo‐flora and fauna assemblages from the lake sediments. Gravity and magnetic surveys revealed anomalies coincident with the crater, but not interpretable from surface lithologies.Some of the early investigations were of a reconnaissance nature and results and interpretation can only be considered preliminary. Other studies that were carried out in some detail, petrographic investigations in particular, require complementary work for a fuller understanding of their significance. As a result, in 1984 the HISS (Haughton Impact Structure Studies) group carried out a program of detailed geological mapping and sampling, and seismic, gravity, and magnetic surveys in an attempt to improve the definition of the surface and subsurface nature of Haughton, and to formulate a more complete understanding of its formation and subsequent history. Results of these various studies are presented in the eight succeeding papers of this v

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01280.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—The central allochthonous polymict breccia of the Haughton impact structure is up to about 90 m thick and as much as 7.3 km in radial extent. It has been analyzed with respect to modal composition, grain‐size characteristics, and degree of shock metamorphism for the grain‐size ranges 10–∼ 50, 1–10, 0.03–1, and<0.03 mm. The mineralogy of the breccia matrix is dominated by dolomite and calcite, with minor amounts of quartz, other silicate minerals, and rare melt particles. The following lithic clasts have been identified in the 1–10 mm size fraction (averages of vol.% given in parentheses): dolomitic rocks (51), limestones (29), crystalline rocks (10), sandstones and siltstones (3.7), chert (0.7), melt particles (1.9). The mineral clasts (1–0.03 mm) comprise (with decreasing frequency) dolomite, quartz, calcite, feldspar, biotite, amphibole, garnet, opaques, rounded quartz derived from sandstones and accessory minerals. Lithic and mineral clasts display various degrees of shock. Fragments of crystalline rocks are shocked in the 0–60 GPa range; whole rock melts from the crystalline basement are lacking and unshocked rocks are very rare. In contrast, shock‐melted sandstones, shales, and chert were found in most samples. Large clasts of these melt rocks are highly concentrated near the center of the crater. Otherwise, no distinct change of the modal composition with radial range has been observed except that the frequency of limestone clasts increases slightly with radial range. The breccia near the center is more fine‐grained than that beyond about 1 km radius and the sorting parameter increases somewhat with radial range. Except for the high concentration of shock‐melted sedimentary rocks and highly shocked crystalline rocks near the center of the crater, the distribution of shock stages within the lithic clast population is quite uniform throughout the breccia formation. We conclude that the breccia constituents are derived from the lower part of the target stratigraphy (deeper than about 800 m) and that the total depth of excavation at Haughton is in the order of 2000 m. The mixing of sedimentary rocks of the Eleanor River Formation, Lower Ordovician, and Cambrian (∼850 m thickness) with crystalline basement rocks is quite thorough and homogeneous throughout the breccia lens, at least for the analyzed part. This may require an air‐borne mode of emplacement for the upper section of the breccia in analogy to the fall‐back suevite in the Ries crater. A calculation of the excavation (Z‐model) and of the shock pressure attenuation based on reasonable estimates of the energy and crater geometry of the Haughton impact confirms the observed maximum depth of excavation of about 2 km. Shock‐melted crystalline basement rocks, if present at all, must be confined to the very center of the st

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01281.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—About 100 cobble‐sized samples collected from the surface of the central polymict breccia formation of Haughton impact crater, Canada, have been studied microscopically and chemically. Breccia clasts derived from the 1700 m deep Precambian basement consist of 13 rock types which can be grouped into sillimanite‐ and garnet‐bearing gneiss; alkali feldspar‐rich aplitic or biotite‐hornblende‐bearing gneiss; biotite and hornblende gneiss; apatite‐rich biotite and biotite‐hornblende gneiss; calcitediopside gneiss; amphibolite; tonalitic orthogneiss; and basalts. The range of chemical compositions of these rocks is wide:e.g., SiO2ranges from 40–85 wt.%; Al2O3from 7–20 wt.%; CaO from 0.01–25 wt.%; or P2Osfrom<0.01–5 wt.%. Nearly all samples of crystalline rocks are shock metamorphosed up to about 60 GPa. Most conspicuous is the absence of whole‐rock melts and the very rare occurrence of unshocked rocks. The 45 samples examined can be classified into the following shock stages: stage 0 (<5 GPa): 4.5%, stage Ia (10–20 GPa): 9.0%, stage Ib (20–35 GPa): 33%, stage II (35–45 GPa): 29%, stage III (45–55 GPa): 18%, stage III–IV (55–60 GPa): 6.5%. Among Paleozoic sedimentary rock clasts higher degrees of shock than within crystalline rocks were observed such as highly vesiculated, whole‐rock melts of sandstones and shales. Within the northern and eastern sectors of the allochthonous breccia no distinct radial variation of the cobble‐sized lithic clasts regarding abundance, rock type, and degree of shock was observed, with the exception that clasts of shock‐melted sedimentary rocks and of highly shocked basement rocks (stage III–IV) are strongly concentrated near the center of the crater. Based on our field and laboratory investigations we conclude that vaporization and melting due to the Haughton impact affected the lower section of the sedimentary strata from about 900 to 1700 m depth (Eleanor River limestones and dolomites, Lower Ordovician and Cambrian limestones, dolomites, shales, and sandstones). The 60‐GPa shock pressure isobar reached only the uppermost basement rocks so that whole rock

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01282.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—Recent geological and structural investigations support the existing descriptions of the Haughton structure being a dome‐like structure. The central area represents the central uplift and is formed by blocks of rocks belonging to the oldest part of the Devon Island Paleozoic sediment cover. The orientation of the blocks partly reflects this dome‐like structure. The morphologically defined “inner ring” is a structurally distinct zone, predominated by gypsum beds of the Bay Fiord Formation. These beds dip 30 to 40° outward and are asymmetrically folded. The outer radius of this ring is 5 to 5.5 km. A 1 to 1.5 km broad zone follows outward of more chaotically orientated blocks of rocks, which may be considered a megablock‐zone. The next ring‐like zone is strongly faulted (“faulted annulus”) and obviously represents the downfaulted, once uplifted crater rim. Within this structural annulus, a marked asymmetry of the crater structure is expressed by the attitude of the faulted blocks. The structure of the northern and eastern segments of the faulted annulus differ substantially from those of the southern and western segments. This asymmetry may reflect heterogeneities in the pre‐crater geology or a low angle impact. No evidence was found in support of the theory that the Haughton structure represents a

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01283.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—After the impact that formed Haughton crater, 22.4 ± 1.4 Ma ago (early Miocene), the cavity filled with water and began to accumulate lacustrine sediments. These preserve detailed evidence of pre‐impact stratigraphy and post‐impact morphology and development of the crater, as well as of the climatic and biotic regime in which it lay. In this report we formally designate these sediments as the Haughton Formation, of which only a 48 m thick remnant covering approximately 7 km2still exists. Dolomite‐rich, poorly‐sorted silt, fine sand, and mud are the principal lithologies. The formation unconformably overlies a blanket of allochthonous impact breccia forming the floor of the original crater. Presence of a debris‐flow deposit in the base of the sequence indicates that lacustine deposition began very shortly after crater formation.The Haughton Formation contains a moderately diverse and highly endemic vertebrate fauna as well as palynomorphs and plant macrofossils that indicate a cool‐temperate climatic regime. A small percentage of reworked Late Cretaceous and early Tertiary palynomorphs point to the former existence of the Eureka Sound Formation in the drainage area of the crater. In addition, the distribution of the lake beds indicates the absence of an inner ring on the west side of the crater, and the 3° to 3.5° inward dip of Haughton strata implies that the central mass has subsided approximately 300 to 350 m since

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01284.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—40Ar‐39Ar dating of a strongly shocked, vesiculated gneiss clast in a breccia from the Haughton impact structure yields an isochron age of 23.4 ± 1.0 Ma. The sample contains excess

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01285.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—The results of a new gravity survey show that the Haughton impact structure is associated with a 24 km diameter negative Bouguer gravity anomaly with a maximum amplitude of −12 mgal. A local minimum with a half‐width of 2 km and an amplitude of −4 mgal is located at the center of the structure. A positive magnetic total field anomaly with a half‐width of 0.6 km and an amplitude of 700 nT coincides with the local central gravity anomaly. The overall negative gravity anomaly is explained by lowered rock densities due to impact‐related fracturing in the crater area. The central gravity and magnetic anomalies are believed to be due to highly shocked and heated sedimentary and crystalline basement rocks forming the unexposed peak of the central uplift in the Haughton impac

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01286.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—The western flank of the Haughton impact structure was imaged with a reflection profile generating 9.8 km of subsurface information. Ten reflecting horizons were recognized and have been correlatedviaa sonic log with the Paleozoic limestone/dolomite rock sequences. The seismic section is dominated by a dense and complex compound fault system with variable attitudes. These steeply dipping faults penetrated the sedimentary rocks but showed no recognizable extension into the crystalline basement. According to the seismically recognized fracture zones of the western margin, the structure is significantly larger than previously estimated. Reconstruction of the crater on the basis of the seismic information and existing scaling relationships reveals a structure with an apparent diameter of 23.9 km, and an excavated cavity of 10.3 km width and 1.97 km depth. The estimated diameters of the transient crater and the central uplift are 12 km and 11 km respectively. The morphologically distinct ring zones do not have seismically recognizable subsurface signatures. The underlying crystalline basement rocks did not exhibit seismically mappable impact‐related zones of disturbance. In the central interior region, coherent reflection signals are virtually absent. Valuable information for this area was provided by a 10.26 km long refraction profile that indicated nearly uniform velocities (∼5000 m/s) to a considerable depth. Major lateral variations in the velocity field across the structure were not det

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01287.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract—Surface and subsurface structural studies undertaken under the Haughton impact structure study (HISS) project indicate that the 23 Ma‐old Haughton impact structure, (Devon Island, Canadian Arctic) consists of a central basin of uplifted strata, an inner zone of uplifted megablocks at 3.5–5.5 km radius, a complex, faulted annulus of megablocks at 5.5–7.0 km radius and an outer zone of downfaulted blocks. No evidence of a previously suggested structural multi‐ring form was found. The geophysical studies suggest an original diameter of 24 km, slightly larger than previous estimates and the seismic data indicate considerably more faulting in the western portion than has been mapped from surface exposures. Detailed studies of the allochthonous breccia deposits found no major radial variations in lithology and shock levels. The only anomaly is the concentration of highly shocked, cobble‐sized clasts in the central area coincident with the maximum gravity and magnetic anomalies. It is suggested that this local component is related to the highly shocked rocks of the central uplift and may have been shed from the uplift during late stage adjustments. There is no visible central topographic peak of uplifted bedrock at Haughton but studies of the post‐impact Haughton Formation suggest that the center of the structure subsided 300–350 m soon after formation. Breccia studies also indicate the occurrence of shock‐melted sediments, including shales, but no evidence of shock melted carbonates, the most common target lithology. This may be ascribed to the ease with which carbonates are volatilized by relatively moderate shock levels. The large amount of volatiles released on impact helped disperse the highly shocked products leading to the formation of a relatively cool clastic and polymict breccia deposit in the interior, as opposed to a coherent melt sheet. In this regard, the breccia deposit is somewhat analogous to the suevite deposits within the Ries crater. Sedimentological studies indicate that the Cretaceous‐age Eureka Sound Formation was present at the time of impact and that the Haughton area has undergone as much as 200 m of erosion since

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01288.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1988.tb01289.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY