The Lawn Hill annulus: an Ordovician meteorite impact into water-saturated dolomite

Darlington, Vicki, Blenkinsop, Tom, Dirks, Paul, Salisbury, Jess, and Tomkins, Andrew (2016) The Lawn Hill annulus: an Ordovician meteorite impact into water-saturated dolomite. Meteoritics and Planetary Science, 51 (12). pp. 2416-2440.

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Abstract

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of similar to 18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. Ar-40-Ar-39 dating of impact-related melt particles provides a time of impact in the Ordovician (472 +/- 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low-strength, porous, and water-saturated, with all three properties affecting the crater morphology. The water-saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid-Ordovician (470 +/- 6 Ma).

Item ID: 47631
Item Type: Article (Refereed Research - C1)
ISSN: 1945-5100
Funders: Australian Geographic Society, College of Science and Engineering, James Cook University
Date Deposited: 08 Mar 2017 07:32
FoR Codes: 04 EARTH SCIENCES > 0401 Atmospheric Sciences > 040107 Meteorology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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