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Amelia Creek Impact Structure                                                                                                                                            

The Amelia Creek structure is in the Davenport Ranges, Northern Territory, Australia. On the southern, down-range side of the structure there are shock metamorphic features. The structure is in the sedimentary and volcanic rocks in the southern Tennant Creek Inlier. In the centre of the structure there are ramping, thrust sheets trending to the south-southwest with a central syncline (trough) that is canoe-shaped, and runs NNE-SSW for about 5 km, and is about 1 km wide. Associated with this structure are shatter cones, impact breccias and hydrothermal deposits.

On the southern side of the structure are many quartzite beds, that are all oriented upwards, in which shatter cones are prolific (Fig. 1, source 1). It has been suggested that as the beds are all oriented upwards indicates the impact occurred later than about 1700 Ma when the folding occurred locally (Stewart et al., 1984, source 1). On the southern side of the structure the distribution of shatter cones on the surface, approximately in the form of a crescent, about 1 by 3 km, extends for at least 4 km to the south from the central syncline. Throughout the structure the lithologies are similar, but shatter cones are found only on the southern side, that is down range. Along many of the major thrust faults within the structure there are allogenic breccias, and there is evidence of baked margins and shocked clasts.

According to MacDonald & Mitchell, most impacts are oblique, and in very oblique impacts the energy transfer to the target is less efficient, producing craters that are smaller for a given mass and velocity of impactor than would occur for impacts closer to the vertical (Melosh, 1989, source 1). They suggest that oblique impacts develop shallower deformation, and possibly don't develop central uplifts, even with large structures.

They believe the amount of erosion at Amelia Creek is such that it could be about 1 km, and that the Neoproterozoic or Early Cambrian age of the flat-topped hills of the Davenport Ranges, that are made up of exhumed land surfaces may have been buried for much of the Phanerozoic (Stewart et al., 1986, source 1). The age of the structure is still uncertain [at the time of the report] because of the ages of Cambrian and Tertiary found for some breccias in and around the structure, (Stewart et al., source 1). The authors suggest these breccias may actually be impact ejecta and impact breccia.

The authors also suggest that because of the presence of rocks up-range of the structure, that appear to be anomalously deformed, there is a possibility that the Amelia Creek structure may in fact be part of a crater field or a ricochet structure. They comment on the strong similarity of the anomalous deformation around Amelia Creek, as seen on aeromagnetic and Aster images and geological maps, to extremely oblique impact structures on Mars and the Moon. (Shultz & Gault, 1990, source 1).

The authors conclude that the shock metamorphosed rocks at Amelia Creek are the relict of an extremely oblique impact. They base this conclusion on evidence such as an elongated area of deformation, the movement of most of the structural elements in a south-southwesterly direction, and the presence of a central trough and syncline instead of a central uplift, as well as the distribution of the shatter cones, that is restricted to the downrange side of the structure. The mechanics of large impact craters that are very oblique is not well understood (Melosh, 1989, source 1), partly because no other extremely oblique craters, that are exposed, are known on Earth, resulting in a lack of field measurements to put constraints on theoretical models. The authors believe the impact structure at Amelia Creek, that is very well exposed, with the associated impact-deformed rocks of the Davenport Ranges, will probably be a type locality for oblique impacting.

Sources & Further reading

  1. Amelia Creek oblique impact structure with no central uplift
Author: M. H. Monroe
Last updated  28/02/2011



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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading