by: Charles O’Dale
- confirmed impact craters;
- suspected impact craters.
Gravity anomalies in impact structures may result from quite different processes. Small, simple craters in general show a roughly circular negative anomaly (e.g., Brent). A low-density breccia lens at the crater floor, post-impact young crater sediments, and fracturing of the rocks beneath and around the crater may contribute to the mass deficit.
In larger, complex craters with central uplifts and/or rings, gravity anomalies may be complex as well (e.g., Manicouagan). Like in simple craters, rock fracturing and low-density impact melt rocks, suevites and other breccias cause negative anomalies, and post-impact crater sediments may also contrast with the pre-impact target rocks. In addition, relatively positive anomalies are measured, if rocks of higher density were uplifted in the modification stage of the cratering process to form the above-mentioned uplifts and rings. Shock lithification of porous rocks may also lead to locally increased density.
In this context, gravity measurements prove to be a valuable tool in the investigation of impact structures. They are important to detect buried impact structures (as for example the famous giant Chicxulub structure and they can trace the original size of deeply eroded craters where only relics of impactites point to an impact origin.
Gravity potential theory tells us that from the integration over a gravity negative anomaly the total mass deficit can be calculated regardless of the specific density distribution. As in impact structures the mass deficit is related with the kinetic energy of the projectile, impact energy considerations and estimates of the displaced masses are possible. On the other hand, density modeling of gravity anomalies may show details about the internal structure of impact craters (Kord Ernstson 2009).
GRAVIMETRY – Gravity anomaly
Geophysical method to measure variations of the gravity field related with subsurface density variations. Impact structures commonly show pronounced gravity negative anomalies due to the occurrence of low-density breccias, rock fracturing, and replacement of ejected material by post-impact young sediments. In very large impact structures, relative positive anomalies may be produced by the uplift (see; central uplift) of high-density material from the Earth’s lower crust and upper mantle.
2. CONFIRMED IMPACT CRATERS
- Glover Bluff;
- Maple Creek;
- West Hawk.
A gravity anomaly at the Brent Crater produced by the sediments and fragmented rocks in the crater reinforces the meteoritic origin of this crater similar to other structures (see West Hawk and Wanapitei) that have been identified as impact events by similar gravity anomalies. It is interesting to note that in this gravity map that was published in 1960 the magnetic north had an indicated west declination (variation) of 10° 05’ W. Today in 2012 it is 12° 00’ W. The change is due to the drift of the magnetic north pole over the past 52 years (Chavez 1986).
There is a strong similarity between the profiles of the Can-Am structure and the Manicouagan Impact Crater. The coincidence between magnetic and gravity signatures strongly suggests a common source for both fields. This data documents that the Precambrian basement rocks are interrupted by anomalies that clearly outline the circular nature of the structure and provides evidence that the remnants of a complex meteorite crater is situated in the south end of Lake Huron.
A gravity survey of the West Hawk Lake and surrounding region (Halliday and Griffin 1963) indicated a peak negative gravity anomaly of over 6 milligals, illustrated in the Bouguer anomaly map of the West Hawk Lake Impact Crater. The anomaly is produced by the sediments and fragmented rocks under the crater. This gravity anomaly reinforces the meteoritic origin of this crater similar to other craters (see Brent and Wanapitei) that have been identified as impact events by similar gravity anomalies.
3. SUSPECTED IMPACT CRATERS
CAN-AM (PROBABLE) IMPACT STRUCTURE
Kord Ernstson, Gravity surveys of impact structures 2009