GRAVITY ANOMALY

GRAVITY ANOMALY

by: Charles O’Dale

  1. introduction;
  2. confirmed impact craters;
  3. suspected impact craters.

1. INTRODUCTION

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

  • Beaverhead;
  • Brent;
  • Carswell;
  • Chicxulub;
  • Clearwater;
  • Glover Bluff;
  • Holleford;
  • Kentland;
  • Manicouagan;
  • Maple Creek;
  • Montagnais;
  • Pingualuit;
  • Wanapitei;
  • West Hawk.

BEAVERHEAD

Bouguer gravity map of region surrounding the proposed thrust detached root to the Beaverhead structure, East Idaho.

BRENT

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).

Bouguer gravity anomaly over Gilmour and Tecumseh lakes region. The crater lies at the center – Modified from Millman et al, 1960.

CARSWELL

Innes, M. J. S., Recent advances in meteorite crater research at the Dominion Observatory, Ottawa, Canada. 1964

CHICXULUB

Chicxulub crater image of its gravitational field (NASA)

CLEARWATER, EAST & WEST

GLOVER BLUFF

Bouguer gravity map showing the location of the Glover Bluff disturbed area (gravity data from Koenen, 1956). Contours in milligals. B, Ground magnetic map showing the location of the Glover Bluff disturbed area (magnetic data from Koenen, 1956). Contours in gammas.

HOLLEFORD

The Holleford Crater Bouguer gravity anomaly map produced by the Gravity Division, indicates that the gravity magnitude contours were circular and follow in a general way the outline of the impact structure depression. The central gravity low indicates an anomaly of approximately two milligals. This suggests a circular depression filled to an estimated depth of ~250 metres with material which is on the average of a lower density than the surrounding Precambrian rocks.

KENTLAND

MANICOUAGAN CRATER / CAN-AM STRUCTURE

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.

Bouguer gravity map of the Manicouagan Impact. (2009 Kord Ernstson)
Can-Am bouguer anomaly field. G.F. = Grenville front.

MAPLE CREEK

MONTAGNAIS

Bouguer gravity analysis of the Montagnais impact crater.

PINGUALUIT

WANAPETEI

(Dence and Popelar 1972).
Geophysical evidence is in the form of a circular gravity low of ~15 mgal, documented in this Bouguer gravity anomaly map. The low is observed over the north-central, island free area of the lake. This gravity anomaly reinforces the meteoritic origin of this structure similar to other structures (see West Hawk and Brent) that have been identified as impact events by similar gravity anomalies. The data was corrected using water depth measurements from the Ontario Department of Lands and Forests.

WEST HAWK

 

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

Can-Am bouguer anomaly field. G.F. = Grenville front.

Reference:

Kord Ernstson, Gravity surveys of impact structures 2009

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