A possible impact feature in the Okavango Delta
Colin Henshaw
10, Delamere Road, Gatley, Cheadle, Cheshire, SK8 4PH, ENGLAND.
In December 1993 I was working in Botswana and I was interviewed for a possible new contract at Delta Waters International School in Maun. Maun is a frontier town in the Okavango Delta, and is the main urbanised centre for several hundred kilometres. It is very isolated, and until about ten years ago was only accesable by a long dirt road. This isolation impeded the town's development, but after the road was tarred over, it began to boom. This encouraged tourism and the growth of light industry, and the town expanded rapidly. It now boasts an international airport, and several shopping centres. I first visted Maun in the early 1980's when it was nothing more than an assemblage of crude African huts. These are still there, but after the construction of the new road, many modern buildings have now appeared. During the interview, my interest in astronomy was mentioned, since from this many good scientific projects can be developed, which can captivate the interest of children. The headmistress who was conducting the interview then mentioned a suspected meteorite crater in the Okavango Delta which is located next to the town. I was very interested in this, and suggested it might be a good idea to investigate it with a party of interested students. Soon afterwards I received notification that I had been appointed to teach science in the school, and I started in January 1994. During the Christmas holidays I was in England, where I purchased a small metal detector, to locate metalic meteorites.
After I started at the school I began to investigate what was already known about the crater. Unfortunately there were two conflicting stories. One safari operator said it occured in 1978, on August 12th., at 12.30 local time, and that it was associated with a minor earthquake. Unfortunately this could just be a coincidence as the Okavango Delta lies at the end of the African Rift Valley system, and is known to be seismically active. The strongest quake ever recorded in the area was of magnitude 6 on the Richter scale, though it didn't cause much damage. A few buildings were damaged slightly but there were no casualties. This was due to the fact that most of Botswana is covered in several hundred metres of Kalahari Sand, which cushions the effect of any earthquakes. A second safari operator said he discovered the crator in the early 1950's when he first arrived in the country and that it was known to local people for about twenty years before that. One of these safari operators was not telling the truth, though for what reason was unknown.
On April 23rd., 1994 an expedition was organised with a local person who knew the location of the crater. It was located in the centre of the Okavango Delta, in an area known as Khurunxaraga, not far from Beacon and Bobo Islands. It was only about 35 kilometres from Maun, but accessable only by four wheel drive vehicles. The roads into the delta were nothing more than dirt tracks, and this resulted in a journey time of about 2h 45m. The crater was located inside the Buffalo Fence, an enclosure around the delta designed to keep domestic and wild animals apart. Inside the fence the vegetation becomes much thicker as it is not overgrazed by the goats which the Batswana keep in large numbers. Once inside the fence, sightings of wild animals become common.
The feature is not visible from the road due to the thick vegetation, so a tour guide is essential before any intended expedition can succeed. On arrival I set my students the task of investigating the crater. It was found to be only about 22 metres in diameter, and about 3.5 to 4 metres deep. The students used tape measures and strin to make their measurements. The crater was saucer-shaped, and the sides of the crater had suffered a certain amount of gullying. In fact it bore a remarkable resemblance to the Arizona Meteorite Crater, in miniature! The sides of the crater were bare, but the interior was overgrown with grass. The material in which the crater was excavated was soft Calcrete, a substance which is very common in Botswana and formed under desert conditions. It is used extensively in road making. Calcrete is a conglomerate formed from the cementation of sand and gravel with calcium carbonate. Around the crater the soil was sandy. Using the metal detector my students swept the area both inside and outside the crater, but did not find any metalic objects. Nor was their any stony material apart from the usual calcrete which was found everywhere.
Now that the feature had been investigated, it was essential to determine whether it was natural or artifical in orgin. It may just have been a hole excavated by local people to obtain building material. However, people digging a hole would tend to heap excavated material randomly around the hole and there was no evidence for this. The location was very isolated, with no substantial settlements in the area, so there was no reason why anyone would want to excavate in that area. It might be a bomb crater, but the Botswana Defence Force had not been active in the area. It had also been suggested that the South African Defence Force might have off-loaded unused bombs after bombing raids on Angola during the late 1970's. However to produce such a crater would require a very large bomb, and the results of the metal detection programme did not reaveal any evidence of shrapnel. So it was generally felt this explaination could be discounted. Some have suggested it may be a "sink hole," - some kind of subsidence feature. However, if it was, then one would expect other similar features tobe found. This is not the case and the feature appears to be unique. It was generally believed by those who have visited the site that the feature is a meteorite crater. Photographs were submitted to Neil Bone of the BAA Meteor Section, and to Colin Pilinger of the Open University in Milton Keynes, and the possibility could not be discounted. Colin Pilinger pointed out a relationship existing between an impacting body and the size of crater produced, such that the crater is about ten times the diameter of the object. If the impact explaination is correct, that would suggested an impactor about 2 metres in diameter. The impactor would bury itself deep below ground, and if it came in at an angle, would not be necessarilly be located unde the crater. Colin Pilinger pointed out that a large meteorite recovered recently in China excavated a hole 28 metres across and 6 metres deep.
Those who have visited the site and also those who had seen photographs could not discount a meteoritic origin for the crater. It was considered worth preserving as it is, and any attempt to recover the meteorite, should it exist, would irreparably damage the crater. Erosion could be a serious problem, so any future visitors should be discouraged from going inside it.