The Mescal Limestone

The Mescal Limestone is precambrian in age, and has been dated to be 1.2 Billion years old. Occurring mainly in Southeastern Arizona, the Mescal contains limestone, dolomitic limestones, and large amounts of cherts. The Mescal can be divided into three readily recognized members: A bedded lower member, a stromatolite bearing algal member, and a upper argillite member. Many areas of the Mescal have been metamorphosed by adjacent diabase sills over time, and few areas have not been affected by either silicification, or contact metamorphism.


The ridge of Mescal Limestone where our study area was located.

The ground here is covered with algal stromatolites. At this locality near Young, we are in the middle member of the Mescal Limestone. It is worth considering that when this member was laid down 1.2 Bya, these were the most advanced lifeforms on the planet. The stromatolites are completely silicified, and are white to pink in color with brown , black and reddish banding.

Dr. Paul Knauth stands on a huge stromatolite bed, with the folds and intricate detail visible. The stromatolites here are more of an algal mat, rather than the individual heads found elsewhere in the precambrian. This is at the Young locality.

The Author next to a thick bed of stromatolites at the Young locality. The beds are very hard from being silicified, and great care must be taken when removing samples for study from flying razor sharp shards.

Resting for lunch at the Young Mescal site. This day, we were getting geared up for Pauls early life on land project, looking for signs of precambrian occupation on the surface of the land, rather than in the ocean waters. Because such a discovery and confirmation would have such profound effects on concepts of early life, the project is very important scientifically.

In the upper algal member of the Mescal at Young, we found a massive bed of crossbedded sandy conglomerate. The crossbedded facies rests on a laminar bedded facies seen here as the bottom foot or so of this layer. The laminar bed represents more of a tidal flat deposit, with a low energy deposition. The crossbedded and more conglomeratic facies represents a very near shore subaerial dune deposit where the energy was much higher.

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