Asandstone is found to overlie a shale. the shale contains a volcanic ash layer that has been radiometrically dated to 84 million years ago. both the shale and the sandstone are cut by a pluton of gabbro that has been dated to 60 million years old. how old is the sandstone layer?

It is between  84 and 60 million years old

Explanation:

Since the sandstone is on top of shale which has ashes 84 million years old and both are cut by pluton of gabbro which is 60 millions of years old we can conclude that the sandstone layer is between 84 and 60 million years old.

Gabbro originated in shale and went into sandstone. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".

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