Oriented, linear-shrinkage cracks in Green River Formation (Eocene), Raven Ridge Area, Uinta Basin, Utah

ABSTRACT The term oriented, linear-shrinkage cracks is proposed for unusual shrinkage cracks in the non-marine Green River Formation (Eocene) along Raven Ridge, northeastern Uinta Basin, Utah and Colorado. Linear-shrinkage cracks are identical to incomplete shrinkage cracks noted by others in the Belt Series (Precambrian) and the Medinan Series (Silurian). Linear-shrinkage cracks are mainly straight (or slightly curved), single cracks, which show preferential orientation. Minor two- and three-segment cracks, whose segments are equal and unequal in length, are associated with the linear-shrinkage cracks. Lengths and widths of 200 single cracks were measured. Lengths range up to 75 mm, with a mean of 21 mm, a median of 17 mm, and a standard deviation of 15 mm. Widths range up to 8 mm, with both a mean and median of 3.3 mm, and a standard deviation of 1.5 mm. The cracks penetrate the beds from 0.5 to 4 mm; crack-casts rise above the upper surface of beds from 2 to 4 mm. Linear-shrinkage cracks are consistently oriented; where associated in the same beds with ripple mark horizons they are parallel to the strikes of the crests. On non-rippled surfaces the trends of cracks are similar to trends of cracks in ripple-marked horizons. Linear-shrinkage cracks yield paleocurrent orientations that are useful where other paleocurrent data are absent. The dominant trends of the single cracks are believed to be approximately parallel to the shorelines. The interpretation is that the linear-shrinkage cracks opened parallel to the shorelines due to very slight, down-slope, gravity movement of homogeneous material.