TURTLE ROCKS: DIAGENESIS OF THE HARTSHORNE SANDSTONE
LeBas, John 1976-
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The “turtle rocks” of the Pennsylvanian Hartshorne Sandstone (Arkoma Basin, northwestern Arkansas) are unusual landforms characterized by polygonally jointed and segmented mounds that resemble turtle shells. Turtle rocks have selectively developed on less than 0.2% of the Hartshorne outcrop belt, and their origins are unknown. Quantitative field and petrographic study showed that turtle rocks are coarser grained, more porous, and more thoroughly fractured than typical Hartshorne outcrop. Furthermore, turtle rocks are overall poorly indurated compared to typical Hartshorne but are preferentially case hardened via iron oxide and amorphous silica cementation. Intersecting polygonal joints create a framework that defines the borders of individual turtle rocks, which have an average maximum length of 3 m and height of 0.7 m. Surface segments have variable polygonal and ovoid geometries but consistently average lengths of 0.3 m. Porosity in both turtle rocks (12.5%) and typical Hartshorne (3.0%) is almost entirely secondary, owing to the removal of unstable cements, rock fragments, and feldspars. The greater amount of secondary porosity in turtle rocks is associated with a greater extent of initial cementation and less overall mechanical compaction related to coarser grain size. The field and petrographic data suggest that polygonal jointing initiated under subsurface stresses, had an increased density in more thoroughly cemented beds, and continued at the surface via case hardening. The development of individual mounds is a result of erosion down-cutting along joint traces and rounding off the outcrop between joints. Surface segments developed and are maintained by case hardening, which appears to be enhanced by greater porosity. Based on the findings of this research, the origins of the turtle rocks are likely explained by outcrop-scale diagenetic variations resulting in part from differential depositional characteristics.