DETERMINING FLUVIAL STACKING PATTERNS IN THE LOWER CASTLEGATE SANDSTONE (CAMPANIAN, HELPER, UTAH) USING LIDAR IMAGING

Fluvial architecture analysis focuses on the distribution of channel sand bodies within fine-grained overbank materials. In deposits generated by fluvial systems where floodplain mudstones are scarce, such as sandy braided streams, other elements must be used to analyze stacking patterns. The lower interval of the Castlegate Sandstone exposed near Helper, Utah, was deposited by sandy streams and lacks continuous floodplain deposits. The size and distribution of bar forms, discontinuous muddy intervals, and scour surfaces can be seen in photo panels and form the basis of architectural analysis. In order to analyze stacking patterns with a degree of accuracy unobtainable with photo panels, LIDAR (LIght Detection And Ranging) data were collected from >5 km of the lower Castlegate Sandstone where it crops out north of Helper, Utah. Field observations were made to facilitate interpretation of LIDAR images.

Preliminary results suggest the study interval can be broken into two parts: the lower 30-40 m, characterized by bar clinoforms with maximum heights of 2-3 m and a high density of scour surfaces, and the upper 50-60 m of the section characterized by bar clinoform heights of 4-5 m and fewer scour surfaces. There are two possible explanations for the observed stacking pattern. An increase in sedimentation rate either caused by an increase in sediment supply or an increase in accommodation might affect the relative amount of channel incision vs. aggradation and the resulting deposits would appear less amalgamated. Alternatively an increase in channel flow depths due to increased water supply or a change in river morphology could increase the spacing between scours resulting in a less amalgamated appearance. Other external factors, such as tidal influence may also impact stacking patterns in the lower Castlegate Sandstone. Using clinoform height as a minimum estimate of paleoflow depth, these results suggest that river flow depths increased through time during deposition of the lower Castlegate Sandstone, and scour spacing is in part a result of these changes.