Distinguishing Between Primary and Secondary Emplacement Events of Geologic Deposits Using Rock Size Populations.

Geologic interpretations of emplacement origin based on geologic mapping and dimensional analyses remain constrained by limited high-resolution images and topography. Geochemical data provide mineralogy but require additional context for the mode of emplacement to be understood. An important key to understanding past and potentially future geologic behavior of Earth and other bodies in the solar system is to interpret surface deposits. However, interpreting the origin of a geologic surface is difficult, especially in instances where ground observations are not possible, or where additional evidence that could provide spatial and temporal context for the deposit is ambiguous. This difficulty is compounded in terrains where emplacement processes such as lava flows and rock avalanches can occur in the same material and locality. Furthermore, post-emplacement processes such as glaciation, fluvial, and aeolian activity may contribute additional complexity to interpreting the origin of a rocky surface. To significantly advance our understanding there is a need to identify geologic characteristics that are uniquely diagnostic of a particular process of emplacement. Based on our work on Earth, we have determined that rock size distribution may be one such diagnostic, allowing distinction between primary and secondary emplacement processes. In this context, volcanic, glacial, fluvial, aeolian, tectonic, and impact processes can be thought of as being either primary (capable of both creating and degrading rocks) or secondary processes (only capable of break existing rocks). Measuring rocks in the field provides the best resolution but is time consuming. Thus, we have examined whether LiDAR offers an improved way of obtaining such data since it allows topographic profiles to be extracted for any given site of interest. We have determined that it is possible to resolve rocks at ~1 m based on analysis of data over Mission Creek, CA and Chaos Jumbles, CA. Field and LiDAR measurements are being examined to determine if geologically valuable information is lost with only 1 m resolution and its significance to our statistical approach to examining rock size distributions. These quantitative terrestrial field experiments will provide a unique diagnostic for distinguishing between surfaces on other planetary surfaces.