Transitional flow deposits in submarine lobe strata : the Cretaceous Point Loma Formation, San Diego, California
Transitional flow deposits (TFDs) include a spectrum of subaqueous sediment-gravity flow deposits that do not fit conventional models for turbidites and debrites. Examples of TFDs are common in submarine lobe strata of the Cretaceous Point Loma Formation in San Diego, California. The cross-flow orientation and exceptional quality of this sea cliff exposure presents a unique opportunity to document axis-to-margin variability within individual beds. TFDs in the Point Loma Formation are typically composed of a lower interval of structureless sandstone that is overlain by meso- to micro-banded sandstone with high mud content. This upward transition from sandstone with lower mud content to overlying mud-rich sandstone is characteristic of TFDs worldwide. Cross-flow exposure reveals a corresponding change in geometry: a lenticular 'core' of structureless sandstone is overlain by an extensive, mud-rich 'drape' of banded sandstone. This internal shift in both composition and geometry is interpreted to reflect deposition from two distinct stages of transitional flows. Initial deposition of coarser sand was spatially restricted while flows were energetic and able to efficiently segregate particles of different size. Following this initial stage of deposition and energy loss, flows deposited muddy sand over a wider area; this transition is interpreted to reflect a change in current behavior where reduced flow velocity coincided with decreased mixing and poor segregation of suspended sand and mud fractions during deposition. The organization of TFDs within the outcrop varies with scale. Multiple TFDs stack to form lobe elements with minimal lateral offset between bed axes. As a result, both individual beds and lobe elements show relatively simple transitions from sand-prone axes to mud-rich margins. Compensational stacking of lobe elements introduces additional complexity at larger scales, where vertical alternation between sandy and mud-rich units reflects significant lateral shifts in position (ca. 500 m to 1 km) between the axes of successive lobe elements. Bed thickness frequency analysis shows how differences between distributions of vertically measured thicknesses relate to the spatial variability in outcrop. Developing a better understanding of these relationships is critical for applications in both industry and academia in which workers are frequently limited to using one-dimensional data sets.