GEOMORPHIC CHARACTERISTICS AND CLASSIFICATION OF GLOBALLY DISTRIBUTED, HOLOCENE, BRAID DELTAS AS A GUIDE FOR INTERPRETING NON-VEGETATED TERRESTRIAL LANDSCAPES ON EARLY EARTH AND MARS

Braided streams are relied on as present-day analogues for barren, pre-Devonian, fluvial landscapes on Earth. Pre-vegetation stratigraphic packages are commonly interpreted as multi-channel river deposits, the result of reduced sediment cohesion before the emergence of rooting plants. Fluvial systems on Mars formed in the absence of vegetation and despite examples of meandering rivers, braided morphologies are known, so braid deltas should have prograded into impact-basin lacustrine settings. Recognized as a unique depositional environment for decades, little detailed information is known about the range of braid-delta morphotypes. Here, we present the major geomorphic characteristics of ~800 globally distributed, Holocene, braid deltas, collected via a survey of marine and lacustrine shorelines. Each site, selected from ~2400 preliminary locations, is >500 meters in channel-belt width with a gradient <2 degrees. Braid deltas occur at all latitudes (with the exception of the Antarctic), but 75% are found in periglacial settings between 60º to 90º or -45º to -60º latitude when normalized to land mass.

We propose a classification system to explain braid-delta morphology using three parameters: hydrology, either perennial/intermittent or ephemeral, dominance of river, wave, or tide energy, and how confinement changes (or remains similar) from the feeding fluvial system to the delta. Nine types of braid delta are detected in the dataset. Notably, the majority of present-day braid deltas are formed by perennial/intermittent rivers (67%), river-dominated (50%), and not radial in morphology (63%). Tide dominance, with large tidal bars, is only observed in eight perennial systems. After developing a classification, ~40 km of reach length across nine type-localities representing each class of braid delta were mapped using ArcGIS to depict common facies relationships. A reduction in channel count per meters transect distance, by ~0.01, typically occurs <100 meters from the coastline at mapped sites. Unstable on weekly timescales, some braid-delta environments experience rapid change in morphology during river flooding or wave reworking. Thus, the strata of ancient braid deltas are expected to preserve interbedded terrestrial and marine facies that must be described in totality before classification.