Abstract Upon their discovery in 2000, Martian gullies were hailed as the first proof of recent (i.e. less than a few million years) flowing liquid water on the surface of a dry desert planet. Many processes have been proposed to have formed Martian gullies, ranging from liquid-water seepage from aquifers, melting of snow, ice and frost, to dry granular flows, potentially lubricated by CO 2 . Terrestrial analogues have played a pivotal role in the conception and validation of gully-formation mechanisms. Comparison with the terrestrial landscape argues for gully formation by liquid-water debris flows originating from surface melting. However, limited knowledge of sediment transport by sublimation is a critical factor in impeding progress on the CO 2 -sublimation hypothesis. We propose avenues towards resolving the debate: (a) laboratory simulations targeting variables that can be measured from orbit; (b) applications of landscape-evolution models; (c) incorporation of the concept of sediment connectivity; (d) using 3D fluid-dynamic models to link deposit morphology and flow rheology; and (e) a more intense exchange of techniques between terrestrial and planetary geomorphology, including quantitative and temporal approaches. Finally, we emphasize that the present may not accurately represent the past and that Martian gullies likely formed by a combination of processes.

Abstract The Los Angeles basin is a Neogene rift containing a nearly ideal petroleum system. Highly organicrich strata accumulated slowly during tectonic rotation, followed by rapid subsidence and burial beneath thick successions of submarine fan deposits and nonmarine sediments. Late Miocene and Pliocene slope-channel and basin-floor fan sandstones are the main reservoirs. Most petroleum accumulations have been found in faulted anticlines that are associated with the principal structures of the basin and that have been greatly enhanced and modified by transpressional tectonics during the past 6 million years. The basin’s 68 named oil fields probably originally contained more than 40 billion barrels of oil in place. In spite of many years of production, large volumes of technically recoverable petroleum remain in undiscovered accumulations, as additional recoverable oil in existing fields, and possibly in source-rock system reservoirs. Additional large-scale development is problematic, however, owing to the complexities of oil production within a modern megacity.