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ABSTRACT The Santa Rosalía basin (Baja California Sur, México) contains a rich record of late Cenozoic volcanism, faulting, and sedimentation that provides a crucial constraint on the timing of marine flooding from the Pacific Ocean into the nascent Gulf of California oblique rift, yet the precise age of the basin is uncertain. Previous studies used reconnaissance paleomagnetic data and a 40Ar/39Ar age of 6.76 ± 0.90 Ma on the intrabasinal Cinta Colorada tuff to estimate a depositional age of ca. 7.2–6.3 Ma for the marine Boleo Formation and initial flooding of the central Gulf of California. Here, we present a large (n = 2091) detrital zircon U-Pb geochronology data set from the Boleo Formation that indicates a maximum depositional age of 6.35 ± 0.21 Ma for pumiceous sandstone at the base (below the basal limestone), a revised age of 5.86 ± 0.06 Ma for the Cinta Colorada tuff in the middle, and a maximum depositional age of 5.70 ± 0.21 Ma for the top. Detrital zircon age spectra suggest a local provenance for the Boleo Formation involving recycling from underlying Oligocene–Miocene strata in proximal source areas. Integration of detrital zircon ages with existing paleomagnetic data suggests that the lower ~30 m of the Boleo Formation accumulated during normal-polarity subchron C3An.1n (6.27–6.02 Ma), and the middle to upper Boleo Formation was deposited entirely during reverse-polarity chron C3r (6.02–5.24 Ma). We therefore reassign the depositional age span of the Boleo Formation to ca. 6.3–5.7 Ma. Although not preferred, a minimum-duration depositional model from ca. 6.1 to 5.8 Ma is also permissible if a consistently high sedimentation rate of ~0.4– 1.0 mm/yr is inferred. This revised younger age for the Boleo Formation implies marine incursion in the central Gulf of California at ca. 6.3 Ma, ~1 m.y. younger than previously thought. We envision that regional marine flooding occurred during a very short (<100 k.y.) event that inundated a narrow tectonic trough over a distance of at least ~1000 km along the plate boundary from the central Gulf of California to the Salton Trough and reaching into the present-day Lower Colorado River Valley. This study also demonstrates the utility of large-volume and large-n detrital zircon studies in establishing the ages of sedimentary successions deposited over very short time spans (<1 m.y.) and/or during relative lulls in magmatism and geomagnetic reversals. 

Abstract Despite forming under different flow conditions, the geometries of tidal and fluvial channel planforms and planform transformations display significant overlap, hindering efforts to differentiate them geometrically. Although studies have demonstrated that globally, tidal and fluvial planforms are statistically distinct based on meander metrics, there are currently no machine‐learning methodologies for classifying channels as tidal or fluvial that do not focus on meander‐specific geometries. In this study, we present a methodology for classifying channel planforms as tidal or fluvial using statistical representations of channel planforms and machine‐learning algorithms. Using a dataset of 4294 tidal and fluvial channel segments (63 channel reaches), we trained three machine‐learning classifiers (Logistic Regression, Multi‐layer Perceptron, and Random Forest) across 69 trials to identify the machine‐learning algorithm and variables that perform best at classifying channel reaches. We evaluated the performance of the classifiers at three thresholds based on the percent of channel segments correctly identified in a given reach (>50%, >66% and >75%). At the >50% classification threshold, all three classifiers attained a 95% reach‐scale accuracy during individual trials. However, at higher classification thresholds, the RF classifier performed best. Feature importances from the RF classifier indicate that measures of the central tendency and minimum/maximum of the normalized radius of curvature convolved with normalized width of channel segments play a key role in differentiating between the planforms, with normalized width also contributing to the difference. This indicates that the relationship between width and radius of curvature is more important than width or measures of curvature on their own. This result likely reflects the downstream funnelling of tidal channels and the limitation on the sharpness of bends associated with increased width. These methods have potential for application in the study of channels preserved on relict geomorphic surfaces and in mixed‐energy settings.