Multiple, Discrete Channel Incision Episodes in Kasei Valles, Mars, Recorded by Alluvial Fan Stratigraphy

Introduction:

Ancient Mars (Noachian to Early Hesperian > ~3.5 Ga¹) may have experienced a warm, stable climate conducive to extensive fluvial activity, before transitioning during the younger Hesperian and Amazonian (3.5 – 0 Ga¹) to a cold, hyper-arid state². This climatic shift may have been punctuated by multiple transient warm and wet episodes³. During this transition, catastrophic floods, likely driven by rapid ice melt and/or bursting groundwater aquifers, carved outflow channels independently of global-scale climate forcing, most prominently in the Circum-Chryse region⁴. Alluvial fans, which record subaerial stream and/or debris flow processes, are widespread in Hesperian and Amazonian-aged impact craters, yet their relationship to outflow channels is largely undocumented⁵. This study explores whether the formation of Kasei Valles, Mars’ largest outflow channel⁶, was punctuated by lower-magnitude, local-scale fluvial processes associated with alluvial fan deposition. Using high-resolution orbital datasets (HiRISE, CTX, HRSC and MOLA), we mapped the distribution of alluvial fans, examined their surface morphology, measured surface slope angles and investigated their stratigraphic relationships to the broader channel.

Observations and Interpretations: 

Alluvial Fans in Kasei Valles:

A 4° × 4° survey of Kasei Valles identified a total of 108 deposits, including 58 bajadas and 32 alluvial fans that were previously undocumented (Figure 1). Fan deposits were most prevalent around the Sharanov ejecta blanket, whereas bajadas dominated increasingly channelised terrain, particularly within the southern channel, where steep topography and limited lateral confinement likely promoted fan coalescence. The fan catchments, characterised by spurs and gullies that vary in configuration, scale, and degree of entrenchment, are consistent with fluvial erosion and weathering rather than mass-wasting processes⁷. The fans also exhibit low-gradient surface slopes, with a median angle of 7.57° derived from 103 deposits. This, together with distributary channels and low-relief ridges, preserved on their surfaces, and sedimentary layering exposed in their margins, supports an alluvial origin for many of the observed fans, with streamflow and/or debris-flow processes likely driving their development⁸.

Figure 1: Topographic map showing the distribution of alluvial fans and bajadas across Kasei Valles^9,10.

Alluvial Fan Deposition between Flooding Events.

The stratigraphic relationships between the alluvial fans and flood-eroded surfaces within Kasei Valles provide insight into the relative chronology of channel incision and fan formation. As an exemplar of this, we present a localised study area centred ~ 22.816°N, -65.954°E in the southern Kasei Valles channel (Figure 2). Here, two sets of flood-eroded surfaces, S1 and S2, are preserved at different elevations along the southernmost channel wall, separated by a flood-incised scarp. The S1 surfaces lie at higher elevations and predate the erosion of the S2 surfaces. A significant portion of the S1 surface is overlain by alluvial fans, hereafter referred to as the F1 fans, which coalesce into bajadas. This relationship, together with the truncated distal margins of the F1 fans delineated by the flood-incised scarp, indicates that the F1 fans were deposited after erosion of the S1 surfaces but prior to the incision of the S2 surfaces. The F1 fans are further dissected by a series of backwasting gullies that function as source catchments for the F2 fans preserved at lower elevations within the channel (i.e., along the current channel floor). The F2 fans coalesce into bajadas that overlie portions of the S2 surface and are distally embayed by flood lavas. This relationship indicates that the F2 fans postdate deposition of the F1 fans and erosion of the S1 and S2 surfaces but predate emplacement of the flood lavas. This stratigraphy suggests that Kasei Valles formed through multiple discrete flooding events, with intervening quiescent periods of sufficient duration to allow alluvial fan formation¹¹.

Figure 2: CTX oblique view of a channel wall in Kasei Valles showing two flood-eroded surfaces overlain by alluvial fans. Dashed lines mark F1 (black) and F2 fans (white), delineate gullies (brown), and trace the truncation of S1 surfaces (blue).

Stratigraphy of the Alluvial Fans, Flood Surfaces, and Flood Lavas.

Similar stratigraphic relationships are preserved at several sites across Kasei Valles, recording multiple, discrete flooding events punctuated by intervals of alluvial fan formation. At these sites, alluvial fans overlie flood-eroded surfaces positioned at various elevations along the channel wall. In some cases, the fans are truncated by flood-incised scarps. A significant proportion of the alluvial fans in Kasei Valles overlie flood-eroded surfaces and exhibit no evidence of truncation by subsequent channel incision, suggesting that their formation postdates the cessation of major flooding activity. Similarly, many alluvial fans on the current channel floor are distally embayed by Late Amazonian flood lavas emplaced between ~128 and 281 Ma¹². Together with the cessation of major flooding activity at ~1.0 Ga¹², this relationship constrains the formation of alluvial fans on the channel floor to the Middle–Late Amazonian and aligns with proposed timelines for fan formation on Mars¹³.

Conclusion:

Our findings indicate that Kasei Valles was incised by multiple, temporally discrete catastrophic flooding events, with intervening episodes of alluvial fan formation driven by local-scale, non-catastrophic fluvial processes such as streamflow and debris flows. This juxtaposition highlights the marked contrast between the environmental conditions required for outflow channel erosion and alluvial fan deposition, suggesting that Mars’ climatic transition was punctuated by transient warm and wet periods conducive to alluvial fan formation within an overarching trend of cooling and drying.

References:

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