The effect of sea level changes on the morphology of mountain belts

Abstract

We have examined quantitatively the effect of large (>200 m) long‐period (>50 m.y.) sea level changes on the development of the landscape of old mountain belts. On the basis of the available data we have assumed that the rate of denudation of these old landscapes is proportional to the average regional elevation trueY¯ by a constant (K ≈ 10−4/1000 years). The results are as follows: (1) In the absence of large sea level changes it will take in excess of 300 m.y. and more likely more than 450 m.y. to degrade a Himalayan size mountain belt ( Y¯=5000 m ) to a peneplain ( Y¯=10 m ). For an old mountain belt that has been eroded to a regional elevation of 500 m, it will take an additional 220 m.y. and more likely 390 m.y. to degrade to a peneplain. (2) In a system in which the river valleys are graded with respect to base level, the rate of downcutting is enhanced by the rate of uplift caused by the isostatic response to regional denudation. With sea level (base level) rising the rate of downcutting is decreased by the rate of rise. However, transgression of the river valleys cannot take place until the rate of river valley uplift caused by isostatic response to the denudation is equal to the rate of sea level rise. Under usual conditions of sea level rise (nonglacial) this will not occur until the landscape has been degraded to a regional elevation of 60 m or less. However, under these conditions the decrease in the rate of downcutting in the river valleys greatly enhances the rate of degradation. As a consequence, older mountain bells already eroded to a regional elevation of 500 m or less may be further degraded to a peneplain ( Y¯≤10 m ) during an episode of persistent sea level rise at rates of several tenths of a centimeter per 1000 years for several tens of millions of years. With sea level falling, the landscapes evolve toward an equilibrium surface at which point the rate of degradation is equal to the rate of sea level fall. (3) We have applied these concepts to the post Triassic evolution of the ridge and valley province of the Appalachians. Sea level probably rose 300 m during the interval from 205 to 95 Ma, which was sufficient to degrade those areas with a regional elevation of 500 m or less to a peneplain. Subsequent to this time, from 80 to 10 Ma, sea level has fallen approximately 300 m, sufficient to cause emergence of the former peneplain to a regional elevation ( trueY¯ ) of 215 m which is approximately equal to the present‐day regional elevation of the AppalachianS since the ridge and valley province near Harrisburg. We have made a further test of this by attempting to restore the remanent peneplain surface (i.e., the Schooley Mountain surface) to its former elevation in the upper Cretaceous and thus estimate the height that sea level had to be at that time. By infilling the valleys with country rock up to 457 m, which is the present elevation of the presumed former peneplain remanent surface, and making the appropriate isostatic adjustments, we may conclude that at a minimum, sea level was 277 ...

Weathering instability and landscape evolution

“…The relationship between isostatic uplift or subsidence and elevation are indirect, with any feedback effects operating through denudation and aggradation. Additional complexity is due to the isostatic responses to erosional unloading and depositional loading (Ahnert, 1984;Morisawa, 1975;Pitman and Golovchenko, 1991). In addition, geomorphic processes such as erosion and deposition may be self-limiting independently of their relationships with isostasy.…”

Late Cretaceous and Cenozoic sea‐level estimates: backstripping analysis of borehole data, onshore New Jersey

“…Finally, sequence stratigraphic data suggest that some very large magnitude third‐order sea‐level changes of 100 m or more occurred (Haq et al , 1987). The only known mechanism for such large, rapid eustatic variations is continental glaciation (e.g., Pitman & Golovchenko, 1991). As such, these events are particularly problematic when they occur during greenhouse climates, such as the Late Cretaceous.…”

Large‐scale relief development related to Quaternary tectonic uplift of a Proterozoic‐Paleozoic basement: The Armorican Massif, NW France

“…For these reasons, the preferential development of eroding landscapes during Quaternary times [Summerfield, 1991 a] can be linked to the specificities of this geological period, because the onset of glaciations around 2.5 Ma [Schackleton et al, 1984] is related to a strong climate change and is also responsible for largeamplitude and high-frequency eustatic variations. The same question arises for the Cenozoic as a whole [Molnar and England, 1990;Pitman and Golovchenko, 1991] since this era corresponds to a period of long-term climate change and sea level fall [Haq et al, 1987], which potentially favored erosional fluvial processes.…”