A million years worth of deposition: where and how much?

The theme for this month’s geoscience blog carnival, The Accretionary Wedge, was to speculate in some way about the geological future of the Earth. I was the host for this month — you can read my summary and find links to the other contributions here.

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I was originally inspired to propose this theme after reading the book Year Million, which is a collection of essays from scientists, technologists, futurists, etc. speculating a million years into the future (most of the essays, not surprisingly, are about the fate of the human species and our civilization).

So, I decided to stick with the million year time frame for my own post. A million years seems to be the basic unit of temporal measurement that many geologists use (unless you focus solely on the Quaternary or solely on the Archean). Finally, since I’m a sedimentary geologist, I decided to explore where deposition of sediment might occur over the next million years. Or put another way — if I were transported a million years in the future, where would I go to find an archive of sediments (or sedimentary rocks) recording that interval of time?

What is fun about pondering this question is that it requires an attempt to grasp the depth of geologic time. I’ve grown accustomed to writing about and discussing with colleagues intervals of time that range from thousands of years to 100s of millions of years in duration.

Think about the surface of the Earth right now — in our current snapshot in time. Sediment is being deposited in many places … in rivers, floodplains, desert valleys, coastal areas, and in the ocean. But remember, this question is about where sediment is deposited over the scale of a million years. Some sediment might be deposited in a mountain stream after a flood, but would it remain there for a million years? Probably not. Similarly, a field of sand dunes might be accumulating near a mountain front in a desert right now. Would this pile of sand remain for a million years? Depends on the setting … but probably not.

So what this question is really asking is where would deposited sediment be preserved? Sedimentation is temporary across a huge range of time scales — a deposit might last only a year or so, only to be wiped out by the next hurricane — or, it might get buried, buried some more, and be preserved for 10s to 100s of millions of years.

At the scale of a million years, the places on the Earth where sediment accumulates, is buried, and preserved are called sedimentary basins*. Put very simply — we need to find a location that has a long-lived supply of sediment combined with a place to put it (e.g., subsiding basin). Continental margins where major river systems meet the ocean (e.g., Mississippi, Amazon, Niger, Ganges-Brahmaputra, Indus, etc.) are the best candidates for finding thick sedimentary successions recording the next million years. Continental margins with large delta systems continually subside under the weight of the sediment, which provides space for more sediment to accumulate, creating more subsidence, and so on — this is how deltaic sediment several miles thick can be found in one location.

Continental margins are certainly not the only place where sediment might accumulate over a million years. Rift basins (e.g., East African Rift Valley), “pull-apart” basins (e.g., related to transform fault zones like the San Andreas), foreland basins (e.g., Himalayan foreland, Andean foreland, etc.) among others are all possible locations where subsidence and sediment supply combine to create a stratigraphic succession.

How much sediment will accumulate over a million years?

My main point in posing this question is to point out that there is no simple answer. Obviously it depends on which basin one would look — it depends on where exactly within that basin as well. Sedimentary basins are not necessarily tectonically “quiet” places — there can be episodes of nondeposition and removal of accumulated sediment (i.e., unconformity development). Sediment accumulation rates can range several orders of magnitude (from essentially nothing to a lot) even within a single sedimentary basin. Certain locations on the Bengal submarine fan, which is the single largest pile of clastic detritus on Earth, has some of the highest short-term rates known (several to 10s of meters per 1,000 years). But, if you remember this post, it is dangerous to extrapolate short-term rates to longer time intervals (and vice versa). When looking at rates determined at a million-year measured interval, the highest rates in the Bengal fan are on the order of 100 meters per million years^.

My guess is that 100 meters of sediment accumulated over a million years is likely the maximum value. I didn’t take the time to look up rates in basins around the world (or look up a paper that has already done that) but I’d guess that somewhere on the order of several to a few tens of meters per million years is reasonable.

The caveat in all of this is that the processes and associated rates over the next million years will be generally similar to the past million years. Even if catastrophic events that occur on timescales of 10s to 100s of millions of years (e.g., large impacts) were to happen, a million years is enough time for the Earth surface system to respond and adjust.

So … to make a long story short. A million years worth of deposition could be as much several 10s of meters, but only in a few select and localized places.

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* I highly recommend a resource I’ve mentioned before — the 1995 Busby & Ingersoll text Tectonics of Sedimentary Basins, which puts many aspects of sedimentary basins (e.g., depositional systems, stratigraphic organization, subsidence mechanisms, provenance, etc.) within the context of plate tectonic processes and plate boundary types.

^ Einsele et al., 1996, The Himalaya-Bengal denudation-accumulation system during the past 20 Ma: The Journal of Geology, v. 104, p. 163-184.

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