The Monterey Submarine Canyon - continued

A couple of weeks ago, on the road in California, I did a brief post reflecting on the dramas going on beneath the waves of Monterey Bay. Given the complexity of the Monterey Submarine Canyon, and the scope of the work done on this huge and dynamic feature, that post was the briefest of introductions; among the comments, Diggitt very reasonably asked about the origin of the canyon, since the corresponding onshore feature, the estuary of the Elkhorn Slough (great name) is a puny drainage, hardly capable of sculpting the submarine topography of the canyon. What with getting distracted by other things, but particularly by becoming diverted into the fascinating literature on the many studies of the canyon, it's taken me rather longer to follow up than I had intended. But here, still only scratching the surface of the material available, are at least some answers - still brief highlights, but I'll include some references for readers who would like to look deeper (so to speak).

It was only fifty years or so ago that Francis Shepard, the first geologist to devote his life to marine geology and an internationally renowned figure in the science, began to define and unravel the mysteries of submarine canyons. But one of the key mysteries remained unsolved for some time - many canyons are clearly related to onshore drainage systems, but, even allowing for a more exaggerated erosional capability during the dramatically lower sea levels of the Ice Ages, most of today's onshore rivers are dwarfed by the scale of their offshore equivalents and, like Elkhorn Slough, seem totally incapable of creating such submarine topography  - the Monterey Canyon is, after all, equivalent in scale and relief to the Grand Canyon. Today, we know much more about the origins and processes, but many mysteries, enticingly, remain. For example, it was long thought that, given the inadequate capability of Elkhorn Slough, the Monterey Canyon must be essentially inactive, a mere shadow of its former self with little or no sediment movement down its axis, little erosion, little activity at all - and that therefore it would be a relatively safe one to examine from the point of view of threats to expensive instrumentation. The canyon quickly demonstrated how wrong this view was, and that regular massive torrents of turbulent, gravity-driven, flows of sand, mud and rocks hurtle down the canyon at speeds in excess of a hundred miles an hour, sweeping away everything before them. The image below (from MBARI, the Monterey Bay Aquarium Research Institute) shows a heavyweight steel base for instruments that was carried fifty meters down the canyon floor from where it was originally placed.

 

There are two basic questions that have formed the basis for extensive studies over recent years: what are the workings of the canyon today and what is its history and origin? The first question is, in many ways, easier to answer than the second, but I must point out that sand (of course) tells us tales that allow us to address both. Submarine canyons behave in many ways just like their surficial, subaerial relatives: the geometries of tributaries and channels are similar, and at any one point in the canyon system at any given time, erosion, sediment transport and deposition may be occurring, serving to create an ever-evolving complex. As sediments are flushed out of the lower reaches of a canyon, the transport velocity drops and therefore so must the sediment - on land, the canyon may end in in a lake and a large delta will form; in the submarine realm, a canyon will end on the wide expanses of the relatively flat sea floor and, shaped very much like deltas, submarine fans will be deposited. It is the history and interactions of successive flushes of sediment and deposition of fans that provides us with part of the story of a canyon's history. For the Monterey Canyon, there are two distinct groups of fan deposits, the older group further out than the younger one. The older sediments seem to have started accumulating perhaps as long as 20 million years ago, fed by a system of canyons, not just the ancestor of today's Monterey Canyon. But we must remember what a tectonically turbulent setting this is, caught between the San Andreas and parallel fault zones offshore - a lot has changed in 20 million years, including moving a long distance northwestward. The younger stack of fan sediments originated within the last million years and continues its activity today; those million years include dynamic changes to the tectonic architecture and topography of this part of California, and the huge fluctuations in sea level during the the Ice Ages - no wonder it's a complex system.

Around a million years ago, California's geography was quite different: much of the Central Valley was a vast freshwater lake, Lake Corcoran, that drained at its southern end into the ancestor of the Salinas River, a much mightier feature than its descendant, cutting through the Coast Ranges and draining into Monterey Bay; it is proposed that it was this river and its massive cargo of sediment torn from the Sierras that initiated the Monterey Canyon as we see it today, quite possibly aided by ever-recurring fault movements. Around 500,000 years ago, upheavals along the San Andreas permanently closed off the river's route - the Monterey Bay drainage system would be emasculated and Lake Corocoran would find a new outlet northward into San Francisco Bay. Elkorn Slough and today's Salinas River, while still players in the canyon's game, are not what they used to be. For today's dynamics, I've annotated a Google Earth image:

 

The Monterey Canyon forms, understandably, the boundary between the Santa Cruz and the Southern Monterey Bay littoral cells - it's the "sink" that permanently sucks sand out of both(for more on littoral cells, see "Beach Nourishment and Sediment Budgets"). General littoral drift, the direction sand transport along the shore, is shown with the orange arrows. For the Santa Cruz cell, sand is transported generally down the coast, all the way from Half Moon Bay (west of the El Corte de Madera Creek tafoni of another earlier post), only to pour over the sides of the canyon and be swept out to sea. South of the canyon, transport along the shore is more complex - in part as a result of the canyon's topography influencing waves and currents. Three "sub-cells" have been identified, with different senses of sediment transport, influenced too by the Salina River. Interestingly, before 1910, the Salinas flowed northward, parallel to the shoreline, and emptied into Elkhorn Slough, but it then broke through the dunes separating it from the ocean to flow essentially along its current route, where we keep it firmly in place. But the shape and size of the sand accumulation of its mouth suggest that this may have been its preferred long-term itinerary during the ice ages.

The Santa Cruz cell has been estimated to deliver, on average, 223,000 cubic yards of sand (12,000 dump trucks) per year, 85% of it from rivers (much diminished under our control); some of this gets caught up behind the structures of the harbour at Santa Cruz. The Southern Monterey Bay cell, although much smaller, includes the input from the Salinas River and shifts 840,000 cubic yards of sand a year, 58% from rivers, the rest blown in off the coastal dunes. Moss Landing sits at the boundary of these two littoral cells, at the mouth of Elkhorn Slough: the head of the canyon yawns only a few hundred meters out to sea, and dredged material dumped near the head disappears quickly - it would seem that the canyon mouth is eroding shorewards, potentially threatening Moss Landing.  

Ongoing processes in the canyon have been studied in many different ways: ROV's (remotely operated vehicles, miniature submarines laden with instruments) roam its topography, seismic profiles enable us to peer through the layers of sediment and reveal the complexity of the fan systems in cross-section, cores of the sediments are taken, and instruments measuring currents and other data are moored (often temporarily) along its length. ROV images are useful for hunting down equipment that has been swept away, but primarily reveal extraordinary details of events in the canyon. Below, from the paper Trail of Sand in Upper Monterey Canyon (see the references at the end) are images of cobbles flushed down the canyon (their rock types easily correlated with the onshore geology) and sand ripples on the floor of the canyon (the black bars are 10 centimeter scales).

 

Perhaps most incredible is the detailed imagery generated by multibeam sonar that creates digital maps of the canyon system at high resolution. Below is one example, from Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry; it shows the upper canyon system in startling detail - the base of the canyon is marked by giant sand waves, several metres high and tens of metres in wavelength, formed by the immensely strong currents. This is one of a time-series of images that have enabled definition of changes day-to-day as avalanches scour the canyon walls, sediment gravity flows deposit sand banks and erode channels, sand waves move on, and meanders change their shape.

 

It has been estimated that around 400,000 cubic yards of sediment is delivered to the canyon head on average each year, to be flushed episodically out to sea. But this is only part of the story - equally huge volumes of sediment are supplied to the canyon by collapse of its walls and by deeper ocean currents. The size of those flushes varies enormously; once again, nature follows a power law - lots and lots of small events, a few massive ones. Truly massive, catastrophic, flows occur only on a scale of years and something sufficient to move sediment all the way down into the far reaches of the canyon seems to take place on a scale of every hundred years or so; the last one of these may have been associated with the 1906 earthquake.

We have much still to learn about this great and dynamic topographic feature, but what we do know hints at the turmoil going on every day beneath waters of Monterey Bay - visit, turn your back on the commercial claptrap of Cannery Row, stare out to sea, and wonder.

[The resources on the Monterey Canyon are vast, many of them originating from the MBARI, the USGS, workers at California State University at Monterey, Stanford, and other research institutions. Brian at Clastic Detritus (who has also posted on the canyon) kindly provided a link to a USGS paper by Fildani and Normark (the latter being one of the long-term canyon workers), http://walrus.wr.usgs.gov/reports/reprints/Fildani_MG_206.pdf; some of the material for this post has also been taken from two Geological Society of America papers: Trail of sand in upper Monterey Canyon: Offshore California, by Paull and others at MBARI (Geological Society of America Bulletin 2005;117;1134-1145) and Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry by Smith and others at California State, Monterey (Geological Society of America Bulletin 2005;117;1123-1133). If any readers are interested, but cannot access GSA journals, please let me know and I can email a copy. General articles can be found at the MBARI site, this one (which includes the images at the top of the post and of the shifted instrumentation package) being particularly good,  and here's a summary of the Trail of Sand paper. Fantastic data on California coastal sediment movement and sand budgets can be found in the report,  http://www.dbw.ca.gov/csmw/pdf/Sand_Budgets_Major_Littoral_Cells.pdf, downloadable from the Coastal Sediment Management Workgroup site that contains all kinds of other goodies.]

Comments

[Tomo H]

Very well written and informative. The Monterey canyon system is still a bit of a mystery, but better understanding of of the dynamic nature of the canyon helps throw light on forces involved on a geologic scale.

There are a handful of geologists who also point out that the Pajaro river is periodically blocked by uplift along the San Andreas fault. Lake San Benito forms behind the dam (it's last shoreline can still be picked out along Cienega road)and finally bursts through, scouring on a large scale. It is thought to have happened many times through the Pleistocene into the Holocene. I never see this sequence mentioned in MBARI papers.

Fascinating stuff.

[Sandglass]

Thanks - an interesting update!

[George Woodward]

Does someone know the origin of the name "Elkhorn" for the slough, the canyon, or the valley? I've tried, without success, to find that information.

[Brian Ackerman]

George Woodward - Joseph and Charlotte Roadhouse in 1852 settled on the north side of Estero Grande, where the Packard Ranch is today. The Slough was call Roadhouse Slough during that era but he named it Elkhorn. Perhaps because there were tule elk in the valley and/or the shape of the slough resembles an elk antler.