How are turbidites formed?

Decoding Underwater Avalanches: How Turbidites Tell Earth’s Story

Ever heard of an underwater avalanche? That’s essentially what creates turbidites – a fascinating type of sedimentary deposit formed by what we call turbidity currents. Think of it as a muddy mix of water and sediment suddenly deciding to take a rapid downhill plunge. These geological formations aren’t just cool to look at; they’re like time capsules, giving us clues about ancient climates, long-gone ocean currents, and even major geological shake-ups.

So, what kicks off these underwater avalanches? Well, a few things can trigger them. Imagine a river dumping tons of sediment into the ocean – sometimes, that build-up gets too heavy and unstable, leading to a slide. Or picture this: a sudden earthquake rattles the seafloor, stirring up sediment and creating a dense, fast-moving slurry. A classic example? The 1929 Newfoundland earthquake – it triggered a massive submarine landslide that turned into a powerful turbidity current. Even big storms or strong ocean currents can stir things up enough to get a current going. Sometimes, it’s just a case of a slope being too steep, with sediment piling up faster than it can settle, eventually giving way.

Once a turbidity current gets rolling, it’s like a runaway train, picking up speed and sediment as it barrels down the slope. The faster it goes and the more sediment it carries, the wilder the ride – and the more interesting the turbidite it eventually leaves behind. As the current hits flatter ground, it starts to slow down, and that’s when the sediment starts to drop out, layer by layer.

Now, here’s where it gets really neat: turbidites often show a “fining-upward” sequence. What that means is, the bottom layers have the coarser stuff, like sand and gravel, and as you move up, the sediment gets finer and finer, like silt and mud. It’s like nature’s way of sorting things out as the current loses steam.

Back in 1962, a guy named Arnold H. Bouma came up with this idealized model to describe these layers, and it’s now known as the Bouma sequence. It’s a classic example of how low-density turbidity currents form deep-water marine sediments. It’s not always perfect – sometimes layers are missing or jumbled up – but it gives us a framework for understanding what happened during the flow.

The Bouma sequence is characterized by five distinct layers, labeled A through E, from bottom to top. The bottom layer, A, is the Graded Bedding, and consists of a coarse-grained interval, often gravel or sand, that grades upwards to finer material. The next layer, B, is the Parallel Lamination, and is composed of sandstone with plane parallel lamination. Layer C is the Cross-Lamination division, and consists of sandstone with cross-lamination and convolute lamination. Layer D is the Laminated Silt and Mud, and is made up of laminated silt to mud. The final layer, E, is the Mudstone, and is the uppermost layer consisting of massive, ungraded mudstone.

Where do you find these turbidites? All sorts of underwater spots! Deep-sea fans are a big one – these are like giant sediment aprons at the foot of continental slopes. Turbidity currents can also travel for miles across abyssal plains, spreading sediment far and wide. Submarine canyons, those deep gashes in the continental slopes, act like highways for these currents, funneling sediment from the coast to the deep ocean. You’ll also find them in deep sea channels, river mouths, and even in deep lake basins.

So, how do geologists spot a turbidite? Well, they look for telltale signs like that fining-upward pattern, sharp, uneven bases (where the current has eroded the underlying sediment), and cool structures like flame structures (where layers get all twisted and distorted). Sometimes, you’ll even see “sole marks” – little impressions left by debris dragged along the bottom by the current.

Why should we care about turbidites? Because they’re more than just pretty rocks! They can tell us about past earthquakes, give us clues about the shape of ancient coastlines, and even record major storm events. They’re like a geological diary, helping us piece together Earth’s history, one underwater avalanche at a time. They can also tell us about tectonic settings and terrestrial events.