I’m working on a region for my world, and I have a very distinct idea of what I want the physical landscape to look like. Below is a description of what I'd like for the land, and an approximation of how it might have been formed.
Location. The region is on the equator of an earth like planet. It’s a large coastal tropical lagoon of approx 20,000-25,000 square kilometers, smack on top of a volcanically active fault. The fault has formed sharp mountains and cliffs on the border of the coast that isolate it from the mainland, and it's an approx 50km from that border to the sea. The entire coastline is subject to seasonal torrential rains, up to three meter tides, and high humidity. It is in short, very wet.
Karst: The lagoon is home to a large variety of corals, many of which have evolved to grow right up to the shore. The coral built up massive limestone deposits, which water erosion and tectonic plate activity fragmented into a patchwork of cliff plateaus and karst formations, with limestone pillars and arches rising as high as 12m or even 15m meters over the water. There are also a variety of lower platforms and mushrooms shapes, and deep sinkholes, leading down into a network of flooded limestone caves. Some of the pillars also have their own pools on top, flooded sinkholes left by the rain. These may let off waterfalls in heavy weather.
Columnar Basalt: Below the karst, a lot of the ground is made up of stairs and platforms of crumbling columnar basalt, brought in by multiple volcanic eruptions over time. These formations clusters around the broken up limestone plateaus, overlooking beaches of black sand colored by the basalt, and many interlocking pools.
Coral Pools: Coral grows in the pools, and at low tide much of it is exposed like a strange garden. At low tide I'd like most of these pools to range from about 5m to 20m across, and 1m to 10m deep, excluding sinkholes. At the highmark much of the area is flooded over.
Volcanoes: While most of the basalt lava came from the large bordering mountains, some of may have come from volcanoes located inside the karst region itself. If so, these might form small islands in the lagoon.
- Is this region geologically possible? Not necessarily plausible, I'm fine with assuming ideal conditions so long as there's an adequate explanation. If not, is there a way to tweak it and make the landscape to look as close to the above description as possible? If yes, would the geology described imply any other natural features that I should also include in this area.
Here are some potential problems I identified while researching. This is my first crack at constructing a geology, and I’ve been learning as I go, so I have little notion whether any of these items are actually worth worrying about.
- 1) I haven't found a real world limestone karst with columnar basalt. I'm worried this could be because limestone has a cooling effect on lava, but I've read conflicting things about how that plays out practically in a lava flow, and nothing about the relation of it to columnar formation.
- 2) I’m not sure if if the water will be ideal for coral growth. It might be that the basalt sediment and the heavy rain make the water not clear or salty enough. It doesn’t have to be earth-coral of course, so I could posit a hardier version if evolutionary plausible. More simply, I might also draw on a source of extra salt or even some kind of natural filtration system for loose sediment … but I have no idea if that’s even necessary, or if so how to go about it.
- 3) I'm still very much playing with the size and proportions of the region. Is there anything I should know if I make it bigger, play with the proportions, etc.?
This is a very long question, and one that I’m invested in, so I’ll give out a bounty if I think one of the answers is extensive, and helps me get me closer to the region that I'm trying to build. (I don't know if people are motivated by the points, but I'd still want to express thanks.)
BOUNTY EDIT: Bounty goes to Will, but other answers are naturally still welcome, particularly if they suggest other likely features of the landscape given the geology. Side note, I wish you could give out a bounty w/out the 24 hour wait time.