For most offshore-fishing Louisianans, the existence of hard rock banks off their coast is unknown except for the one commonly called the “Midnight Lump.”
Reef fish fishermen, red snapper fishermen, pretty much limit themselves to red snapper and mangrove snappers. For that, they pull up to an offshore oil and gas platform, or if they are real adventurous, an artificial reef made up of toppled platforms, drop their hook and hope for the best.
For them, the rest of Louisiana’s offshore coast is a barren, open-bottom wasteland of sand or mud.
Boy, are they wrong.
Louisiana has hard bottoms, and a surprising amount of it, even it is a bit offshore. The 220 million tons of sediment delivered to the Gulf by the Mississippi River long ago buried any open hard bottoms. Off of the Texas-Louisiana coast, those sediment deposits are up to 49,000 feet thick.
But those huge, bottom-burying deposits are also the father of Louisiana’s present hard bottom — massive rocky banks that loom up from the Gulf’s floor. The sediments cover a layer of rock salt almost 10,000 feet thick. The weight of the overlying sediments has caused the salt beneath them to flow and push up through weak places in the sediment, forming dome-shaped diapirs, as scientists call them, or banks or “lumps” in fishermen’s terms.
Best known is Sackett Bank, known to fishermen as the “Midnight Lump,” near the Mississippi River delta.
On the far western end of the state, offshore of the Texas/Louisiana border are the East and West Flower Gardens, the highest banks. They come near enough to the surface to be encrusted with true corals.
Offshore banks on the Texas/Louisiana Shelf come in two flavors: midshelf banks and shelf-edge banks. Mid-shelf banks typically rise from depths of 260 feet or less, and are 13 to 164 feet high. Their tops and sides are limestone, claystone and siltstone.
These banks, nearer to shore, are heavily affected by turbid water and sediments, and are less productive of reef fish.
Named midshelf banks include Sonnier Bank, Fishnet Bank, Claypile Bank, 32 Fathom Bank, Coffee Lump, Stetson Bank, Phleger Bank, and 29 Fathom Bank.
Shelf-edge banks are the big-boys of offshore structure in the Gulf of Mexico. They rise from deep waters (often 600 feet deep) near the edge of the continental shelf. Like midshelf banks, they are capped by carbonate rocks.
Ninety-five species of reef fish, plus many other fish, have been observed on shelf-edge banks.
Shelf-edge banks include Ewing Bank (Pellegrin’s wonderland), East Flower Garden Bank, West Flower Garden Bank, Sackett Bank, Geyer Bank, Rankin Bank, Elvers Bank, MacNeil Bank, Appelbaum Bank, Bright Bank, McGrail Bank, Alderdice Bank, Rezak Bank, Sidner Bank, Jakkula Bank, Bouma Bank, Parker Bank, Diaphus Bank, and Sweet Bank.
More than 130 of these flat-topped, steep-sided, mountain-shaped banks are found along the Louisiana-Texas coast. Pellegrin recommends using Google Earth to locate the banks and to become familiar with their outlines.