There is not just one sedimentary rock layer.
And because of this. Know a great deal about how sediments deposit and what they look like in the rock record.
Remember, there will be details to explain. If there is a noachian flood deposit it will carry some unique features.
here is paper by dr. Walt brown mentioning a few of these "vast blanket sedimentary rocks," not "one" deposit, but so many of them are thousands of miles long, that views have originated about a global sized sea (paleozic sea) they call it.
"In our southwestern states, where they are well exposed by erosion, but also across the continent and everywhere in the world, thousands to hundreds of thousands of square miles of flat, horizontal strata, from a few feet to hundreds of feet thick are found. At no location on the earth may the production of similar sedimentary deposits of like extent be observed today. These formations are composed of sandstone, graywacke, shale, conglomerate, limestone and other types of rock. Some of them extend for thousands of miles, spanning whole continents. For example, the St. Peter sandstone, composed of clean quartz grains, has been traced in twenty states from California to Vermont.11 The Shinarump conglomerate in the Southwest covers some 125,000 square miles,12 and another conglomerate blanket is reported to extend from New Mexico to Saskatchewan and Alberta.13
A continental blanket of clean sandstone, before being deposited, required a steadily flowing current traversing a great distance to separate the sand from silt and gravel. A continental blanket of conglomerate required a continent-sized maelstrom of water in violent, chaotic motion to dump an ungraded mixture of material of all sizes across thousands of miles of terrain. Tremendous water action such as that which would be produced by a global flood seems to offer the only reasonable explanation for the observed facts."
https://www.cs.unc.edu/~plaisted/ce/flood.html
some of these deposits are so big that other theories have developed:
namely the a paleazoic sea covering most of america and more: below is a clip of some theories regarding the cambrian period (500million years ago), regarding why such sedimentary rocks are all over.
"In North America, the era began with deep sedimentary basins along the eastern, southeastern, and western sides of the continent, while the interior was dry land. As the era proceeded, the marginal seas periodically washed over the stable interior, leaving sedimentary deposits to mark their incursions. During the early part of the era, the area of exposed Precambrian, or shield, rocks in central Canada were eroding, supplying sediment to the basins from the interior. Beginning in the Ordovician Period, mountain building intermittently proceeded in the eastern part of the Appalachian region throughout the rest of the era, bringing in new sediments. Sediments washing from the Acadian Mountains filled the western part of the Appalachian basins to form the famous coal swamps of the Carboniferous Period."
more info here:
https://en.wikipedia.org/wiki/Paleozoic
sorry it's so big, but here is the theory to explain why global flood deposits exist, mainly, not a global flood....but a sea covering most of the planet.
