Figure 34. Absolute ages of the formations in Alaska Basin. Shaded parts of the scale show intervals for which there is no record.
| STRATIGRAPHIC SCALE | ABSOLUTE TIME (Years ago) | ENLARGED PIECE OF YARDSTICK SHOWN ON [FIGURE 19] | ||
|---|---|---|---|---|
| 2 | ||||
| PALEZOIC | PENNSYLVANIAN | ? | ||
| 300 million | ||||
| MISSISSIPPIAN | MADISON | |||
| DEVONIAN | DARBY | |||
| 3 | ||||
| 400 million | ||||
| SILURIAN | ||||
| ORDOVICIAN | BIGHORN | |||
| 500 million | 4 | |||
| CAMBRIAN | GALLATIN | |||
| GROS VENTRE | ||||
| FLATHEAD | ||||
| 600 million | ||||
| PRECAMBRIAN | 5 | |||
Figure 35. The first invasions of the Paleozoic sea.
A. In Early Cambrian time an arm of the Pacific Ocean occupied a deep trough in Idaho, Nevada, and part of Utah. The land to the east was a broad gently rolling plain of Precambrian rocks drained by sluggish westward-flowing streams. The site of the Teton Range was part of this plain. Slow subsidence of the land caused the sea to move eastward during Middle Cambrian time flooding the Precambrian plain.
B. By Late Cambrian time the sea had drowned all of Montana and most of Wyoming. The Flathead Sandstone and Gros Ventre Formation were deposited as the sea advanced. The Gallatin Limestone was being deposited when the shoreline was in about the position shown in this drawing.
C. In Early Ordovician time uplift of the land caused the sea to retreat back into the trough, exposing the Cambrian deposits to erosion. Cambrian deposits were partly stripped off of some areas. The Bighorn Dolomite was deposited during the next advance of the sea in Middle and Late Ordovician time.
