For a long time, the base of the Cambrian was identified by the first-occurrence of shell-bearing organisms such as trilobites.
But in the 1970s, a distinctive group of small shelly fossils was found below the first trilobites in Siberia and elsewhere, and dated at 544 my. This small shelly fauna includes sponge spicules, brachiopods, molluscs, and possibly annelids.
The base of the Cambrian is now placed at the oldest occurrence of feeding burrows of the trace fossil Phycodes pedum, and dated radiometrically at 542 my using uranium-lead isotope dates from rocks in Oman coinciding with a chemical anomaly known as the "negative carbon-isotope excursion."
During the Cambrian, there were no vascular plants on the land, so the landscape was barren. Erosion would have been active and severe without plant roots to hold the soil.
After the Neoproterozoic glaciation, the sea transgressed onto the craton.
Shoreline (beach) deposition produced a vast apron of clean quartz sand.
Carbonate deposition occurred farther from land.
In the Grand Canyon region, the Lower Cambrian Tapeats Sandstone is an example
of the sandy beach deposits
unconformably overlying Precambrian rocks.
The Tapeats Sandstone is overlain by the Bright Angel Shale, an offshore deposit.
The Bright Angel Shale is overlain by the Muav Limestone, indicating deposition farther
from the land.
These rocks form a transgressive sequence.
Cross section of Cambrian strata exposed in the Grand Canyon. The red lines are trilobite zones, which approximate time lines.
Note that these sedimentary units are diachronous (cut across time lines). In each case, the sedimentary units are older in the west than in the east.The three facies (sandstone, shale, and limestone) coexisted and migrated laterally as sea level rose. The Bright Angel Shale is Early Cambrian in the west, and Middle Cambrian in the east.
Near the end of the Early Ordovician, the seas regressed (glaciation) and the Muav Limestone was exposed to subaerial erosion and a widespread unconformity developed.
November 11, 2005