Abstract: Paleozoic Depositional Sequences; Comparisons and Contrasts in Magnitude and Amplitude of Sea-Level Changes

Charles A. Ross, June R. P. Ross

Using third-order depositional sequences, sea-level fluctuations within Sloss' Paleozoic Sauk, Tippecanoe, Kaskaskia, and lower and middle parts of the Absaroka megasequences may be grouped into seven intervals of general sea-level highstands and lowstands. Similarity of depositional sequences within each interval suggests that patterns in sea-level changes, and possibly the causes of these changes, remained fairly consistent through time intervals of 15 to 35 my.

Sauk third-order sequences show a general rise of sea levels (15 my) which flooded the cratonic shelves. This was followed by a long interval (35 my) of high sea levels. The Sauk was terminated by an interval (15 my) of generally low sea levels (Whiterockian Stage).

The Tippecanoe is divisible into three intervals. The Mohawkian highstands (15 my) ended with a major lowstand. Succeeding Cincinnatian cycles have greater amplitude (7 to 8 my). Latest Ordovician low sea levels lasted about 2 to 3 my. Early and Middle Silurian (25 my) flooding of cratonic shelves was gradual. After the Middle Silurian, sea level fell and remained relatively low during earliest Devonian (20 to 25 my).

Third-order sequences in the Kaskaskia covered the cratonic shelves by the Middle Devonian and flooded most of the craton by the Frasnian (15 to 20 my). Famennian and early Tournaisian cycles had relatively large fluctuations of sea levels. Visean sea levels remained relatively high and then fell abruptly near the end of the Mississippian. These low sea levels continued through the Early Pennsylvanian (20 my).

By the Middle Pennsylvanian (20 my), Absaroka depositional sequences gradually transgressed onto the shelf. They were internally complex, relatively short (1 my), and with large amplitudes (100-200m). Low sea levels near the end of the Pennsylvanian and the end of the Permian divide the Absaroka into lower (Pennsylvanian), middle (Permian), and upper (Triassic) parts.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994