FASCINATING IOWA CORES: RIFTS AND CRATERS

In 1986 Amoco Oil Company drilled a petroleum test well near the small town of Halbur in west-central Iowa. The well penetrated a flanking basin of the 1.1 Ga Midcontinent Rift System, achieving a total depth of 17,851 feet (5,441m) before ending in undeformed 1.28 Ga gabbro. While cuttings were collected every 10 feet, five core samples totaling 73 feet (22m) were collected from the Precambrian “Red Clastics” interval at depths ranging from 8,834 feet (2,692m) to 16,058 feet (4,894m), including a section of organic-rich siltstones and shales that could have served as petroleum source rock in the past. These samples represent some of the deepest rock cores collected from the Midwest region of the United States.

The Manson Impact Structure (MIS), once thought to be the dinosaur-killer, is a 24 mile (39km) wide complex crater that formed 74 million years ago. Core samples from the MIS show a chaotic array of breccias that underwent a wide range of thermal exposures during the crater’s formation. Cretaceous shale clasts with melted rinds alongside Precambrian granite clasts floating in a matrix ranging from silty clay to fused glass can be seen in these exquisite cores.

A second meteorite impact structure was recently identified beneath the northeast Iowa town of Decorah. The Decorah Impact Structure (DIS) is a simple crater measuring approximately 3.5 miles (5.6 km) wide that formed about 460 million years ago. The crater was originally identified by the presence of a unique shale deposit that only exists within the crater. The Winnishiek Shale is a thinly laminated, dark gray shale that hosts a multitude of exquisitely preserved fossils, garnering the designation as a Lagerstätte deposit. The Winneshiek Lagerstätte has produced the oldest described eurypterid, Pentecopterus decorahensis, along with many other fossils that are still being researched. Beneath the Winneshiek Shale is a thick breccia unit that likely represents the floor of the crater. These core samples are the first to penetrate this mysterious geologic feature.