ABSTRACT

Calcitic nodules in the poorly indurated, quartzarenitic, Jordan Sandstone of northeast Iowa exhibit large areas of crystallographically continuous calcite and frequently have one or two hematitic shells surrounding an indurated core. Hematitic shells on nodules immediately subjacent to the soil horizon lack calcite cement suggesting its removal by acidic vadose solutions. Geochemical data for near-surface environments prompts interpretation of the origin of nodular calcitic and hematitic bodies in the Jordan Sandstone as follows: 1) partial cementation of the sandstone by calcite as "sand crystals" at selected loci; 2) solution of the distal projections of the calcitic "sand crystals" followed by; 3) episodic precipitation of hematitic shells at the pH-buffered interfaces of the nodu es; 4) continued removal of calcite near the soil horizon and; 5) subsequent phases of hematite precipitation within previous hematite shells.