Origin of Nodular Structures in Devonian Limestones of the Cracow Upland, Southern Poland

Abstract

The origin of nodular structures in limestones, whether by mechanical and/or chemical processes, poses serious interpretational problems for sedimentologists. Outcrops of Upper Givetian-Lower Frasnian slope-facies carbonates of the Debnik area, Cracow Upland, have been studied to determine the genesis of their extensive nodular structures. The following types of features have been recognized: (1) nodular and wavy limestones composed of isolated, ellipsoidal, peloidal nodules within marly matrix; (2) flaser-nodular limestones (with solution seams); and (3) conglomeratic nodular limestones composed of irregular micritic fragments plus rounded intraclasts and plasticlasts. Oxygen isotopic analyses of the nodules fall into three distinct groups. The first ranges from {minus}4.4 to {minus}5.0{per thousand} {delta}{sup 18}O(PDB) and includes Type 1 nodules and lithified nodules of Type 3; the second grouping ranges form {minus}5.0 to {minus}5.3{per thousand} and includes some Type 2 nodules and Type 3 plasticlasts; the third group, ranging from {minus}6.0 to {minus}7.4{per thousand}, contains samples form intervening marls and residual seams plus some Type 2 nodules. From these field and laboratory observations, one can conclude that nodular and wavy limestones originated by early diagenetic concretionary cementation and subsequent pressure solution, conglomeratic-type nodular limestones were produced by bioturbation and erosional reworking of partially cemented deposits; flaser limestonesmore » formed by pressure solution during burial diagenesis. Thus, the nodules in these rocks were formed over a long time period and by several processes, a conclusion which probably applies equally to many other carbonate units.« less