MICROSTRUCTURAL ANALYSIS OF STROMATOLITE LAMINATIONS IN A LATE CAMBRIAN SHALLOW SEA, ALLENTOWN FORMATION, PENNSYLVANIA & NEW JERSEY, USA

The Upper Cambrian Allentown Formation in eastern PA and northern NJ developed on a transgressive, passive margin in subtropical to tropical southern paleolatitudes. The unit’s mostly dolostone lithologies include a variety of facies over a range of mud to gravel clast sizes. Sedimentary structures such as rip-up clasts, ripple marks, and mudcracks suggest fluctuating subtidal to intertidal conditions and storm event stratigraphy within a shallow, marine environment. Notable among the unit’s facies are variable microbialite horizons, often in association with ooid-rich layers. Among these are large stromatolites which occur at multiple levels in the formation and are laterally persistent over outcrop scale.

Allentown Fm. stromatolite samples were collected from roadside exposures in and near Easton, Pennsylvania. We targeted large (>50 cm across), thick (>10 cm) domal stromatolites along beds and tied them into measured stratigraphic section for depositional context. Samples were cut and polished into thin sections for examination of lamination thickness, lamination contacts, color, and grain size and content. We noted uninterrupted sections with conformable laminations as well as erosional hiatuses associated with coarse-grained, disordered layers.

Our stromatolite samples exhibit mostly lighter color laminations dominated by dolomitic micrite as matrix between large dolomitic intraclasts, dolomitized ooids, and rare quartz. These contrast with the prominent, darker color and finer-grained laminations that occur less frequently. These associations occur in multiple stacked sets with ~15-20 light laminations bracketed by dark horizons. Contacts between conformable laminations are often irregular, reflecting microtopography at the time of deposition but micro-unconformities beneath disordered, coarser-grained layers are interpreted as “event” horizons.

Using stromatolites as environmental recorders, we used multiple statistical analyses to search for stochasticity or periodicity and address possible paleoclimate and paleoceanographic causes of lamination patterns within the samples. We also note the frequency of erosive events that affect or destroy lamination growth and consider how these may be records of the passage of tropical storms in Cambrian paleogeographic context.