First constrained optimization (CONOP9) analyses on Ordovician graptolites from the Central Andean Basin.

HERRERA SÁNCHEZ, N.C.; AND TORO, B.A

Buenos Aires

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

CONOP9 is an automated graphic correlation program that is multi-dimensional and examines the paleontological data from multiple stratigraphic sections simultaneously. It proposes sequences of first and last appearance datums of fossils; it also rejects impossible solutions (constraint) and searches by sorting through many possible solutions for the best (optimization). This methodology has been applied by many authors in North America, Scandinavia, Gondwana, and the Argentine Precordillera to correlate a number of stratigraphic sections, revising the geological time scale and building biodiversity curves. These researches have been mainly focused on conodonts, foraminifera, chitinozoans, and trilobites. However, the graptolite composite range charts and biodiversity curves obtained with CONOP9 are scarce and some results disagree with those traditionally accepted. In the framework of the Ph.D. thesis of one of the authors (N.C.H.S.), we applied CONOP9 to 23 sections with 69 records of graptolite taxa from the Central Andean Basin for the first time, to obtain a composite range chart and analyze regional biodiversity patterns for the interval comprised between the early Tremadocian (Tr1) to the early Dapingian (Dp2). The composite range chart shows the first and last appearance datums listed for each taxon in a calculated composite section based on the correlation of individual studied sections. CONOP output also includes a histogram that shows the local ranges of each taxon as a function of the position within each of the composite ranges and uses an interval-free method of analyzing species biodiversity. In the resulting composite range chart, we identify the Aorograptus victoriae, Sagenograptus murrayi, Hunnegraptus copiosus, Tetragraptus phyllograptoides, Paratetragraptus akzharensis, Baltograptus cf. B. deflexus, Didymograptellus bifidus, Azygograptus lapworthi and “Isograptus victoriae” biozones, corresponding to the late Tremadocian (Tr2) - early Dapingian interval, based on the range of key taxa or biostratigraphically significant taxa. Certain stratigraphic ranges such as those of Rhabdinopora parabola, R. flabelliformis, Anisograptus matanensis, Adelograptus altus, A. tenellus, and Bryograptus kjerulfi, either partially or completely, occurred in unlikely positions in the range chart. It could be related to poorly correlated fossils, restricted stratigraphic ranges to once or twice sections, monospecific horizons, or artificially truncated single taxon last appearance datum at the top of a section. CONOP9 tends to float these to low diversity areas to minimize unobserved coexistences; their succession, therefore, does not represent what is occurring in some local sections. To avoid the influence of these inconsistencies on the biodiversity curve, it was restricted only to the early Floian (Fl1) to early Dapingian taxa. The first diversification phase occurred in the early Floian (T. phyllograptoides and P. akzharensis biozones), during which the diversity experienced a steady increase and reached the acme. This is in agreement with the graptolite biodiversity curves previously proposed for the same interval in Baltica, the Yangtze region of South China, and Australasia. The study provides interesting results to verify the biostratigraphic framework previously proposed for the Central Andean Basin, but also suggests that probably more data need to be included in future research to solve the remaining questions related to problematic or poorly distributed graptolite taxa.