Context : Bright knots along arms are observed in K-band images of many grand-design spiral galaxies .
They are identified as young starforming complexes using K-band spectra which show strong Br _ { \gamma } emission .
Their alignment with spiral arms and ages < 10 Myr suggest that they are associated with a starforming front linked to a density wave in the galaxies .

Aims : Ages may be estimated for the youngest starforming complexes using NIR broad band colors and Br _ { \gamma } .
A different angular speed of the density wave and material would lead to an azimuthal age gradient of newly formed objects .
We aim to detect this gradient .

Methods : Deep JHK-Br _ { \gamma } photometry of the southern arm of the grand-design spiral galaxy NGC 2997 was obtained by ISAAC/VLT .
All sources in the field brighter than K=19 mag were located .
Color-color diagrams were used to identify young stellar complexes among the extended sources .
Ages can be estimated for the youngest complexes and correlated with azimuthal distances from the spiral arms defined by the K-band intensity variation .

Results : The extended sources with M _ { \mathrm { K } } < -12 mag display a diffuse appearance and are more concentrated inside the arm region than fainter ones , which are compact and uniformly distributed in the disk .
The NIR colors of the bright diffuse objects are consistent with them being young starforming complexes with ages < 10 Myr and reddened by up to 8 mag of visual extinction .
They show a color gradient as a function of their azimuthal distance from the spiral arms .
Interpreting this gradient as an age variation , the pattern speed \Omega _ { \mathrm { p } } = 16 km sec ^ { -1 } kpc ^ { -1 } of the main spiral was derived assuming circular motion .

Conclusions : The alignment and color gradient of the bright , diffuse complexes strongly support a density wave scenario for NGC 2997 .
Only the brightest complexes with M _ { \mathrm { K } } < -12 mag show a well aligned structure along the arm , suggesting that a strong compression in the gas due to the spiral potential is required to form these most massive aggregates , while smaller starforming regions are formed more randomly in the disk .
The sharp transition between the two groups at M _ { \mathrm { K } } = -12 mag may be associated with expulsion of gas when the first supernovae explode in the complex .